1967 Norton Atlas

9

964mike

Joined
Jun 24, 2013
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I am rebuilding a 67 Norton Atlas and looking for 20 over pistons. The orginal pistons are dished. All I can find are the flat top used on the Commando. Will I get in trouble with too high of compression with these pistons? Or is there a source for the original type.

Thanks
 
no flat tops will be fine but if you want a smoother more flexible motor try to find some dished ones
 
baz said:
no flat tops will be fine but if you want a smoother more flexible motor try to find some dished ones
Click to expand...

Has anyone here actually seen or found ANY new dished pistons, ever, anywhere ?
Unobtainium...

They were phased out of production (long?) before the Atlas ended.
Anyone know when ?
Were they even available in oversizes ??

Everyone ? is using flat top pistons.
 
The pistons that came in my '67 G15 basket case were flat top and standard bore - bike is S/N G15CS/1235xx, July, '67 production. I can't say for sure if those pistons are the original ones to the bike, but the engine of a G15 or N15 is identical to an Atlas of the same period.

Dished Atlas pistons are about 7.5:1. Flat tops are about 8.5:1, but a thicker head gasket will drop it down.

Can't say what balance factor would be ideal for that frame, but my P11 is balanced to 82%, and the G15 will be 84%. P11 is good anywhere below 75mph - I'll know what the G15 is like in about 75 days.

P11 pistons are also flat top and .020 over. Bike is really easy to kick - easier than my 850. Same basic engine, clutch and trans as a '67 Atlas with coil ignition.
 
I have searched the web exhaustively and found only one original Hepolite dished top, a LH. For anyone interested, try http://vintagemotorcyclepistons.co.uk. They have some dished tops listed for AJS (will fit Atlas), but beware...they have some listed having those dreaded slots under the oil rings.

I have given up my search and will replace the 10.5:1 compression ratio pistons in my Atlas with Commando flat tops. I understand those to be 8.9:1 CR and with a base shim plus gasket of .057 overall thickness should drop the CR to 8:1. Ignore the thickness of the gasket under the shim...CR is determined assumming it is there.

I just went to Old Brits website...they list nothing for pre commando pistons.
 
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