The BSA Workshop > Twins

RGS inner primary chain case

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robin smith:
As far as I am aware, despite the many RGS options offered by the factory, a fully enclosed final drive chain case was not one of them!

Therefore, why would the short inner primary chain case 42-7515 with the drilled boss to accommodate such a case be fitted, as seen on some RGS labelled machines, apposed to the longer type with the un-drilled boss 42-7505? 

Perhaps the owners of the bikes you have seen could tell you?

I think you will find that all the swinging arm twin chaincase backs have a casting number 42 7503 on them.

The 1954-55 A7 A10 (rod brake) parts book shows the part number as 42 7503.

The 1954-57 book shows it as 42 7515.

The 1958-59 book shows it as 42 7515.

Same in the 1960-63.

The RGS parts service bulletin, G21,  also shows it as 42 7515. (see photo)

So which part number is short and which is long?

I suppose if the owner is happy with his bike does it actually matter?

robin smith:
Having owned and ridden a Gold Star fitted with Super Rocket engine for almost 40 years. During that time, having lost count of the times I have had to explain, this is not in fact Rocket Gold Star!

Then attempting to point out the difference between a genuine RGS and my GSR! Consequently, the question of what is the correct type of inner primary case as arisen at times, hence my enquiry on this forum.

I totally agree with you “If the owner is happy with his bike does it actually matter?”

But obviously to some it does!  Particularly with so many so called replicas in circulation.

robin smith:
It would appear surprisingly, by the lack of response on this particular subject. I should merely shrug my shoulders when asked this question. 

In spite of the wealth of knowledge you guy’s have out there!

The RGS was the end of the line for the pre-unit twin and I would not be at all surprisd if the answer is that they use the last stocks of whatever chaincase was avaible in the latter days if the production run to get bikes out of the door for the least cost.


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