QuestionI am rebuilding my 1275 and would like to know; i had a valve hit a piston after the last rebuild and think it was from planning the head. it has been years but i think the fix was mill the top of the piston on each side. i am boring the block to .060 with high compression pistons and a performance cam. what is a good easy way to check the clearance after i put the new pistons and cam in? also, is there a good way to set timming after i install an adjustable cam gear getting the most preferred timming. i know a little about engines but not an expert.
AnswerHi Bill,
Any time you modify an engine with high lift cams, high compression pistons, block decking and shaving the head you are in danger, so I always used modeling clay in small beads in the places that are in danger. Then put the head on with an old head gasket and pull the head down tight but not up to torque. Then only install push rods and set the valve clearance in one cylinder and hand rotate the crank. The small beads of clay will be compressed with the hand rotation of the crank. If I find it OK (.040") I check the other cylinders too because most heads when machined are not perfect so one end can be closer than the other end.
Remove the clay and take a razor blade and cut a cross section of each bead of clay and measure it. I like to use .040" as my minimum clearance. Talking to other engine builders I have heard as low as .020" as minimum but that scares me so I stuck with .040" and have used it on engines that turned 9500 RPM with no strike damage.
You should lighten and weight balance your pistons anyway but if you do any machining, it is more important then ever. I usually lighten my piston pins and weight balance them too as well as total rod weight and also upper half of the rod too. You would be shocked at the RPM that can be picked up if you do that.
A good example is the 1100cc Midget/Sprite vs the 1100 Sedan. both cars had the same engine, cam, head and twin SU carburetors. In a long drag race the Midget will run off from the Sedan because the Sedan was heaver and less aerodynamic, but about third gear the Sedan will pass the Midget and run off from it as it winds up. The Midget was lighter and gets off the line quicker but the Sedan engine was balanced at the factory and would turn much higher RPM. Yes, I know. but that is what some mechanics do when out road testing cars. At the time we didn't understand why until we talked to a factory rep and found out the Sedan was balanced. We knew the importance of balancing a race engine but were surprised to hear that a mild small Sedan was factory balanced.
Keep in mind that cast pistons and factory rods can take just so much and as you increase the RPM you increase the strain. A domestic V-8 turning 7000 RPM measured the strain on the rod at TDC and BDC and found the strain to start and stop the piston to be measured in tons. So when you increase the RPM of an engine you should lighten the piston, pin and upper half of the rod and balance each. Polishing critical areas will help prevent fatigue fractures. Factories and large race shops have other methods to slow down fractures but most individuals and small shops don't have that available.
Good luck,
Howard
