Help with machining and decking info.


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By ZOEGA - 15 Years Ago
I'm doing some work on my 292, here's what's in my plans:

-Taking 2,2 milimiters (0,086'') from the block.

-Takin 1,5 milimiters (0,059'') from the heads.

-Gasket thickness: 1,3mm (0,051'')

-Cam with 7,8 milimiters (0,307) lift.

-Stock flat pistons.

-Stock/Standart head valves.

-Cilinderheads volume: 72cc (before the machining)

-Standart rocker arms.

I'm worry about the piston hitting the head valves, am I right?

What's the estimated compression ratio?

Thanks!
By Ted - 15 Years Ago

I'll make some assumptions here and with that here’s a shot in the dark.  You didn’t state a head casting number so I’m guessing that the heads if they have the C1TE or similar combustion chambers they will take 0.0068” of mill to remove 1cc.  If milled 0.059” then the final cc will be ~63.3cc’s assuming the heads were actually 72cc to begin with.  If you’re using advertised or published cc values, then that number could be off.  The thickest head gasket I have on file is a McCord gasket at 0.048” thick with a 12.79” long fire ring.  Assuming your gasket is 0.051” thick and the fire ring is 13.0” long (same as a Best Gasket brand), then the cc’s for the head gasket comes out to 11.08cc.  You didn’t mention if the block was decked so that the pistons are at zero deck or flush but for calculation purposes, I put it there.  Using a standard bore with the stock stroke, the compression ratio comes out at 9.03:1.

 

With the lobe lift at 0.307” and using 1.43:1 rockers, lift at the valve with 0.019” lash comes out to 0.420”.  Although piston to valve clearance itself is expected to be more than adequate, checking the clearance at the edge of the cylinder at full lift is warranted simply due to the amount of mill on the heads and especially if the intake valve size has been increased.  If the intake valve sizes have been increased, notches in the cylinders at the intake valve locations may be necessary.  With stock intake valve sizes, there should be adequate clearance with the lift values you plan on using.  With the heavy mill on both the heads and block, the intake surfaces of the heads will need a significant amount taken off there so that the intake bolts and ports align.  Using a piston with a greater compression height or lowered pin will alleviate part of this problem by reducing the amount of material that must be taken from the decks to get the compression ratio up.