Atlas pistons in a Commando?

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Apr 15, 2004
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Here's a question you guys probably don't hear every day! :lol:

I bought a set of "Norton 750" pistons off evil-bay. They're NOS japanese-made and appear to be very high quality. They're std size so I may or may not be able to use them but the price was very attractive so I bought them anyway.

The box says "750cc Norton Atlas". I'm told most people rebuild Atlas engines with Commando pistons for the higher compression ratio (8.9 vs 7.5) and presumably better performance. I guess that's why these were so cheap :?

But what if I put these in my bike anyway? Perhaps a little less compression is actually a good thing with today's poor quality fuel? Would there be a big performance drop?

Debby the bargain hunter
Atlas pistons in a Commando engine

Hi Debby,

The only issue about this was using Hepolite pistons for Atlas engines with a slot cut at bottom of the oil control ring groove. In Commandos the piston crown had a tendency to break off with this type. If your Japanese pistons are solid, don't have the slot, you should be able to use them.

One thing you won't know is the weight comparison to the original pistons that the crankshaft balance factor was made to. You might want to take the crankshaft, connecting rods, rod bearings and pistons with gudgeon pins and rings to an automotive machine ship for balancing. Let them know the balance factor you want, don't know it is for the Commando engine off the top of my head. This will keep the engine as smooth running as possible.
Thanks David! These pistons are the improved design with holes drilled in the ring land, no slots at all. So I guess they should be ok.

Was thinking of just doing a top end job for now although a full teardown would be a good idea considering what I've found on this bike so far. Of course if I find a worn out cam when I pull the barrel I suppose I'd be going in all the way. If so, the bike will probably be down until next spring :(

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