Drive Side Bearing

[olev]

The motors I've worked on in the past were singles and 2 strokes, and had roller and ball bearing bottom ends. No one seems to have a good word to say about BSA twin bottom ends. I'm in the process of building a low stressed star twin (a7). This motor has a roller bearing drive side main.
Brian has mentioned replacing this bearing with a ball type a couple of times on the forum (without expanding too much). Apparently this type of bearing can be locked to the crank and end play is less of an issue??
A gentleman on the web site 'piled-arms.com' (who admits to being controversial) has quite a bit to say on this matter and though it concerns A50's and A65's I suspect it also concerns A7's and A10's.
He also mentions a 'barrel roller' bearing which may be the best of both worlds.
I have an excellent contact in the bearing industry and can probably source one of these barrel roller bearings. Having said all of this I'm not a fan of being the first to try new tricks.
It would be nice to get some views on this.
cheers

[Brian]

Interesting subject and one that will no doubt create some differences of opinion.

There are good arguments for both type of bearing. The roller has a greater ability to deal with the load on the primary (driving) side than a ball does however the ball removes the end float problem. I have also read the article dealing with A50/65 bottom ends and it raises some interesting questions. I guess the question is just how much does end float play a part in the reliability of the bottom end. A roller bearing that has been properly shimmed to remove end float would appear to be better but the problem is that as the engine temperature increases so does the end float. I have had the top end off a hot motor that had no end float cold but had about .004" hot.

The early A10 engines had ball bearings, I am not sure exactly when they went to roller bearings. Obviously the later engines (road rocket etc) put out more power than the early plunger motors so no doubt that was why they changed to roller.

My plunger A10 is still running on the original bottom end and has a ball type bearing in it. When I restored the bike the bottom end was perfect so I put it all back as it was. I dont know for sure but I think it has done about 50,000 miles.

I have often thought about a barrell roller bearing as this would seem to be a very good compromise, best of both worlds. However I have never tried one.

I would have no hesitation in using a ball in a standard A10 Flash or A7 engine but would use a roller in anything that had higher performance.

I have a A7 engine to do in a few months so may well use it as a test bed for a barrell roller.

I think any of the three types would be fine in a A7, what does every one else think?

Brian.

[fido]

I think you are being a bit unfair on BSA twin bottom ends, Olev. It is really just the timing side bush that can sometimes give trouble. If they were that bad you would not see so many of these bikes still on the road. People would have either scrapped them or fitted Triumph engines to make Tribsas.
I didn't realise the early engines were ballrace but then, as I may have mentioned before, my A7 has never been apart since I got it many moons ago.

[olev]

According to Bacon's book 'BSA twin restoration', BSA went back to a ball bearing drive side main in the unit construction engines from 1962 - 65. They also used a shim cup and shims in these engines. At present this is the method I am inclined to use in my bottom end rebuild. He also talks about an oil seal outboard of the drive side main bearing of all engines except those built between 1951 - 53. I can't find this oil seal on any exploded views of the engine that I've looked at. I assume its between the bearing and the case??

Fido,
I'm not 'having a go' at BSA bottom ends. Over the years since they were first built there has been leaps in technology and metalurgy. BSA also used the same basic bottom end in their twins to the end. I would like to incorporate any improvements into my rebuild. Bugger it, I will have a go at their bottom ends. A cheap, sad, gutless, unreliable villiers 'A' series engine from the same era has a bearing configuration that makes a BSA twin look agricultural. (but I'd rather have my BSA -ok?)

[Brian]

I had a villiers engined bike once, I sold it to a fisherman - he needed an anchor.........

I totally agree with Fido, there is nothing wrong with the BSA bottom end and there are lots of them around that have done many thousands of miles. The main argument is that they should have had a bearing and not a bush on the timing side, but, a bush provides much more bearing surface area than a ball or roller bearing and as we all know the automotive (car) world has used plain bearings for about 100 years.

Out of all the modern motorcycles available one of the ones best know for being able to do huge mileages without rebuilds is Moto Guzzi, and guess what sort of main bearings they have.

The BSA bottom end does require some care in assembling correctly but will last a lot of miles when done properly.

So Olev, build your bottom end as per BSA specs for your bike year, no modifications, and it will last a very long time.

A A10 bottom end shouldnt be compared to a A65 either, they are a different design. I have had A65's and A10's and the A10 engine is far superior as far as smoothness and longlivity.

Just as a side note, going back to that Villiers engined bike I had. It was a 125cc James and when I sold it I vowed I would never own another Villiers product again, and I havent. That was 40 years ago. A couple of years after I sold it I bought my first A10.

I'm glad you would rather have your BSA, good choice!

[bsa-bill]

Hi Olev - the seal does go out board of the bearing, it's the last thing you tap in on that side before assembling the cush drive.
Haven't got my spare book at hand but I think it's in there, my book is for 59 onwards so there could well be a difference.

All the best - Bill

[A10Boy]

As we all know, on the STD engines the bush was fine if it had been installed and reamed out correctly. If there was a lot of wear in the bush it could leak oil pressure and starve the big ends. The harder you treated an engine the more critical this was, so the more tuned the engine the more likely it was to go bang especially the high performance A65's.
There is a big difference between plodding around on a 35bhp Flash and racing the idiot off an almost 50bhp lightning.

I would stick with the std bottom end on your A7 it will be fine unless to want to race it. Just keep changing that oil.

[olev]

You got something against fishermen, Bill?
My inclination now is to rebuild it to original specs due to the comments received. Thankyou.
I'd appreciate some views on my latest theory though.
The crankshaft is supposed to have only 1thou or so freeplay. I thought that when the engine was running the crank would just slop about within this 1thou. But having a better look now suspect the action of the oil pump against the worm should push the whole crank up against the drive side bearing.
The attached picture of my timing bush would seem to put the lie to this though?

[RichardL]

I don't think the oil pump/worm drive factor comes into play, as there is unlikely enough lateral leverage from the pump drive to affect the big forces happening at the crankshaft. Also, I don't think I would characterize movement of +/- 0.0005" as "slopping about." It seems to me that keeping the tolerance small will limit the lateral inertia that can develolop when the cush drive bonces about. The more play, the bigger the thump that both bearings (timing side and drive side) and the worm drive must take.

Anyway, Olev, I say, don't use that bush (kidding, of course).

Richard

[A10Boy]

+/- 0.0005 is about the same as a knats Knacker and as manosound says it wont slop about, the oil will take this up when its running.

[olev]

You are correct of course. 'slop about' was never the phrase to use.
The reason for the enquiry was that I'm still inclined to use a ball bearing main in the rebuild.
If the forces acting on the crank push it towards the drive side then all is ok.
If they act in the other direction, as this bearing is locked to the crank they are likely to push or flog the bearing out of the timing side case and jam the crank against the timing side bush. As the bearing is not captured by a circlip or plate and the problem doesn't seem to arise in the a10s and a65s that use ball bearing mains, it seems probable that the crank does migrate to the drive side.
I read an excellent write up about this somewhere on the internet but can't find it now.
I recall the gent who wrote it thought the action of the cush drive had no effect (which seems strange to me) on lateral forces. The ones he mentioned were the oil pump and something to do with the alignment of the primary drive. There may be others. I'm confused and need realignment.
Manasound, about that bush - maybe a shot of silicon.

[beezalex]

AFAIK, no A10 ever used a ball bearing.  All parts manuals as well as press releases show a roller bearing from '49 on.  It wasn't until the unit twins that BSA switched to a ball bearing...and then back to a roller.  The bottom end issues associated with the unit twins appear to result from oil supply/cleanliness and higher power/rpms associated with these bikes.  That said, in both engines, thrust is a secondary concern as long as it is not outrageously excessive (I ran an A65 with .020" endplay for 10,000 miles with no issue other than noise at idle)  as long as the primary chain is in alignment.  I think the importance of endplay is overrated.  Clean oil, a healthy pump and clear drillings are much more important.