1967 Norton Atlas

jms said:
Who makes the new stronger type and what are the actual differences? I know that the bike originally came with the dished pistons which translated to a lower compression engine than the higher compression commando type. Here‘s an oddity these pistons of mine measure at the skirt 2.865 a clear 10 thou under the standard 2.875. To be clear the standard piston size should be 2.875 and the bore of the cylinder 4-5 thou bigger than that correct?
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I've just measured the original pistons I removed from my commando at about 35000 miles - 2.871" & 2.8715" which makes yours quite worn but thinking about it, if you are building a show bike that is going to be treated gently and infrequently, it should be ok to re-use them after checking for cracks, otherwise replace them. I seriously thrashed my commando through the gears every time I used it and the pistons stayed together. Measure the bores to see if you need a re-bore or just honing.
 
baz said:
This Is the type of piston you don't want
the weak point is the slot in the piston in the oil control ring groove
The stronger type are drilled
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So Baz the picture of the piston you posted is broken correct? it looks like a continuous groove behind the oil control ring. Mine looks like this with the rings removed. 4 short slots behind the oil ring
 

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jms said:
So Baz the picture of the piston you posted is broken correct? it looks like a continuous groove behind the oil control ring. Mine looks like this with the rings removed. 4 short slots behind the oil ring
Click to expand...
No it's not broken but it's the type that break
The picture I posted was of an early commando piston that has a slot behind the oil control ring instead of holes drilled
 
Old thread, I know but I have a short story about slotted pistons.
After thrashing my Atlas on a long weekend ride of several hundred mountanous miles, I was about 100 miles from home when the bike started running on one cylinder and vibrating differently. Parked the bike at a farm, rode pillion home with one of the other guys and went back later with a pickup to fetch the bike.

On tear down, one piston remained at the top of the bore while the engine was turned over. Pulled it all apart to find that a slotted piston had failed, leaving the skirt and pin intact. The bores remained unscratched so I honed them and fitted new drilled pistons sourced from Austrailia. I think they came through Rabers. The failed pistons were 0.030 oversize so these slotted pistons had gotten into the spares supply chain. The Austrailian pistons had very tight pins and had to be bored on the pin fitter. I don't recall what the cylinder clearance was. Once installed there is a slight piston slap when cold but goes away in a few minutes. Thirty years on and much more thrashing it still runs fine.
 
Well I guess that confirms it. Don't use slotted pistons! Thanks for the story. I recently found a pair of original Atlas dished pistons so that's what I'll be using. It's sort of hard to believe that with all the R&D that goes into manufacturing these things that failures like that present themselves. See the red Commando of mine just to the left here... it started off life as the 76 Fastback Commando off the line in 1968. Guess what still had the " Widowmaker frame" when I bought it in 2010. I had to weld the appropriate frame remember in. Hard to believe that Norton went from the sturdy Featherbed frame to one that had the potential to collapse at the headstock
 
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