Just had an expensive rebuild on my A65 race engine. This included a new timing side bush (all bronze type) supplied to size from SRM. The bush was fitted in the case then honed to + .0015" clearance by a reputable car race engine specialist. The engine was run up to temperature (10 mins @ 2000 rpm) 5 or 6 times. I took it to 3 Sisters race circuit to run it in and did about 7 to 10 gentle miles before a non engine problem put a halt to it. I then took it to a dyno to complete the running in and final set up of carbs and ignition timing. The dyno man was just warming the engine up on the dyno when within 2 minutes at low revs it seized solid. Removed from dyno and on initial inspection found we could move the crank slightly to and fro and the pistons were moving. I have now stripped the engine and found that the timing side bush is seized onto the crank.
When removing the sump hardly any oil (about an egg cup full) came out yet when the engine was stripped there was plenty of oil getting everywhere and the new big end shells are perfect and no signs of oil starvation anywhere.
Any ideas on the lack of oil in the sump and reason for the seized bush would be very much appreciated.
I'm not an expert but 0.0015" clearance seems way too small to me. I had a Thunderbolt crank re-ground and new bush like you many years ago and the engine told me he would machine it with a clearance of 0.005" (from memory).
I would contact SRM and speak to Geoff. He will advise the correct clearance.
There should not be too much oil in the sump, the scavenge gears of the pump being larger capacity than the feed gears.
Thanks for the reply. it was Geoff at SRM that said .0015 to .002. I have seen .0015 on other forums as well but cannot find anything in my workshop manual.
Cheers Julian, I was thinking (hoping) that might be the case.
Was the bush honed as a single case or was it set up with the drive side case and line honed? Any misalignment would cause tight spots and that may have been the cause of the seizure. Also did you check the correct alignment of the oil feed hole to the annular groove in the new bush as any misalignment would impede the oil flow to the bush. In my manual it gives the crank to bush clearance of 1 to 1.5 thou
Hi,
From the manual, the crankshaft timing side, when new is supplied 1.4985 - 1.4980", the bush is 1.5000 - 1.5005", making the tolerance of 0.0015" to 0.0025".
Best wishes...
Obviously differences in manuals my manual states 1.4995 - 1.4990 for the bush fitted so what i am now wondering is what actully is the recomended clearance
The bush was honed whilst in one case not line bored. I didn't fit it so unsure about oil hole alignment but I understand the big ends rely on oil through this bush so surely they would be starved of oil if it wasn't lined up?
My manual is a Haynes manual and doesn't give clearances so thanks for the info from the manuals.
Hi,
I have BSA workshop sheet A5065 GD3, copy attached.
Best wishes...
Here is a shot from my 1971 A65 manual GD3
I am wondering if bsa changed the tolerances later in production
Thanks for the info gents but, unless I am being even thicker than usual, I can't see where we get the clearance from - especially after a regrind.
Whatever the final ground diameter of the crankshaft journal is the bush internal finished diameter needs to be larger by 1.5 or 2.5 thou dependant on which manual you use. Thats where your clearance is from. The manuals dont give the clearance as a specific figure its a case of adding the clearance figure to the journal diameter figure to determine the internal diameter of the bush
Hi Shedman,
Yours is A65 only, I agree it's probably the later OIF model. What is the first regrind diameter of the timing side from your 1971 manual?
Best wishes..
First regrind is given as1.4885" - 1.489"
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
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...
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?
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...
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.
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.
SRM should have specified line boring more clearly. I am reasonably sure any workshop manual would have emphasised the need to line bore.
"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.
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?
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.
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?