The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => A7 & A10 Engine => Topic started by: olev on 21.10. 2008 13:37

Title: A7 camshafts
Post by: olev on 21.10. 2008 13:37
I am restoring a 51 star twin for my wife to ride.
The motor has a 356 camshaft which needs replacing.
Can anyone tell me the difference between a motor running a 356 and one running
a standard camshaft (a 334 I think)
Title: Re: A7 camshafts
Post by: Brian on 21.10. 2008 23:12

I wasnt sure how to go about putting a explanation about cams into words so I will quote from D.W. Munro's book of BSA twins.

Quote - "Officially there are three "A" camshafts: that fitted to the original A7 and Star Twin up to 1951 (67-695), the A10 camshaft also used on present day A7 models (67-334), and the sports camshaft (67-356) for the Star Twin, the Shooting Star, and the Road Rocket.

When dealing with compression ratios we stressed the importance of having cams to suit the ratio. A high compression engine works harder than a low compression one: it develops more power, and to do this it must be able to breathe more freely. The cams therefore usually have higher lifts and longer periods. They also have more overlap, which means that the inlet opens earlier and the exhaust closes later, so that there is a longer period at the appropriate top dead centre during which both valves are open at the same time. This has a profound influence on the extractor effect, which is discussed further in the notes dealing with exhaust systems.

To sum up these remarks, it simply means that sports cams are necessary for the sports performance expected of high compressions. If ordinary cams are used the engine will be stiffled, and consequently unable to give of its best." - end quote.

Your bike being a Star Twin would have had a 356 cam in it as standard.

Hope this is of some help.

Title: Re: A7 camshafts
Post by: dpaddock on 22.10. 2008 02:41
Actually, there are also "officially" the -357 and the -358 camshafts, neither of which pertains to the A7 - only to the A10 derivatives.
Title: Re: A7 camshafts
Post by: olev on 22.10. 2008 02:50
Thanks Brian,
As the bike was primarily for the wife, I intended to build something tractible and smooth and thought the 334 cam might do a better job of this.
Thinking about it, maybe the 356 cam is best. Its not as though we're talking fireblades.
Don't string me up but i intend to fit a 26mm mikuni for the same reason.
Title: Re: A7 camshafts
Post by: Brian on 22.10. 2008 05:23

I would go with the standard Star Twin set up. The 356 cam and 7.25-1 compression ratio pistons. Depending on what pistons are available you could go slightly higher, 7.5-1, but I wouldnt go any higher than that. This should give you what you want, reasonable performance but a nice tractable motor.

My 51' A7 has this set up and runs beautifully, dead easy to start and goes well.

The original carby will work well provided it is in good nick but the mikuni should also work ok once you have got it jetted correctly. You could also consider a concentric, whichever you prefer.

Hope it all goes well for you.


Title: Re: A7 camshafts
Post by: olev on 23.10. 2008 11:22
Thanks for that Brian,
< runs beautifully, dead easy to start and goes well >
sounds good to me.