Piston/valve clearance 993 rsr cam to 3.8 pistons
 

To check piston to valve clearance using 993 rsr cams and 993 rsr 3.8 domed pistons how far should I advance rocker to check clearance? ie: set timing with proper lash and then screw in rocker adj screw 1 add'l turn (1mm) and rotate engine slowely to confirm no interference.

You can use the rocker method. However I would not rotate the motor with the rocker screwed in.
Start with the valve correctly adjusted then rotate the crank stopping every 10 degrees. Then screw in the valve to check the clearance at each location of the crank.

Or.

Fit a degree wheel, TDC the piston then position the piston at 20° BTDC. Fit one head with the valves loose. No springs, retainers etc. They will not fall as the piston will stop them. Hold the exhaust valve up with a rubber band or similar so it does not interfere with the Intake valve. Now with a dial indicator measure the valve drop from the seat to the piston. Do this every 5° or 10 ° from 20° BTDC to 20° ATDC. Do the same for the exhaust valve. This becomes the total distance the valve can travel from the seat to the piston with that piston and that head/head seat. Regardless of cam or cam timing.

Now ask the cam supplier to give you the valve lift at whatever cam timing you are going to use at those same piston positions. This then becomes the actual valve lift at those piston positions with the cam timing you are using. Subtract these numbers from the numbers you measured and this is the actual clearance you have at each piston position. If your valve lift is greater than the seat to piston, you are in theoretical trouble. Deeper pockets or a different cam/position.

This becomes very helpful when you change the cam timing or the cam LSA numbers. You have the total possible lift max numbers and any cam change is just a mathematical exercise to know the clearances.

Thanks Guys... Interesting technique Neil.

Btw, what would be considered an acceptable "minimum" piston to valve clearance? I'd heard 1mm.