Big Boyz Head Porting  


Measure everything! I see engines that get built without checking tolerances and measuring everything. S&S didn't properly measure my pistons and cylinders, I'm glad I did. If you are building the engine don't go by what someone else tells you. If someone else is building the engine, I am not suggesting that you stand over their shoulder and measure it.
In most instances when using a bolt in cam and stock heads it really doesn't require checking piston to valve clearance BUT it's always a good idea, good practice, good habit and good technique.
The way I check piston to valve clearance is called "claying".
Take some good modeling clay and roll it in your fingers until is it about .150-.200 around (thick). Then lay it across the valve relief in the piston, perpendicular to the center of the cut. Do this for all 4 valve reliefs. Then assemble the motor, heads, rockers, lifters, pushrods, head gaskets etc. Then turn the motor over. Then disassemble the motor and the clay will have been pressed down by the valves. Then measure the thickness of the crushed part of the clay. Then you will have your piston to valve clearance.

Picture 1) Clay layout

Picture 2) Valve Compressing clay (simulation)

Picture 3) Clay after compression

Picture 4) Measuring clay

Picture 5) Read measurement

1) Using light spring in place of the valve spring will make turning the engine over much easier.
2) Turn the engine over by hand, slowly in case there is any interference between the valve and the piston. This will prevent damaging any components like the valve, guides, pistons, rods and bearings.
3) Lubricate the face of the valve to prevent it from sticking to the clay. Pam spray cooking oil works well.
4) If larger than stock valves are used, check clearance to the side of the valve relief pocket as well.
5) Only turn the engine over 2 full rotations so each valve only compresses the clay once.
6) A modified set of lifters that don't collapse are handy.
Here is an example of using a high lift cam and not measuring. The smaller valve relief cut into the piston was done by Slamming the valve into the piston. In the cross sectioned piston, the shiny part is what I am referring too.

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