GPM pistons +0,20

Hi All.
I have two couple of GPM pistons +0,20.
They look like the same but one couple has the top flat and the other has the top al little bit domed.
What is this diference?
Thank you.
Piero

Piero,

the only domed Commando pistons I've seen are AE (Hepolite) 'Powermax'.

http://www.accessnorton.com/piston-t12643.html

I have some high compression Dunstall GPM pistons, but these are flat topped, so I couldn't say what you have there!

Hi Andy.
I can only confirm that one couple of pistons have a little bit domed, they are not very flat.
The domed is not high like your AE powermax, but less.
Ciao
Piero

The difference in height from the centre of the small end to top of piston is given in the books for 750 pistons as Flat top. Piston number 18145. Compression ratio Atlas 8.7-1 Commando 8.9-1. 1.5429 inch / 39.19 mm.
Dome top. Piston number 18554. Compression ratio Atlas10.5-1. Commando 10.25-1. 1.7398 inch / 44.19 mm.

I assume your pistons simply give different compression ratios BUT DO NOT EVER believe the compression ratios given in the books....if you really want to know what they give fit them into a motor and measure it.....and then fall about laughing at the values given in the books and start thinking on how to increase it ! Mind you as one very well known Gentleman said to me not long ago..'When I bought my std 650SS it was timed at 118mph but after fitting all the go faster goodies it went slower being times at 110mph.....I then removed all the go faster goodies'.