Measuring Piston To Valve Clearance

When doing this test what would be considered the best Measurement? When will it become to close or too far?

Hi Brain,

There is no exact number to this question although the general rule is you at least .080" clearance on the intake valve and 0.100" on the exhaust and 0.050" for side clearance.

This will depend on factors such as rod material used, camshaft profile, piston rock etc

Yup, some will run it tighter, as the valve reliefs reduce the compression, but it isn't that much and, unless really chasing that last few hp... You do need to make SURE your valve springs are up to the rpm you are going to use, including possible over-rev's on 'enthusiastic' downshifts - see next paragraph.

It is not uncommon to run more exhaust to allow for slight valve float - this is because the piston is 'chasing' the exhaust valve(s) and it is possible to have contact, whereas with the intakes the piston is close to BDC, so no chance of contact.

What I would suggest is that you also check with the inlet advanced, say, 5 degrees and the exhaust retarded the same, so you will be confident of clearance if you need to swing their timing a little. Retarded and advanced, respectively isn't a problem.

As you may gather from what Chris said, because of variations in engine spec's the closest approaches may not be at TDC, but a little before or after - to be safe, check from, say, 20 degrees BTDC to 20 ATDC.

As an aside, there are 'fast road' types of camshafts that supposedly fit engines without modification - still a good idea to check if you're fitting them to an engine that isn't in complete OEM condition - the head and/or block may have been heavily machined at some point and significantly reduced the original clearances. Or aftermarket "OEM spec'" pistons may have been fitted that have smaller valve pockets.