Valve to piston clearance

Easy to find the spec for a small block Chev.  .100" or thereabouts.   What about an overhead cam like a BMW 2 valve? Less reciprocating mass, fewer parts. Is less clearance possible on that?

3.5 M30, specifically.

My Bentley repair manual does not list a spec for that and some quick googling didn't find anything either for the M30. For the M10 someone mentioned 1.15 - 1.5 mm. I'd go ahead and measure it if you are concerned, I doubt you'll find they are that close. In the past we could access the BMW factory service manuals online but they have shut that down.

What are the details on the motor? What's it going in? I've got an M30B34 block with Carillo 11:1 pistons with an M30B35 head/intake on it with a 290 Dbilas cam in my E28. I did not measure valve to piston clearance when I put the motor together but I guess I should have!

It's stock pistons, reground cam, .020 off the head.  It has 2.25mm on the exhaust cam, which seems adequate to me, but based on Chevrolet numbers, is a bit tight.

Not my engine.

.080" intake, .120" exhaust typically for a sbc. I seem to recall that most tend to not go much below .080" exhaust if you have a handle on the valvetrain.

Good on ya for building M30s guys! The Bavarian Big Block.

P-V should be greater on the exhaust to allow some margin in the case of valve float (bounce). The valve is running away from the piston as the motor approaches minimum clearance, but seat bounce and rod stretch are both very real at higher RPM. The number to use depends on a lot of factors, but .080 is a reasonable minimum for the exhaust. The intake valve doesn't need as much space. Relative motion is closing at minimum clearance, but flex in the valvetrain will increase the running clearance (more for pushrod motors), and the intake valve will never be close to the piston when it sees some seat bounce. Although big intake numbers are an easy safety margin that don't require deep investigation, they're not always possible without altering valve pockets in the pistons or adjusting cam timing (often to the detriment of power)--an open secret used by engine builders is a minimum intake P-V roughly equal to the head gasket thickness for a pushrod motor. I'm currently running a BBC with 2.3" intake valves at .035, mostly because I didn't want to put a different set of pistons in it when I last cammed it. That number is probably too tight for a higher-revving OHC like the M30 with more rod stretch and stiffer valvetrain.

This is what happened when I first built my M20B25 without checking P-V, running stock short block with a Schrick 288:

Fortunately, the only real damage was the intake valve guides. I took the opportunity to do all the guides and switched to new +1mm valves. I made a cutter out of one of the old intake valves and a chunk of lathe parting tool and notched the intake pockets an additional .030 with the short block still together in the car. No problems since.

Streetwiseguy said:

  It has 2.25mm on the exhaust cam, which seems adequate to me, but based on Chevrolet numbers, is a bit tight.

Not my engine.

That's lot's. I've built Suzuki motors with half that and they rev for real