when facing the seats for the pre combustion chambers should the amount taken off be made up in the gasket thickness or just use stock gasket i have noticed there is copper gaskets and steel gaskets which is best jw45

The different thickness of the gasket washer is for getting the glowplug in the right place for the wiring harness to hook up to it and not interfere with the injector line. If you don't have glowplugs, then the only thing I can think of is not protruding into the combustion chamber enough to interfere with the piston, but it should be over the chamber in the piston and you can't be removing that much anyway.

You could use the thicker washers if you wanted, but should not be necessary except as noted above. Might raise the compression a bit to have the chamber face a little deeper in the cylinder.

I believe CAT has gone to the steel ones, all the old ones were copper and maybe some aftermarket still are? With good bores and chambers, I expect the steel ones are the way to go. I think that the copper might be better on rough counterbores or old chambers.

Kind of depends on how much material you removed with the spot facing. There is some forgiveness with the upper chamber "O" ring seal area. I prefer the copper washers for their corrosion resistance.

the cyclinder head iam working on has a #stamped on it #3F8803 then to the right of it towards the t stat housing it has the letters LG can anyone tell me what machine it came off of the pre combustion chamber o rings are a different size than the one on my machine its machine #9U26429 which is a 1957 model will these heads interchange also on one of the pre combustion chambers it is marked DD does that indicate a size thanks jw45

Check that stamped number again...if it reads 3F9903 that is a casting number for an early 5F1305 part number head.
The 5F1305 head was replaced in 1952 with a 1H7659 (service replacement) unit at serial number 9U11075-up.
The later head uses a different chamber retainer and gasket rings. The old outer ring seal #4F5607 became the new inner ring seal and a new 2H3086 outer ring seal is used.

The LG may stand for the "G" series industrial engine.

DD may be an indicator for after market size.....but they all used the #4F8517 chamber unless it is a glow plug model.

If your unit uses the later style one piece chambers then you would only need the 2H3086 ring seals.

thanks om i will check that # and reply tomorrow

yes after looking close it is 3F9903