The V8 Owners Forum -

as it says,
how deep is the norm when cutting valve reliefs???
3mm??? reason i ask going to make a collar at work which i can adjust when i do each relief.

from what I have read you want 60 to 80 thou between the face of the valve and the piston at closest approach (check from about 15 deg before TDC to 15 deg ATDC between exhaust and inlet stroke and at full valve lift on both valves. and 40 to 60 thou between the rim of the valve and the side wall of the cut out. the larger tollerance with heavier valve train and hydrolic valves (and forged pistons) the tighter end on mechanical flat tappet cam and lifters and hyperutetic pistons.
Sorry Tony no simple answer
really the only check is with plasterceene

like kiwicar says there isn't any easy answer to this as it depends how close your valves are to the piston. They wont all be the same either but I would pick the deepest pocket and make them all the same. Plasticine is really about as much use as fag papers too for doing this, use a dial gauge and soft springs.
Course if you don't care about getting everything as tight as you can 3-4mm should give you clearance whatever cam you have so long as your head/block isn't heavy skimmed.

I'll do 3mm deep cut outs on all pistons and then bolt it all up and do a dummy run with no gasket,
if that dont hit then i should be ok with the gaskets fitted.

thanks guys

its not a case of hitting M8 you need 2mm clearance for when the engine is running for valve float, rod stretch and heat expansion. Valves can miss at idle but hit and bend at revs.

shall i go a bit deeper then mate?

It may be a stupid question but how much will this lower the compressionratio as the total combustion space will be larger with the valve reliefs cut out.

it lowers it very little unless your stupid and run a fly cutter straight across the top if the piston. don't laugh I have seen alleged professional tuning shops do this. On a 3.9 you will loose at most a couple of cc with the deepest pockets. On a Rover you only get a couple of small crescents when it's done properly. Tony is cutting them with a large valve ground as a cutter so he will only have the small pockets. If I remember I will take a pic over the weekend of how not to do it.

I think you know where your heading Tony as discussed over the phone

well,
sunday im going to make a new cutter,
and also open the exhaust ports out a fraction,
then, i think im ready for the install once i strip an old head down for the cutter to go in,
then its cutting time, have made a stop up today also so i can measure each piston to valve face and get each pocket the same, im going for around 4mm then this will give me max clearance for my cam,
still undecided to which head gasket though, composite for sealing and nitrous, or tin to raise the comp ratio up higher.

tin every time for me.

I've just measured the cut outs on the pistons on my new 4.6 engine thats ready to go in. They are spot on 3mm at the deepest point on the piston crown and taper to nothing at the other end of the cutout. Hope this helps.

This is a newly built V8 Developments engine.

cheers mate, looks like its going to be 3mm ish then

Take a look at the forged cosworth one i gave you, you could just replicate that.

bugger forgot about that gav

just found it out will do that, cheers dude,

Tony, if you see daylight thru the releif, you have gone too far