to what engine ? usually needed that cylinders are not to close to each others. in turbo aplication its not as strong as thin, because heat.
i use custom 3mm Cometic HG in my 3T engine, pistons come 2mm over the block. still work with stock pushrods, also cam is grinded, so its got couple mm smaller basic diameter.
My Friend "Los Patentos" in 2TG engine, there is 3T Crank with really long rods and Custom pistons.
bought side O-Ringed steelplate between head and block, bought side got ~1mm copper gaskets. oil is coming outside of the head, they are closed from head and block. plate if about ~5mm thick, not remember.
picture at spacer
http://kuva1.kuvablogi.com/iso/img719714.jpg
and at the block with pistons
http://kuva1.kuvablogi.com/iso/img719739.jpg
have been dynoed ~500 hp
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