I am refurbishing my 1.7. Had the case line bored. Got new 93mm pistons from AA (upping displacement to 1.8 from 1.7). Now, the heads CC out at 58. In calculating compression ratio, do I include the "dished" area of the pistons. Those dishes add 10cc's. The photo shows what it looks like currently, and there is .008 clearance between the flat top of the piston and the top of the cylinder.

I would like to have .040 clearance. Am I simply stacking a few shims to meet the .040 clearance? It seems the compression ratio is hovering at 7 (when I plug the numbers into the calculator online) and I would like to hover between 7.5 and 8:1.

What am I missing? (It's been decades since I have rebuilt one of these so I am rusty)

I'm not trying to compete with Raby and build some monster, just a little over stock. Stock cam; stock fuel injection etc...

Thank you for your knowledge and wisdom that hopefully will be forthcoming. Click to view attachment

10cc ? I'm not in love.
Those must be a bus piston.

So AA is going to swap em out.. flat top.. which is more better... assuming they are better pistons, can you assist with the question posed? I still need to adjust deck height.. now at least we are working with just a 58 cc chamber... but as far as adjusting for deck height, it is a matter of stacking shims, yes?

real low compression and certainly way more than 10cc dish...strange how these pistons are machined?,have a set of AA P&C's but they are flat top,so don't know why that dish is that way.
Wondering and not being rude but why do the same cyl head machining and not go to AA 96mm flat tops ?,low compression in a 914 isn't needed and can be higher ,8.4 up to 8.7:1 stockish , thinking your current setup may make it hard to achieve that without some flycutting and shims.
Good luck.

who did your line bore and how much ?

European motor works in Hawthorne, CA. Work is good. Bearing all fit good, clearances all in spec on crank.

Sorry read my late reply,thinking with flat tops you can set -up better for deck height ,you will have good luck with AA or atleast I have.

The 96mm AA birals I purchased back in early 2012 have 2cc valve pockets.

Yes, add the dish volume and/or valve pocket volume in figuring compression ratio. A maximum of 3 factory barrel shims are allowed per spigot hole.

dont forget to include the head gaskets in the calculation.

Unless you choose to leave them out. This gives some extra compression.

Thanks for all the suggestions. I ended up swapping the pistons for truly flat topped 96 mm pistons and jugs. Made it easy since the heads CC"d at 58. Didn't have to calculate any valve reliefs etc...


Thanks @mark henry for pointing out the dished "bus" pistons... I didn't catch it.

No problem, but I'm surprised no one picked up on the obscure soft rock pun.
Must be showing my age...

I got it but was embarrassed to admit it, ha.