Numbers puzzle

[Seven13]

Hi all, Canada calling after a fairly long absence from the forum. My Matchless CS is 99% complete, and an article will be submitted for Jampot, but before doing so perhaps some of our learned brethren can help solve a numbers question. As can be seen by the attached pic the engine number is 58/G12 05998, club records show the bike was completed and tested on 11/11/57, shipped to Firth's in Canada on 18/11/57 tested by Bob Regan. The dating certificate from the club shows it was a 648c.c, not a 646. There is the mystery. Is this possibly a Cooper specced motor, as '57 was in the midst of the export or die period? Thoughts would be appreciated.

[dave16mct]

For 58 export 646cc one year only. See Don Madden's description in the Tech Articles. You seem to have 60-0n heads and 60-on duplex frame.
Dave.

[SPRIDDLER]

I know zilch about twins but shouldn't the engine number have 'CSR' after the number G12?
There was a 1964 G12CSR of 648cc on sale here:

G12CSR 648cc.JPG

[SPRIDDLER]

For 58 export 646cc one year only. See Don Madden's description in the Tech Articles. You seem to have 60-0n heads and 60-on duplex frame.
Dave.

A bit more background here, esp re. duplex frame as Dave mentioned:
https://www.motorcyclenews.net/tag/retr ... ws/page/6/

G12CSR USA.JPG

[Seven13]

As with any basket case, this one languished for many years under a bench, who knows the what and why of how the bits came together. Club records show the frame to be a CSR, the engine as you can see is a '58 G12, what happened to the original frame and engine that go with what I have will forever be a mystery. However, it would certainly appear that the parts as delivered to me the owner(s) previous to the gentleman i bought it from used it as a CS.

[Seven13]

As a picture is worth 648 words here is the almost finished project in CS format. An article for Jampot will be submitted in due course. Not correct in many areas I am sure, but this is what it arrived at in boxes.

[dave16mct]

Well done, you've made a lovely job of that. I'm sure you're going to have some great rides too! I'm looking forward to your article in the Jampot.
Dave.

[les ward]

As I understand it, the 1958 (N America only) 650s were created by changing the position of the gudgeon (wrist) pin to enlarge the capacity and these pistons only fit those specific bikes. Otherwise they were virtually the same as the 600s. The 1959 650s had newly designed barrels, heads and pistons. This might explain the difference between the stated 646cc and 648cc.
Barrels, heads and pistons changed again for 1960 and it looks like you have these fitted now.

[Andy G]

I never understood the gudgeon pin position discussions...Surely this makes no difference to the swept volume (capacity) of the cylinder ?

[SPRIDDLER]

I don't recall seeing any official factory documentation (e.g. Posters, Adverts, Owners or Workshop manual) stating 648cc, only third party adverts/editorial and on the OP's V5 (which perhaps was a mistake by the clerk who produced that document. )