Different deck heights

[jclayc]

I have a case from cbperformance, 94mm pistons, 74 crank and stock rods. I’m calculating deck height using a dial indicator to find TDC at the center of the piston and I’m torquing the deck tool down to about 8ft lbs on each nut. Measurements were repeated 3x

Using the same cylinder and piston, I get a deck height on cylinder 1 at 0.002, cylinder 2 at 0.006 and cylinders 3 & 4 at 0.013 each. The difference is what I’m curious about.

I know one option is to tear the short block back apart, check the crank and rods (from DPR) and/or get the block re-decked. Short of doing that though, would you get 4 cylinder shims of the same size and just run different deck heights or should I put different shims on the cylinders to get them all of an equal deck height?

Note: this will be a street car using cb perf panchito heads w 61cc chambers.

Thanks!

[Dale M.]

First rough guess is case center bore (crank) is "off center".... Use cylinder shims to equalize everything out, you want a deck height of something like .020 to .030 (minimum) if memory serves me correctly.... But then with your 61cc heads deck height is going to be critical to get desired compression ratio.... What is your target compression ratio?....
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All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ...

[jclayc]

[modok]

You could put .008" shims on one side.

Some of it might be the rods, but it's mostly the case I'd assume.

[vwracerdave]

The crank centerline is not machined perfectly center in relation to the case halves. VW has an acceptable amount they can be off centered.

The safe minimum deck height is .040". Building anything less is a very risky. You can not put different shims under 2 cylinders on the same side. Both cylinders on the same side must be level so the heads sit parallel and seal correctly. Measuring all 4 holes with the same piston is not the correct way to do it. Bolt on all 4 and measure. You can then move around the rods and the pistons to get them the closest. On a street engine they DO NOT have to all be exactly the same. If all 4 holes are less than .010" difference build it and enjoy it.

CR depends on the cam you choose and the octane gas you want to run.
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[jclayc]

Thanks everyone. I will ultimately have a height between 0.04 and 0.06

And bolting up all 4 pistons and cylinders and measuring is something I can do/try. Maybe there is some variance in the cylinder heights(?) that may work in my favor(?)

Regarding both cylinder heights needing to be the same on one side… but not having different shim sizes… is there a tolerance in deck heights on the same size? like having 0.003-0.005 difference in deck heights is fine on cylinders #1 & #2 as long as the cylinders themselves are the same height?

Also,?is there a tolerance for deck heights from bank to bank?

[modok]

Having different deck height will not cause any mechanical failure, it will work as is, but it would be nice to be within .004 IMO, just as a matter of pride in workmanship.

cylinders should be should be even with eachother within .002 max"
(straight edge across the top)
Cylinders should not vary in length, usually they are within .0005"

[RWK]

 If you put same cylinder/piston/rod in different holes and get a different number then your deck is off on 1 of the holes, slim possibility crank throw is off, that would be the only other variable.
The decks on any 1 side should be flat and parallel to one another (zero tolerance) so that the cylinders are straight up and down, not tilted in any direction, this is key so that heads seal properly, 1/4 of 1 degree out is equal to .004 per inch, now stand cylinder up 3 inches or so!
The alum heads do twist and bend a bit when you bolt them down, this takes up some of the inaccuracies if decks are not good, puts some added strain on guides and valve train when heads mate to un-even cylinders.
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