What are these pistons?
Turbo Mini Forums

I bought what I was told was a 1380 block and pistons about ten years ago, last night i was looking at it to see what size bowls i could get into the pistons,

and i noticed its not a 1380, the pistons are 70.6mm (2.78") isn't that +80 thou? also the pistons and rings are marked STD, inside the pistons there is 20659A cast in.

Any Ideas whats going on here?

70.6mm is standard 1275 bore

Sorry I was comparing them to a set of std's and typed the wrong figure,

They are 72.73 (2.863")

is that diameter of the bore, crown or skirt of piston???

crown.

I make them roughly +60's, allowing for a bit of measuring error etc....

So would be a 1330cc engine assuming standard crank stroke.

well id say 73mm if its the crown you have measured

you should measure about 10mm up from the base of the piston on the skirt in the same line as the gudegon pin

This is often called the A measument for some reason.

AlexF

Also remember the piston is smaller than the bore anyway to alow for running clearance so wont measure exactly stated sizes

Better to measure the bore at the very bottom or the very top where wear does not take place

Edited by Sprocket on 21st Sep, 2006.

Well if they are bigger than std,and marked std they could be triumph spitfire? pistons.well they definatly used some triumph pistons to go big bore

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ok just measured the "A" measurement 73.12mm,

so, looks like its a 1360!!??

Edited by Sprocket on 21st Sep, 2006.

ok, now i'm at home i've measured the block, just a shad under 73.5mm, and now i've added it all up the "A" measurement is about 11 thou less than the bore and the crown about 15 thou less than the "A" so it stacks up if you allow the measrements are with cheap digital verniers.

still, strange they are marked STD, I'm wondering if anything has been taken from the top of the block now...

Oh, I forgot to mention that these are new pistons in a fresh block.

found out what they are...

20659A's are 73.5mm powermax's...

Still puzzled by the STD marks though

Hi,
STD means standard size for that bore, you could still theoretically get +0.020 ect for the 73.5mm bore.
Cheers,
Gavin :)

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Ah right that clears that up then,

soooo.... 9cc's dish, I wonder what they can be machined out to.

here they are for any one interested.

http://www.minimania.com/web/Item/20659%2F...S/InvDetail.cfm