Valve to piston clearance?

nm9stheham said
Thanks very much Duane. Looks like the described procedure checks at both 10 BTDC and TDC for exhaust, and both TDC and 10 ATDC for intake. And then says "This gives you your piston-to-valve clearance at the two points where the piston and valve are normally the closest." It does not look to be saying exactly +/- 10 are always the closest.... and that's not the be expected with all sorts different cams. But I'd think you're spot-on that it is going to be somewhere around +/10 degrees from TDC.

I use a similar procedure to what you linked.... I just rotate the engine a couple of degrees at a time starting at maybe 20 degrees BTDC on overlap, with checking springs in place and cam timed, and take note of how far each valve can open at each point. It is pretty easy to find at what degree points partially open valves have the least clearance to the piston, and get a measurement there. With performance type valve-reliefed pistons and modest cams, it is not a problem (that I have seen). Seems like the aftermarket piston manufacturers do a pretty good job of designing for this.

The article mentions tdc, but in my limited experience my last two builds(assuming you have located tdc correctly on your degree wheel) the closest points were at exactly 10 degrees.
One engine was a .585 lift cam that did not have adequate clearance and required custom valve reliefs in the pistons $$$. Any my current build with 12.7:1 compression and .705 valve lifter that clears easily.
Go figure.
My Pistons were a custom Ross. I have to agree that the piston manufacturers have gotten much better. The cost for the Pistons was not cheap. But that cost was offset in the weight accuracy of the pistons( no balancing was required) and no deepening of valve relief costs.
My point in mentioning this is, and I cannot explain the math the way that you do, is that it cannot be correct to calculate piston to valve clearance with the piston at tdc because that is not the closest points, and that the stroke of the engine has nothing to do with it, nor the lobe profile. IMHO But I could be wrong, this is based on my experience that 10 degrees is it.