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Main Bearing Clearance - Cranky Dispositon

Checking main bearing clearances & crank endplay

At the heart of your powerplant is the crankshaft, and this baby not only has to endure the entire rotating mass, but it is subjected to countless spin cycles at varying rpm. It's a tough life, which makes proper installation that much more critical. It's nothing tricky, but taking the time to check tolerances can be the difference between a long -running powerhouse and a short-lived paperweight.

But what is it about bearing clearances that's so important? Simply put, if the main bearings are too tight, they'll burn up, and, conversely, if the clearances are excessively loose, you'll end up with a big drop in oil pressure at operating temperature. Let's not forget about the thrust tolerances. If you don't have enough crank endplay, you could burn the thrust up, which will eventually wear out the thrust on the crankshaft. If this should happen, you can expect to pony up some serious green for major reconstructive surgery, by having to reweld or even replace the crankshaft. All said and done, take the time to measure every journal on the crankshaft and each main bearing, and make sure to have your crank endplay in check.

The better it's matched the more you're freeing up available power. It's cheap insurance to know everything is working in unison and that the rotating assembly isn't fighting itself. We all know engines aren't cheap, so do it once, do it right the first time, and you won't have to worry about your mill giving up the ghost to a major catastrophe.


As a quick tip, unless you plan on installing the cam bearings yourself, be sure to ask the machine shop to insert them prior to installing the crankshaft; if not, installing them after the crankshaft has been set into place is extremely challenging. Also, if you don't have the tools to check the main bearing clearances, then you'll be wise to have your machine shop do it for you. We started by installing the bearings-as if we were getting ready to run the crank, just without the crankshaft-and torquing the main caps to spec.

We measured every journal at (as close as possible) the same spot, then turned the crank 90 degrees and measured again to make sure it wasn't out of round. You'll also need to check the taper, checking end-to-end from one side of the journal to the other. As long as the numbers are within 0.1 inch of each other, the crank is good to go.

Since bearings aren't perfectly round, meaning there are two halves of a bearing, you're actually checking the clearances from top to bottom. Don't bother checking the sides, as the clearance numbers will be larger and give you a wrong reading.

After inserting the bore gauge, we checked each main bearing. It's critical to check both sides of the main bearing for taper by measuring before and after the oil groove in the center of the bearing. Any differences should be less than 0.0005 inch.

We cleaned the thrust bearing on both sides to make sure there weren't any high or low spots. The best way to do this is by placing the bearing on its side over 600-grit sandpaper wrapped around something smooth, like glass or a block of granite, and lightly sliding the bearing in a circular motion.

When installing the rear main seals, rather than inserting them flat, QMP likes to cock them a bit. The reasoning behind this is that when inserted traditionally, there's a small gap between the upper and lower seals after bolting on the crank and main cap. By offsetting it, that split area can be encased in the groove instead, which helps to prevent future leaks.

We liberally spread nonsynthetic 30-weight oil over the rear main seal and all along the mains. While some may opt for synthetic, using conventional oil throughout the assembly will ensure ring seal and proper break-in.

With the crankshaft in place, we then checked the thrust without the rear main cap on. We placed a dial indicator at the front of the crankshaft and set it to 0. Next we gently placed a flat-head screwdriver between the counterweight and the main and applied some tension back and forth for a reading. Generally, on performance applications you'll want no less than 0.006 inch. High-boosted applications or extreme clutchless-type race cars will demand more clearance.

Once we had the "before" reading, we placed the rear main and checked it again. With the main cap torqued to spec, you'll want to be within 0.0005 inch. If it isn't, you'll need to take out the rear thrust bearing and repeat the sanding.

To align the rear cap, we placed feeler gauges between the top and bottom of the bearing flange, or the side of the thrust bearing. We placed the feeler gauge at 0.008 inch between the crankshaft and the thrust part of the bearing. This ensures you'll have the same thrust with and without the main cap. We torqued the cap with the feeler gauges in place, then pulled them out.

We lubed up the remainder of the bearings underneath the main caps and torqued them down to spec. With nothing out of whack, the crank rotated 360 degrees freely, without any tight spots.


Chatsworth, CA



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