Piston AE 063360 ?

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Josh Cox

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Have just pulled AE 063360 pistons out of my engine, 20 thou, am trying to find out the compression ratio for these pistons ?
 
Josh Cox said:
Have just pulled AE 063360 pistons out of my engine, 20 thou, am trying to find out the compression ratio for these pistons ?
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Do they have flat crowns? If so, then they are probably standard ratio, as high compression Commando pistons are rare.
 
I've never seen raised-crown pistons for Commando/Dominator motors. As far as I know, compression mods were done by shaving the heads and fly-cutting the valve pockets in the pistons for clearance.
 
flat top, with valve recess, one piston is 063360, the other 063353.
Piston AE 063360 ?

Piston AE 063360 ?
 
Josh Cox said:
one piston is 063360, the other 063353.
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"063353" would be a R/H +.020 Combat and later "all [750] models" piston (assembly). "063360" is an unknown number.
 
Well a harder question then, the barrels you see above are the Atlas barrels that I have removed, they have a spigot/lip at the top of the cylinder wall. Do the commando barrels have this also ?.

What is the purpose of the spigot. The guy I swapped the barrels with has noted a 3mm difference in the distance between the top of the gudgeon pin and the top of the piston on pistons he believes are Atlas pistons.

His theory is that the spigot is required as it protects the head gasket ?, does this make sense ? ( not to me anyway ). If so, why would they do this ?.
 
Josh Cox said:
Well a harder question then, the barrels you see above are the Atlas barrels that I have removed, they have a spigot/lip at the top of the cylinder wall. Do the commando barrels have this also ?.
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No.



Josh Cox said:
What is the purpose of the spigot. The guy I swapped the barrels with has noted a 3mm difference in the distance between the top of the gudgeon pin and the top of the piston on pistons he believes are Atlas pistons.

His theory is that the spigot is required as it protects the head gasket ?, does this make sense ? ( not to me anyway ). If so, why would they do this ?.
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Yes I believe so, as I think it helps to reduce the combustion pressure on the gasket.

I noticed the two pistons in your photo appear to have slightly different skirt profiles?
 
Josh Cox said:
The skirts are the same, just inside and ouside if you catch my drift, thanks.
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I'm not sure that's standard feature of either Atlas or 750 Commando pistons? Perhaps others can verify whether it is or not?
 
I just had a look at 7 pairs of old pistons in my workshop, they are all the same style as this with the slots and all are symmetrical when it comes to the skirts. I think someone has modified these.
 
Those pistons are the old slotted style, with more slot under the oilring. Their redline is lower than later style commando pistons - if you rev them too hard, the top pulls off the pistons.

Early Atlas motors (and other dommies) had the spiggot up into the head. And the head was machined to suit too. The theory was originally that this kept them together as a more rigid unit. Experience proved that this wasn't needed, and without them, the cylinders and heads are more easily milled to keep things flat for less blown head gaskets. At least, thats the theory.

hth.
 
Rohan said:
Those pistons are the old slotted style, with more slot under the oilring. Their redline is lower than later style commando pistons - if you rev them too hard, the top pulls off the pistons.
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063353 should be the stronger "Combat" piston. http://www.norvilmotorcycle.co.uk/techtalk34.htm

I don't know about the 063360, but I'd guess that is also a non-slotted type?
 
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