Late Model G12 Engine Stamping

[oldandmatchless]

I know that late model G12s did not have the model year stamped, but were the CSR models all stamped as "CSR"?

I have several late G12's - one is stamped "CSRx105**" and the other is stamped "x107**" The later is evidently a very late G12, but I'm not sure it is a CSR.

Both have transmissions that are stamped MA*****R

Any thoughts or opinions?

Thanks!

Rick T

[Ib-Denmark]

I know that late model G12s did not have the model year stamped, but were the CSR models all stamped as "CSR"?

I have several late G12's - one is stamped "CSRx105**" and the other is stamped "x107**" The later is evidently a very late G12, but I'm not sure it is a CSR.

Both have transmissions that are stamped MA*****R

Any thoughts or opinions?

Thanks!

Rick T

My g12 1965 Is stamped:
g12 X10311.
I am convinced that the engine is a G12 standard because I have recently checked the pistons.
(The only differerence between a standard engine and a CSR is that the pistons in a standard are 7.5:1 (43.5bhp) and a CSR 8.5:1 (48bhp))
I have now changed my pistons to 8.5:1 and I dont feel any difference
regards Ib
p.s. If you want to check you can measure the distance from the plug hole and compare with the CSR.
p.s.p.s. As I remember the difference is app. 1 - 1.5mm

[Rob Harknett]

I am sure if an engine is a CSR it would be stamped CSR.

Ib. Having fitted 8.5 pistons in your G12 and feel no difference, what else have you done?
G12 & G12 CSR used a different carb type. I noticed the CSR carb has a larger main jet. Perhaps your G12 also now wants a larger main jet?

[Ib-Denmark]

I am sure if an engine is a CSR it would be stamped CSR.

Ib. Having fitted 8.5 pistons in your G12 and feel no difference, what else have you done?
G12 & G12 CSR used a different carb type. I noticed the CSR carb has a larger main jet. Perhaps your G12 also now wants a larger main jet?

Maybe I feel a little difference
All carb jets are exchanged to the correct ones. I mainly dont do much racing and i guess the difference is more noticeable at high speed.
Dont want to say that the bike is running badly. it is running fantastic as it also did before the new pistons.

[Rob Harknett]

When I was about 17 years old Ib I put high comp pistons in my 55 G3/LS. I noticed a difference of top end speed. Also knocked the big end out pretty quick.
When I collected the bike after my local dealer repaired it. He said here's the piston we found in it. We have replaced it with the correct piston. You can put the hi comp piston back in, but you'll soon be bringing the bike back again for a new big end. He said if you want to go faster buy a 500.

[Peter Morris]

My late Ajay version (M31) engine number has CSR stamped, it's 1966, they don't come any later than that.

[oldandmatchless]

Thank you for the replies.

I have to assume that mine is a “standard” G12. The transmission stamping threw me a little as I was told that all the CSR boxes ended with an R. It's possible the trans could have been changed sometime in the last 50+ years but the number sequence pretty much puts the motor and trans manufacture in the proper date (late 66) sequence.

[JEAN-NOEL]

On gear boxes the final "R" does not indicate a CSR or not. It appears in 63 & 64 (according the versions) for a 17 teeth sprocket on the final drive, when the rims were 18'' instead of 19. In the primary, late bikes had 23 teeth sprocket for Standard, and 24 for CSR ones.

[oldandmatchless]

I did check the primary drive sprocket. It is a 23 tooth sprocket. I think that answers my question.

Thank you!

[oldandmatchless]

In checking the workshop manual and spares manual (for 1962, 1962, 1963, 1964) it appears that the late 650 CSR used a 23 tooth engine sprocket and a 17 tooth trans drive sprocket.

The standard model used a 24 tooth engine sprocket (the manual states add 1 tooth for 18" rear tire models) and a 16 tooth transmission drive sprocket.

Am I misunderstanding?

The motor and transmission I have has a 23 tooth engine sprocket and a 17 tooth transmission drive sprocket.

I have attached a picture of the references from the maintenance manual.