Here are Comp Cams' new Ultra Pro Magnum rocker arms. They are investment-cast 8650 chrome
Greg Smith, Woodland Hills, CA: I have a stout small-block in a '55 Chevy with an aggressive mechanical roller cam that has been in the car now for years. I don't put a lot of miles on the car, but when I do, it's making quarter-mile passes on a regular basis. Last weekend I wanted to check the lash. The cam is a custom grind from Comp with 264/269 degrees of duration at 0.050-inch tappet lift with 0.631-inch valve lift on the intake and exhaust with a 112-degree lobe-separation angle. My Comp cam card specifies 0.026 and 0.028 inch of lash, but it does not say whether it should be set cold or hot. If I set the lash with the engine cold, does the clearance increase or decrease after the engine is warmed up? I've received two different opinions from friends.
Jeff Smith: Great question, Greg. We decided to go to the source, so I called Billy Godbold, Comp Cams' rocket scientist turned cam designer. First, Comp always specs lash as a hot setting. This is probably true of all cam companies because lash is designed to accommodate changes in engine clearances due to thermal expansion. Billy says lash loosens on all pushrod engines as the engine warms up. In general, he says an engine with an iron block and heads will increase the lash about 0.003 to 0.005 inch. An iron-block engine with aluminum heads will grow roughly 0.005 to 0.007 inch, and all-aluminum engines grow the most-anywhere from 0.007 to 0.015 inch or more.
Let's say your new engine has an iron block and aluminum heads and has never been started. Billy recommends setting the lash at 0.020 inch cold, which allows more than enough clearance for the exhaust valve to fully close as the engine warms up. Then recheck the lash after the engine has achieved a normalized temperature. If the lash increases about 0.005 inch for a total of 0.025 inch, this is very close to Comp's hot recommendations and you can adjust accordingly. Billy says you should never try to run too tight a lash when cold-nothing less than 0.015 inch. This is because if the engine ever saw very cold temperatures, it's possible the exhaust valves would not close and would burn up during startup before the engine achieved operating temperature.
This also brings up the concept of how much the recommended lash can be altered. Billy says most flat-tappet and roller cams with lash specs around 0.020 inch can be run as tight as 0.016 inch and as loose as 0.030 to 0.035 inch. The only reason to change the lash would be to determine what effect a change in duration would have on the engine. As an example, Billy likes to work on the intake first since it tends to have more of an effect on engine performance. He prefers to work in stages of 0.004 inch by loosening the lash and then testing the engine. Loosening the lash effectively shortens the duration by roughly 2 degrees. If the power increases (more trap speed in dragstrip testing), it indicates the camshaft duration is too long by at least 2 degrees. In this case, you would add another 0.004 inch of lash and run the car again. In the case of your camshaft, this has now pushed the lash up to 0.034 inch while cutting the duration by roughly 4 degrees. Keep in mind that loosening the lash also reduces lift, but 0.008 inch isn't that much. If the dragstrip trap speed improves again, we would suggest that shortening the duration tends to improve midrange torque perhaps at the cost of top-end power. A trap speed improvement in this case is probably due to the increased torque accelerating the car more efficiently as evidenced by a higher trap speed and perhaps a lower e.t.
While we're on the subject of lash, this offers the opportunity to smash a popularly held performance myth about street-driven mechanical camshafts. It's common to hear someone say, "I don't want a mechanical cam because I don't like setting lash all the time." I think this started with '60s small-block Chevys where the factory used a 3/8-inch fine thread pinch nut. These locking nuts work great-once. After only a couple of adjustments, the nut loses lock tension, it loosens, and the frequent adjustments begin. Today, the smart move is to use a simple poly lock. They don't move if you adjust them properly. As a reality check, I have a John Lingenfelter-built 420ci small-block in my '65 Chevelle that has been in the car since about 1990. Over the course of 20 years of abuse, the lash has changed perhaps 0.002 inch, and I'll attribute that more to how I estimate the pull on the feeler gauge. I still check the lash about once a year just to make sure something isn't wearing out, but it never changes. If the lash changes, something is wearing, bending, or about to break.
The bottom line is lash will not change if everything is happy.
Given that nasty Kaase Boss in this issue, we thought we'd continue the Ford flavor with Rick Stanton's '69 Talladega powered by a 598ci Boss motor. He stroked his Boss with a 4.50-inch crank, and at Westech it tweaked the dyno to 824 hp at 7,600 rpm. With an Isky roller cam and a 1,250-cfm Dominator carb, this beast smokes.