Timing side bush.

[Shedman]

On looking at the figures in my manual again and working back from the first regrind it would only give a maximum clearance of half a thou at best and size for size at worst. This doesnt make sense to me and clearly wouldnt work. Time for a brew and try and get my head round this

[Bess]

Hi Shedman,
                   Yep, something not quite right there, maybe a typo.

I have the the 1962-65 and up to 1971 manuals they are both 1.4985 - 1.4990", the bush is 1.5000 - 1.5005" giving 1 to 2 thou clearance.

Sorry my first post was wrong at 1.4985 - 1.4980,

Best wishes...

[teebee67]

So to recap : 1 to 2 thou means SRM were right to say measure the journal then oversize the bush by 1.5 thou. I am no nearer to understanding why it seized on the shaft. Maybe I have to pay the extra to get it line bored rather than honed or reamed?

[Bess]

Hi,
    Yep SRM were correct. Setting up with the cases together and ensuring the drive side is true (concentric) before reaming/honing the timing side bush is the correct option.

https://www.youtube.com/watch?v=Lg5AcTqq_9g

A link showing the mains being replaced and a machining method on a A10.

Best wishes...

[STAR TWIN]

I think the answer is that your bush was not line bored. Line boring is essential with BSA timing side bushes. Was the bush fitted by someone familiar with BSAs? If he was, he should have ensured that the oil feed holes lined up and he should have insisted on line boring.

[teebee67]

Thanks for all the replies, all very much appreciated. Given me food for thought, it looks like line boring is the way to go though SRM said to fit into case and ream to size.

[STAR TWIN]

SRM should have specified line boring more clearly. I am reasonably sure any workshop manual would have emphasised the need to line bore.

[JulianS]

"I am reasonably sure any workshop manual would have emphasised the need to line bore."


The A65 (and post 1957 A7 and A10) was not intended for reaming or line boring so neither are mentioned in any of the factory A65 manuals. the original steel backed bushes were properly sized to the journals when fitted to the case without any extra finishing, infact reaming, or honing or boring an original Vandervell bush would damage the bearing material.

Yes times have changed and Vandervell no longer make the bushes.

[teebee67]

So what did BSA recommend when the bush needs replacing? I can still buy a steel backed bush (including a new old stock genuine BSA one) but I was told that the solid bronze bushes were better?

[JulianS]

From the manual if the journal was worn by 2 thou or damaged it needed a regrind , not very prescriptive I am afraid. Page from 1969/70 manual attached.

The composte bush cannot be machined as it would destroy the thin deposit of Vandervell  bearing material. The weak point of the composite bush was that if the insert moved, which sometimes happened even though they were pegged together, it would block the oilway to the crank then disaster would soon follow. And of course with a solid bush your can just shave a little off the journal to remove damage or ovility and size the bush to suit.

[teebee67]

I can see that makes sense but surely it is not as good as what I had done:
Journal ground, measured and bearing made to size by SRM
Bush fitted into crankcase then honed to 1.5 thou clearance.
Surely the BSA way is not as accurate regarding clearance and doesn't allow for any distortion when the bush is fitted into the case.

I have now had the journal ground and polished the minimum amount to make it perfectly round and then measured. It measures 1.49". I will have a new bush fitted into the case then either line bore or hone to clearance size. I would've thought this is better than how you describe BSA did it? (Unless I am not understanding it correctly?).

To recap the bush seized solid to the crank. On stripping the big end shells were perfect, and after removing the bush (with a chisel) there was no discoloration of the bush - so no sign of excess heat. No part of the engine showed signs of oil starvation and, except for the bush seizing on the shaft, the engine is in perfect condition.

It has been suggested to me that I go to 2 thou clearance (I know this will reduce oil pressure slightly) instead of 1.5 thou - any comments welcome?