Your right, just measured two blocks and the pistons are.25mm down the bore in the MG metro block and 1.5mm down in the 1275GT .

As I thought.
Can you please measure the total height of both blocks, I think you will find they are the same, or near it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Hmmm no access to anything big enough to measure that. So, approximately 4mm of ring land left if I shorten the GR pistons, hopefully safe enough provided it never detonates or runs lean.
I hope.......

Right, after too long have got the block back from the machine shop, have done a lot of measuring and volume checking and my intended build is never going to work.

Basically I need 9cc in the piston instead of the current 6.5cc, the head and an old gasket measure 22 in the chamber. Can anyone tell me the depth and diameter of the bowl in a 73 or 73.5mm piston?

Any opinions on how thin a piston crown can be and survive?

Thanks in advance, all sensible advice gratefully received!

Dr. Mini already said the GR Pistons will be fine with 1.5mm off the top. But why haven't you contacted GR yet and sent him your information? He built them after all. Or just pm him as he is on this forum too.

I have mailed Graham.
It's not the machining of the top now, its the bowl. If I machine the same from the bowl as off the top I will be getting on for 12.8 to 1 cr. Ok I have a 300 degree cam but I still think its higher than necessary and getting good enough fuel to run that will be a problem. I'd like it under 12 to 1.

Basically the volume in the bowl has to be 50% bigger than it is currently.

Cutting the top of the piston off increases the volume of the bowl so to speak because the piston doesn't come as close to the head. Easy way to get the CC's you need but you lose out on squish. Personally i would play with the head chambers.

just run it on E85 .. problem solved

runs cooler and will never knock .. im running over 14:1 in mine

Beanie

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I am the son of the STIG

HOTBRICKS
http://hotbricks.org.au/

'65 NCF MINI.....
'69 MINIMATIC.... 9 year resto still on the go

I had a motor built by a not so reputable builder. After it was destroyed by a spun cente main bearing, took it to Graham and he measured the comp at 13.5 to 1. He machined enough out of his new pistons to use the existing head and drop comp to about 10.5:1. Happy day's.
Talk to GR

I'm up and running successfully folks, happy enough so far.