Indeed the position of 10-12* before and after TDC are often quoted as the spot of closest P-V approach ( and I have seen a good engine building book suggest checking at 10*-only-) you have to check. My engine with high R/S ratio, big valve, high lift cam had closest P-V at 20 degrees. I checked at every 5 until I was sure where that occurred. If you rely on one "generally accepted" spot, I humbly suggest Mr Piston and Mr Valve may become closely acquainted. 
I use a dial indicator on the head, aligned with the spring retainer, with adequate travel to measure resulting V/P clearance. I install a "light" spring on the valve.
For intake, I start checking at 5 degrees ATDC, then 8, 10, 12, 14, 16, etc; until the V/P begins to open up. On cams with consistent flank motion, the clearance opens up because the piston accelerates away from TDC . . . . . hence my concerns about piston "dwell" . . . . .
For exhaust, I start checking @ 20 degrees BTDC, then 18, 16, 14, etc; again until the V/P opens up.
If you are expecting to have V/P clearances on the order of .100"/.120", by all means use clay or whatever.
On tiny engines I run considerably less clearance than that.
Fordboy
Agree 100%- with one exception-I think it is better to measure these and related things with the springs you are running. With a DOHC direct tappet not so much perhaps but with a pushrod motor, there can be quite a bit of deflection in the valve train and for performance or clearance critical measurements, a light checking spring might fool you. This makes things considerably more difficult particularly when dealing with springs with 400lbs on the seat... 
. My combination might have liked a bit more cam advance than I ran but was limited by P-V clearance even with deep piston pockets (= CR compromise) and led me to range of .042-45" P-V and .032"deck clearance trying to get the performance. Pushing my luck there for sure.....
I will point out I hesitate a lot to disagree with FB about anything.... 
Yes, it is important to point out that high spring pressures can & do cause deflection in a pushrod valve train. I've seen this a lot. Can you use any sort of "stud girdle"? Or are you using shaft mounted rockers already?
With DOHC, the valve train tends to be much "stiffer" and less "compliant" than a pushrod v/t, allowing ease of checking with a light spring.
Here are some numbers for thought. As originally setup, the V/P clearance for the MM's BMC was .055" intake, and .070" exhaust, with adequate valve spring pressure. On the dyno, we advanced the cam to .035" valve to piston clearance. That would have increased exhaust clearance to .090" No problem, either on the dyno, or out on the Salt. Watch the video again if you think Chris "baby'ed" the "Grenade".
Remember that on intake opening, after your minimum clearance point, you can NOT open the valve as fast as the piston moves down the bore. It is why on N/A engines, there is flow "demand" at that point. Exhaust valves on the other hand, are being chased closed by the piston. And the force to control that motion is
decreasing at the same time. Sort of a double whammy. Any significant deviation from the motion required to maintain clearance between the parts, and BANGO, your day can get bad and expensive. I tend to be more conservative on exhausts for clearance, say .065" minimum on small stuff, .080" on the bigs, but it's all about valve train mass . . . . . . . .
I will point out I hesitate a lot to disagree with FB about anything....
Thank you. My main goal is to get everybody thinking about this stuff, (you know, enough to raise their brain temperature
), rather than guessing, and to initiate intelligent discussion about possible solutions to problems.
Now that I think about it, I'd prefer that guys think enough about things first, so that they can AVOID problems to begin with.
Fordboy
Topic: Milwaukee Midget (Read 3270406 times)