If you've been following the buildup of GMHTP's project 2001 Trans Am WS6, you know that we've been hinting at an imminent displacement increase. While bolt-ons like our full exhaust, big intake, and direct-port nitrous system have gotten excellent power increases out of this Pontiac's internally stock engine, there's nothing like a bump in cubic inches to really wake things up.
Unfortunately, pumping up cubes usually means pumping many thousands of dollars in the direction of a speed shop's bank account. Highly competent shops exist throughout North America that will be happy to take your hard-earned cash and turn your late-model, Gen III-powered ride into an impressive performer. This is definitely not a bad way to go.
But putting together one's own high-power, stroked EFI engine can be just as--or more--rewarding than enjoying the end product. After all, anybody with a wad of cash and zero hands-on knowledge can drop his or her car off at one of the aforementioned establishments and drive away a few weeks later with a couple hundred extra horses under the hood. The more unique breed of person dreams of building and installing such an engine at home, thereby gaining the satisfaction of tackling a seemingly daunting task many would not dare. But can such an undertaking really be done in the average suburban garage? Or is this power-hungry do-it-yourselfer destined to spend thousands of dollars extra having a shop build and install this stroked motor instead?
The time has come to find out. We're going to double the fun and halve the cash by building a stroker in our own garage. Follow along as we transform our Trans Am's 346-inch LS1 into a 383-cube LS-wonder.
Stroker Parts Selection
As we've hopefully made clear, there are two interlocking themes to this story: build-it-yourself, and build on a budget. The two go hand-in-hand, as the do-it-yourselfer is likely building his or her own engine in large part to save labor costs.
The question of what one looks for when building a budget-conscious stroked LS1 can't be asked unless one knows how tight cash really is. Someone on an absolute shoestring budget can get away with cost-saving measures like reusing the stock cylinder heads and just having a simple port job done to them. However, when it comes to the parts to actually make the engine a stroker, there are minimums of what must be bought, and the major items include a crankshaft, connecting rods, and pistons.
Before we get into how to go about choosing components, keep in mind the single most important canons of engine building: inform yourself, set realistic goals, and stick with your decisions throughout the stages of selecting parts, engine assembly, and eventual enjoyment out on the road. Ignore these, and disaster is certain to occur. And while we'll guide you in the right direction of understanding stroker LS1 component design and selection, we'd be doing you a disservice if we purported to include "everything you need to know." We just don't have the room to cram it all in. So, do your research! We'll mention some good books to pick up, and of course there's a ton of info to be garnered from visiting LS1-oriented Web sites. It also helps to make sure you pick up a copy of GMHTP every month!
Crankshaft: The Heart of a Stroker
GM uses a 3.622-inch crankshaft from the factory in the LS1; so how much bigger should you go? For our application, we wanted to make sure we would still have a reliable, daily drivable car that wouldn't suck fossil fuel like there was no tomorrow. Therefore, we chose to go with a 4-inch stroke, giving us 383 cubic inches (6.3 liters). Though larger cranks can fit, our choice to forego "throwing in the kitchen sink" is based on a few things in addition to those we've already mentioned. First, clearancing of the block will be minimal, saving us headache. And, keeping our planned moderate nitrous usage in mind, we'll limit our horsepower level enough that we'll be able to safely stick with our stock steel main caps without worry that we should have shelled out cash for some expensive billet units--and the accompanying machining costs to fit them to the block.
The analysis doesn't end there. Absent an adjustment in rod length, increased crankshaft stroke will decrease compression height (the distance between the centerline of the piston pin and the face of the piston). With our plans for nitrous use, we wanted as much ring land strength as possible; the smaller the compression height, the less space there is for the rings, and at a certain point the top compression ring will have to be moved too far toward the face of the piston, exposing it and the now-thin ring land to more heat than they can bear. You'll see these dimensions explained further in the photo captions when we get to piston assembly.
Connecting Rods And Pistons: Adding To The Equation
There are a lot of LS1 stroker crank manufacturers, and that variety extends to the rods and pistons. When choosing each of these components, the money you'll need to spend will rise with the amount of horsepower you're looking to make. It's possible to go overboard for a budget street build--but more importantly, it's possible to "cheap out" and buy parts from less reputable companies, many of whom have their stuff made overseas. It is far better to spend a little extra money on a company with a good name, so you can rest easy knowing the parts will stand the test of your lead-weighted right foot.
Matching all of these items can be a hassle though. Different crankshafts are often manufactured with varying journal shapes as well as counterweight sizes, so this limits the range of rods that can be used. (We are not going to get into the debate on optimum rod length here; it's beyond the scope of this article. You've probably heard it: longer rods decrease piston side loads, while shorter rods increase piston speed and decrease dwell time.) Different piston designs can interfere with the shape of the crank throws at bottom dead center, and different rod designs and lengths also affect piston specifications. Plus, after juggling rod length with piston pin height, you have to throw all of this into your compression ratio equation.
Worst of all, after the hours upon hours of head-scratching and parts-matching, the crank must be precisely matched to the rods, pistons, and all other reciprocating masses using a process called "balancing." This can get expensive; it involves drilling and often adding heavy, pricey Mallory metal in precise locations in the crank counterweights. Think of it like balancing a wheel and tire--only it's much more complex, and the stakes are far higher.
Save yourself the headache of all of the above issues and buy an entire rotating assembly that is already matched, balanced, and ready to go. It takes a lot of potential for error out of the equation--and you'll save money, too, as it's a package deal. Rotating assemblies for high-power, reasonable-budget builds like ours are readily available in many different configurations of stroke and piston type; see the sidebar on the Lunati rotating assembly we chose for this build.
The Rest Of The Engine
Once you've got an idea which rotating assembly you want, the only other major item to put thought into is the cylinder heads. We're not going to dictate what makes a good head and discuss all of the options out there for LS-series engines (see our "Gen III/Gen IV Cylinder Head Buyers' Guide," Jan. 2006). There's just too much to it. What's important from a do-it-yourself perspective is this: combustion chamber size (cc). Along with head gasket thickness, piston valve relief pocket cc, and a couple other factors, this will determine your compression ratio. A cylinder head and a rotating assembly should be chosen at the same time, since not all heads can be had with all combustion chamber cc's. We won't get to install our heads in this segment, but expect an in-depth discussion of cylinder head selection, as well as a helpful sidebar on compression ratio and how to calculate it, soon!
Beyond rotating assembly and cylinder heads, there are other miscellaneous items you'll need, like a camshaft, valve lifters, timing chain, oil pump, and so on. Some of these components can be reused if dictated by a particularly tight budget. We'll discuss these other items in the photo captions and sidebars as we go along. (But we won't get to most of these extras until next time, when we'll also have a complete rundown of all parts used and tally up the cost.) Just remember, we're not doing anything crazy here: just a dependable, streetable build without blowing too huge a wad of dough. With these emphases in mind, let the fun begin!
Starting Off: The Engine Block
We're not going back on our promise that you can build a stroked LS1 yourself. But the one thing that would be highly impractical to do on your own is engine block machining. The tools required for this cost many thousands of dollars; but luckily they are in the hands of your local engine machine shop.
At the Machine Shop: Machining Processes and Options
Fortunately for late-model enthusiasts, GM designed the Gen III block so well that very little machining has to be done to freshen it up. The days of align honing block mains are virtually a thing of the past, and even decking of these blocks is often unnecessary. Generally, all you'll need to have the shop do is install a new set of cam bearings and perform a cylinder hone.
Cylinder honing is simply a term that refers to the precise removal of material from cylinder walls. This stands in opposition to boring, where a large amount of material is removed rather inaccurately. Honing gets the cylinder to the exact size needed and puts the correct surface finish into the metal of the wall. As mentioned earlier, with the aluminum-block Gen III, only a small amount of material can be removed from the cast iron cylinder liners. Simply put, they can't be bored--only honed.
Although rotating assemblies are available that include stock-size, 3.898-inch bore pistons (which would yield 382 cubes instead of 383), sticking with the stock bore size would be a bad idea on all but the most low-mileage LS1 block. Since cylinder taper and out-of-round were distinct possibilities on our 42,000-mile engine, we decided to have the cylinders honed a full 0.005 to accommodate 3.903-inch pistons. This is the best way to go unless you're starting off with a brand-new, never-used engine block.
The machine shop will measure your pistons and decide the exact size of the hone that needs to be performed based on the specified piston-to-cylinder-wall clearance given by the piston manufacturer. This means you'll have to have your pistons in your hands before taking the block to the machine shop.
We should note that while a so-called "deck plate" should ideally be used during cylinder honing (it bolts to the head surface to simulate bore distortion when the head bolts are tightened), it's not absolutely necessary; GM does not use a deck plate when honing blocks at the factory. If your local shop has done enough LS1s and you've heard good things about them, you should be OK--whether they have one of these deck plates or not. While the cylinder tolerances might not be within the kind of exacting specs Hendrick Motorsports looks for, you're probably not leaving that much on the table for a street/strip motor.
Back In Your Garage: Checking And Cleaning
Once you have the block back in your hands, don't just start assembling the engine. Since you couldn't machine the block in your garage, you're going to do the next best thing: double-check the machine work and, after this, make sure the entire block is clean enough for the Queen of England to eat off of.
Preassembly Checks And Fitment Of The Rotating Assembly
We're almost ready to begin assembly of the engine, but before we do, we need to make sure our rotating assembly fits properly in the block. To accomplish this, bearing clearances must be checked, clearance between the rotating assembly and the block must be assessed, and piston rings must be "file fit" to the block.
Checking Bearing Clearances
It is absolutely essential that main bearing clearances be within proper tolerance. If they aren't, reduced bearing life will result. Though the process is a somewhat-tedious "dry run" of the subsequent actual assembly, it cannot be skipped.
Checking Block-To-Rotating-Assembly Clearances
An increased crank stroke pushes the crank journals and connecting rods further outward toward the oil pan rails. In order to check and see whether any part of the rotating assembly is going to contact the engine block, we need to temporarily assemble the crank to the block--this time with oil in the bearings so that it can spin. In this way, a piston/rod assembly can be attached at each cylinder location, and the crank spun to see if there are any areas of interference.
Piston Ring Fitting
With most sets of piston rings, the "top" ring (ring closest to the surface of the piston) and second ring need to be custom-fit to each cylinder. This process involves installing each ring into a cylinder and measuring the gap between the two ends of the ring.
Begin Final Assembly
We strongly recommend following the GM service manual step by step for the entire build. You may be anxious to get your stroker up and running, but doing this--and carefully noting anything you need to skip and come back to--will ensure you don't miss anything (every step is critical!).
We suggest fitting all of the top rings first, followed by all of the bottom rings (or vice versa). This way, you won't be confused when filing the rings: the top and second rings usually look very similar. (With our Lunati set, the only real difference is the ductile iron second rings have a less shiny finish than the plasma-moly top rings.)
Assembling The Short-Block
Now that our entire rotating assembly is properly fitted to the engine block, we can finally start putting the sucker together for good. Though the preceding steps were involved and at times trying of the patience, they are absolutely essential and ensure that this LS1 will perform reliably for tens of thousands of miles to come. So without further ado, let's get to the real deal.
From the bare block, the manual dictates first reinstalling any block plugs that had been heretofore removed. There are several coolant and oil gallery plugs on the LS1 engine, and the service manual dictates where each one is located, what sealant to use on the threads (as applicable), and to what torque to tighten them to.
A crankshaft is a rather heavy item (Lunati's weighs about 50 lbs) so be very careful with the sucker; you might want a helper standing by to help you guide it into the block safely. Also, all journals (both main and rod) are highly polished and cannot be scratched without creating a problem; you'll note that we wait until after the crank is in place to install our ARP main studs, and the reason is so that we don't inadvertently contact one of them while we lower the crank in. Proceed with care!
Connecting Rod And Piston Prep
We're nearing the point where the pistons and connecting rods can be installed into the block. First, though, some preparatory work is in order. Specifically, the rods need to be disassembled, the rods must be attached to the pistons, and the piston rings need to installed and clocked onto the pistons.
Piston And Connecting Rod Installation
With the connecting rods hung on the pistons, it's finally time to shove the pistons down the bores and bolt the rods to the crank--for good. During this process, there will be some special tools involved, but fear not: just as the others we've used up to this point in the build, none of them are out of the price range of the budget do-it-yourselfer.
Connecting Rod Tightening
Once all of the pistons have been installed, one can begin carefully and systematically tightening the rod caps. Lunati uses rod bolt stretch measurement to make sure the rod bolts are correctly "tightened;" this is considered the most accurate method of evaluating whether a fastener has been properly "secured" (a more proper term).
LS1 Stroker Basics
If you're looking to up the displacement of an internal combustion engine, there are only two ways to go about it: increase cylinder bore, or increase crankshaft stroke. In the case of "boring," you're increasing the diameter of the piston, and hence, the diameter of the hole you need to fill with fuel and air. In the case of "stroking," the pistons travel further downward into the cylinder, meaning a deeper hole is to be filled up.
Stroking is very popular when it comes to Gen IIIs since the ability to increase cylinder bore is quite limited. A minimum thickness must be kept in the block's stock cast iron cylinder liners, and there isn't a whole lot to work with: maximum overbore is generally accepted to be 10 thousandths of an inch (even less for earlier LS1 blocks). Though "re-sleeving" is possible--installing cylinder liners that can accept larger pistons--this process gets expensive in terms of machining costs, as well as the price of the oversize sleeves themselves.
It should be noted that we are focusing only on the Gen III LS1 and related engines in this article; the Gen IV (LS2 etc.) is a bit different and uses a larger bore from the factory (4.000 inches). While the majority of the information in this article will apply to the similar-architecture LS2, there are too many technical differences to keep track of and still keep this story readable. Hence, our Gen III-only focus.
But "stroking" an engine is not simply a matter of swapping the crankshaft out and calling it a day. Rather, a different crank will require changes to the entire rotating assembly. The piston still must reach the top of the bore during each crankshaft revolution, so the stock LS1 pistons' pin location would dictate an extremely short rod if used with a stroker crank. Besides, the strength of a stock cast aluminum piston is peanuts compared to any aftermarket forged unit and probably wouldn't stand up to the power levels produced by even the most conservative stroker build. And while the stock GM powdered metal steel connecting rods are an appropriate length for most strokers (6.098 inches), a few obstacles stand in the way of reusing them. Aside from the stock rods being press-fit to the piston pins, nearly all aftermarket cranks are filleted around the edges of the journals, whereas the stock nodular iron crank is not. This means the big end of the rod is simply incompatible with most aftermarket cranks.
The Lunati Rotating Assembly
Lunati, a trusted name in aftermarket internal engine components for the last four decades, now offers a line of LS1 Pro Series Stroker kits. These rotating assemblies are designed to fit the needs of any Gen III build, with varying crankshaft strokes and piston designs to help achieve the displacement and compression ratio the customer demands. These assemblies are a matched set and include Lunati's own Pro Series 4340 non-twist forged steel crank, Pro Billet Super Light connecting rods (also aircraft-quality 4340 forged steel), and 4032 forged aluminum pistons. Based on crank stroke and cylinder bore selected, rotating assemblies yielding cubic inches from 347 all the way to 447 can be selected (with a modified block on the high end of course). The kits also include piston rings as well as main and rod bearings, making them 100 percent complete. We'll discuss the high-tech features of each of these components as we proceed with the build.
We selected PN EA035-383, which with its 4.000-inch stroke and 3.903-inch bore yields the magic Mopar value of 383 cubic inches (hey, at least it isn't a Ford number). As stated in the main text, the 4.000-inch stroke crank will require minimal clearancing of the engine block to fit, and the 3.903 bore will be achieved with a simple cylinder hone. This rotating assembly is rated at a whopping 1,100 hp, which equates to around 900 at the tires--more than sufficient for any street machine. It carries a suggested retail price of $3,800. Not small change, yes, but the kit is a huge savings over purchasing all of the individual items alone (you'd spend an extra $600). Plus, with a typical engine balancing running at least a couple hundred dollars, this money stays in your pocket as well.
And with the rotating assembly being 100 percent Lunati, you can rest easy knowing your bottom end is made of the best quality stuff on the market, all forged and machined in the U.S. of A.
Bearing Clearances, And How To Measure Them At Home
When we say bearing clearance, we're talking about the amount of space between the fast-spinning crankshaft journal and the bearing (whether they be rod or main bearings). This space must be occupied by a certain amount of oil to ensure proper lubrication. Too much main bearing clearance and you lose oil pressure; too little and oil can't properly flow into and out of the space.
While it's possible to assess this by carefully measuring and comparing the bearing bore diameter to the bearing thickness and crank journal diameter, the problem is that all but the most well-equipped machine shop lacks the kind of super-expensive, precise measuring equipment needed to do this. All of the tools in our possession are only accurate to about half of a thousandth of an inch; for each item you're trying to measure and compare, the error only compounds. Therefore, we're going to go with the tried-and-true method of the home hot rodder and professional engine builder alike: Plastigage.
The Plastigage we chose was manufactured by Sealed Power. We needed the type that measures clearances in the range of 1 to 3 thousandths: PN SPG-1. Plastigage is available by mail order or at any machine shop. It's cheap, too; heck, our machine shop gave us our piece for free.
Basically, the Plastigauge gets squeezed between the bearing and the crankshaft journal when the main or rod cap is tightened. By measuring the width of the smooshed piece of Plastigauge, we know the bearing clearance. More smoosh = less clearance for oil to flow through. It's simple, but this stuff is quite accurate and has been used in engine assembly for decades. We've detailed the procedure for checking main bearing clearances in the photo captions; the process of checking rod bearing clearances is virtually identical.
The Lunati Friction Package
We asked Mark Chacon, Lunati's East Coast Regional Rep., about the reasoning behind Lunati's choice to include bearings in its kit that provide somewhat greater clearances than those used by GM.
"As a general rule, high performance or race engines may need different clearances than a factory engine. Such engines will generate more bearing heat than a stock engine, and the engine builder may decide to establish bearing clearance to whatever he is comfortable with. Everyone has their own take here and you will get a different opinion depending on who you talk to. If a given bearing clearance works well for a given environment that an engine is being used in, and no excessive bearing wear has been seen once the engine is torn down, it's been my experience that the engine builder will continue to do what he has had success with.
"That said, my opinion on bearing clearance for a high performance street or race engine would be 2.25 to 2.50 thousandths on the rod bearings and 2.50 to 2.75 thousandths for the main bearings. The engine may need to run a slightly heavier weight oil to keep the necessary oil pressure (10 psi per 1,000 rpm), but this is a good area for compromise: there's simply more protection inherent in a slightly heavier weight motor oil. Sure, it might cost a small amount of horsepower to drive the oil pump, but in my view it's worth it. After all, engine building is nothing more than a series of well-thought-out, strategic compromises.
"Oil volume and pressure are the lifeblood of your engine, and the type of oil used can affect this to some degree. For example, synthetics may cause you to lose some oil pressure because they are slicker and a bit more effective at lubrication. I may be inclined to be a bit tighter with my tolerances if using synthetic oil; but we're not talking big amounts here. Similarly, I might be inclined to go a little wider on clearances in an endurance application; for example, a circle track or marine application where the engine will be at mid to high rpms most of the time. All of these decisions should be based on application as well as the oil type used."
"In my view there are some dangers if you get 'cute' with main and rod bearing running clearances in a street engine. On a short-life, all-out race engine where you're looking to extract every last hp (and with regards to engineering a friction package specific to a total race application), I might want to run some super-light, zero-weight motor oil, and might look at adjusting the bearing clearances to accommodate this. But on the street, who knows when you are going to get stuck in gridlock traffic on a 100-degree day--I like the idea of 20W-50 in my street engine for the extra protection this type of oil will provide. Dry sump engines require altogether different considerations as well. Again, it's all about the environment the engine will be used in.
"On a final note, I feel that I need to give credit where credit is due, for I am but the student: the real expert here is master engine builder and thinker Bob Mendenhall, who took the time 30 years ago to teach a 15-year-old kid the finer points of engine assembly and advanced engine theory. Thanks again Bob!"
We should note that we'll be installing a high-volume SLP oil pump in the next segment of this story, and that we'll be adjusting it to give increased oil pressure. So stay tuned for more on this topic!
Science Behind Simplicity
OK, so turning a ring filer and sticking a feeler gauge in a ring end gap doesn't take a master machinist. But don't overlook the importance of what you're doing: properly determining and setting piston ring end gap is a crucial part of any engine build, and every engine is unique. Different ring designs and materials respond differently to conditions inside the cylinder, and piston design also factors into how much heat the rings actually see. To further complicate matters, the actual ring gap is not constant even while the engine is running. For example, when the engine is started cold, ring end gap is much wider than it is during full-throttle operation.
It's an exceptionally bad idea to deviate from what the ring manufacturer recommends regarding end gap. Too much gap, and efficiency will be lost; an excess of gases will escape from above the piston--gases that would otherwise have added to cylinder pressure. Too little gap, and with heat expansion under full engine load, the ends of the ring will butt together and destroy the ring, cylinder wall, and piston.
The Pro Series rings we received in our Lunati kit consisted of barrel-faced plasma-moly top rings, ductile iron second rings, and low-tension oil control rings (which do not have to be gapped; they're pre-made and work for a narrow range of cylinder diameters). Lunati specified a minimum of 0.004 inches of ring end gap per inch of cylinder bore diameter for the top ring. So:
3.903 inches bore x 0.004 = 0.015612 inches ring end gap
Rounding up, we get 16 thousandths. However, this number is for a naturally aspirated engine; for this ring set, Lunati recommended adding between 2 and 4 thousandths for an engine that would be seeing a moderate amount of nitrous use. The increased cylinder pressures that come along with nitrous add heat inside the cylinder, which reaches the rings and causes them to expand even more. We chose to err on the side of safety and add the full 4 thousandths, bringing the end gap for the plasma-moly top ring to 0.020 inches. As to the second ring, 0.003 per inch of bore was the suggested minimum:
3.903 inches bore x 0.003 = 0.011709 inches ring end gap
Which rounds up to 12 thousandths. Adding the aforementioned 0.004, the end gap for the ductile iron second ring is 0.016 inches.
Remember, always stick to the recommendations given by the ring manufacturer; don't get cute with "tricks" you heard through the grapevine or found on some online message board. If you have any questions that aren't explained in the instructions, call the manufacturer; they'll be happy to clarify! Remember: screw this step up, and you'll either lose power or end up with a blown motor.
As we hope you're beginning to see, a full motor build requires time, patience, mechanical competence--and most importantly, knowledge. Many resources are available to help you get informed on the parts, tools, and skills you'll need to tackle such a daunting task. Ideally, before even thinking about opening your favorite mail order catalog and perusing for parts, you should get a hold of these resources and get informed about what you are getting into.
Start with GM's service manual for your particular vehicle. They are published for GM by Helm and you'll have to contact Helm to get a hold of one. The GM service manual is an absolutely essential reference when it comes time for the actual build (including the preassembly checks we've gone through; they are detailed as well). Do NOT attempt an engine build without it! This book set is a bit pricey-our 2001 F-body service manual, PN GMP01F, came to $135--but it will pay for itself in peace-of-mind, knowing the job has been done right.
A couple of bookstore publications we recommend are Will Handzel's "Chevy LS1/LS6 V-8s" and Chris Endres' "Chevy LS1/LS6 Performance." The former is particularly essential as it details easy engine removal and installation for many late-model GM vehicles--procedures beyond the scope of this story. Though each of these publications contain their fair share of typos and gloss over some areas of LS1 engine building, the combination of the two will be a great complement to your GM manual.
The LS1 Block: Strong, But How Much So?
We spoke at length to Lunati's learned Mark Chacon regarding his opinion on the limits of the stock LS1 block.
"First and foremost, the aluminum LS1 block is the weak link and will probably fail before our Lunati rotating assembly. As to the configuration of the stock LS1 block, the main caps are never going to be a problem on a naturally aspirated application because of the limited cubic inches the stock block can support (with its max 3.905 inch or so bore). When the stock block with stock main caps will start to be a problem is once a power adder is thrown into the equation. Nitrous will put the most stress on the caps due to the large amount of shock it places on the rotating assembly when the system is first engaged. Generally, turbocharged engines will be easiest on the block because of the gradual way they build boost, and supercharged applications will fall in between the two. And of course, you have to look at the amount of power these items are going to add. If it's going to be substantially more than a naturally aspirated LS1 would normally make, then you need to start looking at buying a set of billet main caps."
"But even with the billet caps, the aluminum-block LS1 can start to have problems around the 950 horsepower range. It's hard to say though: these failures I've heard of could be due to assembly error, a poor tune, and so on. It's for this reason that many big-power-adder racers go to iron Gen III blocks at this level because it is perceived as stronger. But this, too, is debatable. The limits of the LS1 might have more to do with bore capability than block strength, and it all depends on who you talk to."
So to sum up, reusing the stock LS1 or LS6 block is not going to be a problem for the typical naturally aspirated or mild-power-adder street/strip ride (ours included). If you're looking to go with really big power and want to stay reliable, you're going to have to blow cash on billet mains and eventually an upgraded block (perhaps even a C5R or Warhawk). In light of our budget-themed, home-grown build, we chose the stock block and mains; but be guided accordingly.
Why Rod Bolt Stretch?
Although it's common to speak of bolt "tightness" when discussing how to properly secure a bolt, what's really happening is better described as "pre-loading" the bolt. Entire volumes can be written on the physics of bolt pre-load, but for our purposes, just know that if a bolt isn't pre-loaded sufficiently, external forces working on the pieces being clamped together will exceed the bolt pre-load, giving the bolt more internal force than usual. When these external forces are periodic in nature--as is especially the case in reciprocating masses like connecting rods--the fastener's internal force itself fluctuates, which under the right conditions will "fatigue" the fastener and eventually break it. So the idea is to give the rod bolts enough pre-load that the external forces on the connecting rod will never affect the internal force pre-loaded into the bolt. Too much pre-load is similarly bad and will cause premature bolt failure or failure of the threads in the connecting rod itself.
Connecting rod manufacturers differ in the methods they specify for securing their rod bolts. Some use a torque specification only, others use torque-plus-angle, and some--including Lunati--use the rod bolt stretch method. While it is the most time-consuming of the three, rod bolt stretch gives the most accurate reading of bolt pre-load. Any reliance on torque specification simply gives a reading of how much force is required to turn the head of a bolt, and this "tightness" can be influenced by outside factors like thread and bolt head lubrication. Therefore, torque--and to a lesser extent, torque-plus-angle--won't always accurately correlate to the force the fastener is actually clamping the parts together with.
Bolt stretch directly measures the elastic deformation that the fastener undergoes as it clamps down harder and harder. It's analogous to stretching a rubber band: the more it is stretched, the more force it pulls back with. By measuring this stretch and knowing the physical properties of the bolt and its material composition, we get a direct indication of how much force it is holding the rod together with. See the accompanying diagram and equation for clarification.
Though rod bolt manufacturers obviously design and calculate these things very carefully, as an estimate of our rod bolt preload, let's plug some rough real-world numbers into our equation. Assuming a 30,000,000 psi modulus of elasticity for alloy steel, a 0.150 square inch cross-sectional area for a 7/16 bolt, and a 1.25-inch bolt length (to the middle of the threaded region; the diagram shows it as the overall length for clarity), the Lunati-specified bolt stretch value of 0.0050 inches gives a fastener clamping force (pre-load) in the vicinity of 18,000 pounds. With two bolts per rod, that means the suckers are being clamped together by something on the order of 36,000 pounds!