I found some odd pistons that appear to be original on my 1964 Atlas. They are significantly concave in comparison to the flat one's I have seen and the part numbers are: 24282 (left) and 24283 (Right). My spare parts catalog and Bacon's book indicate #24246 and 24247. Other numbers stamped on the Piston heads are (L63 and K63) and both are also stamped with an "A"
As I am in the middle of a rebuild I was wondering what the story is behind these apparently lower compression pistons? Any implications for my rebuild? thanks in advance...Ben
I think the original early Atlas standard compression pistons had concave crowns?
The following paragraph is an extract from Steve Wilson's 'Norton Motor Cycles' book.
Quote: ----------------------------- "Other changes for the 750 [from the previous 650 specification] included concave pistons and a different shaped combustion chamber, giving lower compression. It should be noted that Doug Hele had been emphatic that the compression ratio on this stretched design should on no account be raised above the 7.5:1 cr he originally specified. The raise to 9:1 shortly after the move to Plumstead, and the subsequent Combat Commando follies, were all in direct contravention of this distinguished developer's recommendation." -------------------------
Both the 24246/24247 numbers in the parts book describes the part as "Piston, complete" so these numbers are likely to be the part numbers for the piston assemblies (piston/rings/pin/circlips) and maybe not the part number for the individual pistons.
The 'A' markings are probably the bore grade size.
Some technical concequences are: Unless reworked or you use nonstock flat top commando pistons...the commando pistons are heavier than stock atlas. The effective balance factor goes down. You have more up/down thrust induced, while front/back remains the same. The frame which acts like a tuning fork and therefore will react differently, i.e. vibrate at a different RPM. Rebalance may be desired....but to what??? The higher compression will make it more peppy. You will have to retune for the increased engine pressures/compression.
dynodave BSA 3 1961-1963 Ducati 3 1992-2002 Norton many 1951-1975 87 Serv-Equip 100HP MC brake dynamometer,
Thanks Gents...I am in the hunt for the concave pistons! Still working on determining the range of my rebore. Gary, sorry to hear about the gearbox debacle! Let me know if you remember the brand of the Italian replacement pistons. thanks..ben
Yes but do you spend all your riding time playing at biy racer as we did in our youth and thus had handlebars several inches in diameter most of the time...while the engines stayed in one piece that is!!! Thinking about it having the lumps in bits so often probably saved us from suffering white finger!!! Tee hee.