What's the minimum safe valve head to piston cutout clearance, for a road engine?

I guess that would have to be at TDC plus the range of what? 5 degree crank of crank rotation either side of TDC?

It's easy to set the crank at TDC but as I haven't got a block/head on the bench to play with, can someone say if it's easy to also set the camwheel to its position corresponding to TDC? Block and head separated. Measuring distance perpendicular to deck, to
a) highest edge of valve above head deck, and
b) deepest point of valve cutout below block deck
and allowing for gasket thickness between.

They say that crank camwheel and head camwheel must be a matched pair - can someone explain why? Sure the two must be synchronised at TDC or mid stroke or wherever but doesn't the wheel-to-camshaft adjustability take care of that?

The matched pair stuff is to do with the different pitch crank and cam wheels. I also believe there are some with different numbers of teeth. They need to match otherwise the belt won't fit and/or your timing will be out.

What is your reason for wanting to know the minimum cut out required? The 8v fire is non interference so won't touch at any point (assuming standard specs of course)

Ah thanks - if it's only about number and pitch of teeth, and exact 2:1 diameters follow from that, that's easy.

There's no 8v in this job - it's an ex-1242 mk1 Punto 16v ELX85 head on a Punto Grande 1368 block.

That ex-1242 head is a near-identical casting to the Punto Grande head, same valve c/cs despite the 1242 block having equal bore c/cs but the Grande having different (irregular) bore c/cs. The Grande pistons have their cutouts offset, in order to match up with the valve c/cs of the head. And the valves heads are same diam, in the 1242 and the Grande. so the Grande cutouts are correct size, correct c/cs, to receive the valves.

That just leaves worrying about valve lift, and going from the 1242's 1.5mm thick composite gasket, to the preferred 0.5mm thick steel gasket of the Grande. I don't know but guess that both those factors taken together are going to reduce valve to piston clearance.

On a previous build we thought we'd checked it and test run v gd until suddenly valve contact - but I suspect wrong valve timing. Need to check it again properly now.

i think the only way to be 100% sure is to assemble everything with some plastigauge or similar on the piston top. turn the cam by hand and see what indentations you have in the plastigauge to work out your clearances.


if you turn by hand then you shouldnt have any issues with damage.


i appreciate you said you havnt got a head to play with, but even with measuring using standard specs, this wouldnt account for a skimmed head or similar.