Camshafts Etc

[Rob Neal]

Have I now managed to work out why my A65 won't run properly?  Would appreciate any comments on the following.  Engine  number is A65-253 which I believe makes it an early 1962-63 Star originally fitted with a single carb head.  However my bike has been modified at some point with a twin carb head. The engine has had a recent complete rebuild and I know is fitted with a new cam part number 68-103 as the one it came with (P/N unknown) was completely shot.  Whilst this is correct for the engine number the parts lists show that later twin port head models were fitted with 68-0473 cams. Also the inlet push rod part numbers seem to differ with 68-370 for the 62-63 Star and 68-0365 for the later twin carb set ups.  This info is gleaned from the Dragonfly on line parts lists. Several questions:

1. Cams - will a twin carb run OK with a 68-103 cam? Guessing not with the probs I've been experiencing.
2. How to tell the difference between the inlet pushrods 68-370 and 68-0365?
3. Will a single carb head accept the same down pipes as a twin carb head?

If I do have the wrong cam/head combo I think it will be simpler to fit a single carb head as a cam change will involve a complete engine strip down (not sure I can face that again!!).

As I say I THINK this may be the root cause of the problems I've been trying to solve over the last 12 months. Thanks for taking the time to look at this and looking forward to your wise words.  Cheers. Rob

[DAVE BRADY]

Hi,

My parts books go back to '65 and show the 103 is for the 500cc Royal Star and the 473 for the 650s.
It is possible that the very early 650s had the 103 as it is quite close to the A10 cam.
The twin carbs will work but not to best effect so a single carb head may be a good option - only one carb. to tune as well.
The pipes will fit. Not sure about the pushrods.

Dave.

[limeyrob]

What do you want from the bike?  A single carb head on the gentle (103) cam will be a nice but relatively slow bike that will not get the best from the over-square engine. It may feel a bit like an A10. If you want power (and fun) go for the Spitfire cam, 9 or 10:1 CR, 32mm carbs and long intake trumpets.  I had this on my late A65 and it was under 6s O-60, it really got ongoing once the revs were up and pulled like a train above 5000 rpm.
I did a lot of miles on it and kept it for about 30 years but I wouldn't want to ride it now. Keeping it in the powerband was great but so many parts cracked with high frequency vibration.

[Allan G]

The early Star and Rocket models had the 103 cam. It's a decent cam. It'll pull strong from low rpm, if you get the revs high enough then the motor will see a new lease of life and go really well.

One advantage with the 473 cam is cranking compression is lower due to the increased valve overlap.

My 823cc A65 was hard to crank over when I put the 103 in it. Ended up changing it to a 473 and starting became much easier. My A50 still sports the standard cam (I believe at least - it's one motor I have yet to rebuild)

It all depends how you want to run the bike. A65's work best when they are revving. I prefer the 473 cam, and if it was my money I would fit that. I'm even considering fitting the 357 (same cam profile) into my Golden flash.

The A10 356 cam is the same profile as the 68-103

[Gup holland]

I also got an early A65 with 62/63 engine. My set up is a 473 cam , twin carbs 30mm.
I have run this now for 30years now. I runs great.

[Rob Neal]

Thanks for the advice as always.  From this it looks as if the best option would be to fit the 473 cam.  I was a complete BSA novice when starting this project and should have perhaps bought a complete 'straight' running bike rather than a bitsa with a seized motor.  Oh well, we live and learn!  Back to the spanners for me then!!

[Rob Neal]

Can anyone point me in the right direction?  I'm looking for any published info which gives the cam lift dimensions for the 68-103 and 68-473 cams fitted to A65 engines.  I can only find info that says the 103 is a 'softer' profile.  Very technical! 

[Allan G]

Assuming you're a member of the owners club? Log in to the members section and pick any of the WSM (work shop manuals) from around 1966 onward. In the general data section you will find the cam details for both cams.


Look under A50 for your cam profile as they used the 103 for ALL A50 models from start to end.