Technical advice on engine please.

[Firept]

Ok just finally putting the engine back together today and I have the fitted heads, not tightened down or gasket fitted when rotating the engine the valves (inlet) just kiss the pistons probably by about 10 to 15 thou is this correct?
What should the clearance be between the piston and valve, does any one know?
If I fit a cheap thin gasket it clears (only for testing) and if I fit thick head gasket then all is OK and the valves clear the pistons.
I have put some engineers blue on the valves and rotated the engine and the pistons are clean so I know it clears but by how much I have no idea and as above is there a definitive figure.
All the valve timing appears to be all spot on so I don't have any concerns here but would like to know that when the engine is hot the pistons are not going to hit the valves.
I can think of only 2 ways of finding out:-
1. Set an inlet fully open and set piston fully up and measure the gap from block to head or
2. Put some bluetak / soft wax on the piston and with the gasket fitted turn the engine over open up again and measure the thickness of bluetak or wax, any one got a better idea?
If really necessary I could cut valve recesses in the pistons but if not needed I rather not. or am I worrying unduly.

Help from the wise sages please.

Paul.

[Westfaling]

Paul
Have you done a compression ratio check. I'd be as concerned about that in choosing the head gasket thickness. I had this much clearance on the valve seats and with flat top pistons I needed thick head gaskets to keep the C/R down to 9.3:1.
Nick
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[flying farmer]

A thin gasket is 40 thou thick when compressed which will give you 25 thou clearance if your valves are touching the pistons by 15 thou. This is the clearance I am using on my efi engine, but due to the fast road cams opening the valves earlier I had to pocket the pistons to acheive this. My compression works out at 10.1 : 1 with domed pistons and pinking has not been a problem, in fact I am using 4 degrees extra advance though the fast road cams do have an influence on this.
It is probably worth measuring the combustion chamber volume, and are you using dished or domed pistons?
The domed pistons reduce combustion chamber volume by about 4cc, the thick gaskets increase it by about 3cc over the standard thickness gaskets
Neil

[Firept]

Neil./Nick thanks for the info.
The volume of the head is 35.5ml and the block / piston is as standard whatever that is as I have not measured it.
It is fitted with the flat top (concave) pistons.
my concern was with one head that has been repaired and has had a very light skim about 10 to 14thou.
There was never a problem before with the set up as it was, therefore I am assuming that with the thick gasket it should all still be OK.
I have balanced out the chamber sizes in the head so they are all the same they were about 1.5% different from the left to right head now less than 1% difference.
Any ideas as to what is an acceptable "clearance" between the valve open and the top of the piston?.

Paul.

[Bob Heritage]

I've alway understood as a general guide, that with the valve closed if it does not protrude the surface of the head then standard gaskets are O.K. If it does then you need the thick ones.

[peterrc]

Hi Paul, have a look at this thread

http://www.stag.org.uk/forum/showthread.php?15318-Cylinder-Head-Thickness/page2&highlight=cylinder

The advice from Tony H covers everything.
If it wasn't so b****y cold today I would be doing the same job.
Peter, Cupar

[Firept]

Bob / Peter.
Thanks for the info and the link.
OK all now clear.
The valves do not protrude below the surface of the head when shut so panicking about nothing.
Heads will get bolted down tonight!

Engine rebuild almost finished now, gearbox is next.
Paul.