November 2010 Chevy High Performance Q&A - Performance Q&A

Kevin McClelland Oct 12, 2010 0 Comment(s)
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As for the amount of preload to put on a new tappet with no oil is a little tricky. You will need to do this by feel, and the easiest way is to slowly rotate the pushrod with your fingers and tighten the rocker adjusting nut until you take up all the clearance and feel a slight drag to the rotation. Loosen and tighten across this point a couple of times to ensure you have found zero lash. Then you can preload the lifter to a specified amount. The factory setting is one full turn past zero lash. Plunger travel in the tappets varies by manufacturer, but GM with its factory tappets, the one-turn setting puts the plunger in its midpoint of travel. This gives the most leeway for wear and temperature variations that affect the lifter preload. Now, for performance use we'd prefer the tappet to have less preload. You wish to run less preload because when the engine gets up at high engine speeds, the lifter may try to pump up and keep the valves from seating! You will find that most performance camshaft companies will recommend adjusting the valves to a half turn preload. In some very high rpm applications, I've run the pre-load down to as little as an eighth to a quarter turn. You must be very accurate in your adjustment with this type of setting-and willing to adjust the valves frequently. The best compromise is to adjust the valves to half turn preload.

We hope this gives you all some clear guidelines for adjusting your hydraulic valves. This method can be used on any engine big or small, or cylinder count. Also, this is the same method you will want to use for finding the adjustment point on solid lifters. Lash on!

Aussie Racer
Q: First of all, thanks for your magazine. It was the inspiration for the '83 Holden Commodore I'm currently building. I built a 383 with a 3.75-inch stroke, 5.7-inch rods, 10.8:1 compression, Victor Jr. 64cc heads, and 750-cfm Holley HP carb. The cam is a solid flat-tappet with 0.518/0.536-inch max lift and 290/298 duration. Here in Australia, 98-octane fuel is premium and available everywhere. What ignition timing would you recommend? Also, I'm running MSD everything, including the 6AL and distributor with vacuum advance. Do you think you could shoot me an output estimate? Thanks for even reading this.
Jason Lacey
Victoria, Australia

A: Glad to read neat stories about our gearhead friends from Down Under. You guys have some very cool cars to mod, and with 98-octane at the pumps, you can get very entertaining with your engine builds. Thanks for the photos-love the yellow paint!

With the good fuel you have, we'd start at 36-38 degrees total timing without the vacuum advance connected. After setting the timing, connect the vacuum advance to a ported vacuum source on the Holley carb. Limit the vacuum advance to 10 degrees. This will get you right in the ballpark for spark advance.

As for power, from the photos it looks like you're running a Victor Jr. single-plane intake. The headers appear to be in the 15/8-inch primary range. With these cylinder heads and the generic cam specs you supplied, you may be in the 480-500 hp range. We'd put the torque in the 450-470 lb-ft range and should push your Commodore nicely. The Commodore looks to be a unibody construction and looks rather light, so the 383 should give you quite a thrill ride.

Body Pains
Q: I need help putting my '69 Camaro back together, since I didn't take it apart. I need an assembly breakdown of the steering column and another for under the dash (hoses, wires, cables, vents). I have a factory assembly manual, which has neither of these. I'm hoping you have an answer.
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