Thread: GT 40 P heads Biggest cam with stock rockers

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Sorry Bobby, your question concerning max lift is unanswerable. There are just too many variables and each motor has to be measured on its own.

Stock rockers will not be the limiting factor. Coil bind, rocker slot interference at the stud, retainer to seal interference and valve to piston interference will be the limiting factors.

Get a set of checking springs and install them on one cylinder position with the retainers and locks you got with the heads. Like you are going to shoot an arrow from a bow, push the spring down out of the way and position the valve stem between your fore finger and middle finger and place your thumb on the valve stem tip. Pull and twist while pushing the head away from you with the other hand. What we are trying to do here is to seat the retainer and locks onto the valve stem so that you get good results from this operation. Bolt the head on with an old head gasket using just a couple of bolts snugged down. Roll the crank over so that you are about 30 degrees before TDC on that cylinder. With a dial indicator on the intake valve retainer, push the valve down until it touches the piston and read the travel on the indicator. Write it down. Now roll the crank clockwise until you are at 20 degrees before TDC and do it again and write it down, then again at 10 degrees and write it down, then at TDC and write it down, then at 10 degrees after TDC and write it down, then at 20 degrees after TDC and write it down, then at 30 degrees after TDC and write it down. Deduct 0.080" from the smallest figure you wrote down. That will be the max lift for the intake valve.

Now, move your dial indicator to the exhaust valve retainer and repeat the operation. Deduct 0.100" from the smallest figure you wrote down. That will be the max lift for the exhaust valve.

When mounting the dial indicator, bend down to the level of the indicator and eyeball it in both planes, from the front of the motor and from the side of the motor. You want the indicator stem to be parallel with the valve guide bore. Also, push down only on the valve stem. Pushing on the retainer will dis-lodge it.