470 BLOCK NEEDS SKIMMING

My merc 470 block needs a skim

My merc 470 block needs a skim. I am going to have the pistons skimmed at the same time as there is no tollorance to play with between the block and top of pistons.
Has anyone got any idea how much tollerance I have to play with between the valves and top of the pistons. Im having about 22 thou taken off.??

"The pistons, if are flush to

"The pistons, if are flush to the block are set to 0 deck hieght and that is the was they should be left.
If you are having .022" "milled" off the heads you should try to find a new head gasket that is thicker than the original.

If you can not, then reassemble the head and do a clay test before machining. and measure the clay to see what the piston to valve clearence is now to determine if .022" will be a issue.

If you can not do this have the machine shop do it.

If there is no interference with the .022" removed then all it will do is increase your compession. you will need to increase your gas octane probably to at least to 91 ctane and maybe retard your timing all to avoid detonation."

Further what Kghost said... do

Further what Kghost said... do not mill (skim) the pistons! There isn't any excess meat in them to remove.

I'm curious as to why you need to mill the block. Usually there is so much mass in them that they do not distort. It's usually the heads that need milled.

"If you're gonna broach th

"If you're gonna broach the block (not the head) and you take the same amount off the pistons, you shouldn't change the valve-piston clearance.

If you're doing the HEAD, following Kghost's words should keep you safe.

Why do you need to take off 0.022" - that's a whole lot of metal?"

The machine shop have said tha

The machine shop have said that they will get away with milling away as little as possible but .22 is worse case.
The piston sleeve on no:3 piston has a pit in it on top and keeps blowing gasses directly into the water jacket.
Not sure whether in the past something has been dropped on it or its corrosion. Either way it need levelling off.