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Atlas Piston variations? #71271
09/14/07 12:19 am
09/14/07 12:19 am
Joined: May 2007
Posts: 24
Madison, wi
madcitybinx Offline OP
BritBike Forum
madcitybinx  Offline OP
BritBike Forum
Joined: May 2007
Posts: 24
Madison, wi
Hello folks,

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


1970 BSA Lightning
1964 Norton Atlas
1953 Ariel SQ4
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Re: Atlas Piston variations? #71272
09/14/07 12:45 pm
09/14/07 12:45 pm
Joined: Mar 2006
Posts: 3,689
Norfolk, UK
L.A.B. Offline
BritBike Forum member
L.A.B.  Offline
BritBike Forum member
Joined: Mar 2006
Posts: 3,689
Norfolk, UK
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.

Re: Atlas Piston variations? #71273
09/14/07 1:53 pm
09/14/07 1:53 pm
Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
D
Dave Comeau Offline

Crew Chief
Dave Comeau  Offline

Crew Chief
D
Joined: Aug 2001
Posts: 1,269
Hamilton, Mass. USA
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,
Re: Atlas Piston variations? #71276
09/15/07 5:14 pm
09/15/07 5:14 pm
Joined: May 2007
Posts: 24
Madison, wi
madcitybinx Offline OP
BritBike Forum
madcitybinx  Offline OP
BritBike Forum
Joined: May 2007
Posts: 24
Madison, wi
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


1970 BSA Lightning
1964 Norton Atlas
1953 Ariel SQ4
Re: Atlas Piston variations? #71278
09/24/07 9:41 pm
09/24/07 9:41 pm
Joined: Jul 2006
Posts: 894
newcastle australia
trevinoz Online content
BritBike Forum member
trevinoz  Online Content
BritBike Forum member
Joined: Jul 2006
Posts: 894
newcastle australia
When I rebuilt my '66 Atlas I used Commando pistons, Italian GPM. Had the crank dynamically balanced to 70% and it runs sweet as a nut. Still vibrates but not destructivally.

Re: Atlas Piston variations? #71279
10/07/07 10:47 pm
10/07/07 10:47 pm
Joined: Dec 2002
Posts: 706
B
beltdriveman Offline
BritBike Forum
beltdriveman  Offline
BritBike Forum
B
Joined: Dec 2002
Posts: 706
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.


Moderated by  Dave Comeau 


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