The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => A7 & A10 Engine => Topic started by: Brian on 14.04. 2011 08:06

Title: Drive side main bearing
Post by: Brian on 14.04. 2011 08:06
There has been much written and discussed when it comes to the bottom end of the A series engines. Most of the discussion relates to the timing side bush and its ability or inability to do its job. There are many needle roller conversions available but these are expensive and not always totally successful, its a lot more complicated than just fitting a bearing on the timing side.

Now I believe the timing side bush is not as bad as most think however the bottom end design does have some inherant problems. The BSA crank is one of the only ones where the crank is not "trapped" meaning it can float from side to side and relies on shimming to minimise the float. The other problem is the oil pump is driven by a helical gear on the end of the crank and this in effect pulls the crank to the timing side and especially when the engine is cold and the oil is thick this can amount to a reasonable amount of pressure being applied to the face of the timing bush. As the mileage increases so does the end float in the crank allowing the crank to "shuck" or hammer back and forth. Now lots of people have different theories about just what effect the end float has, some say it doesnt matter and up to .030" is ok, others say once it gets over of the .003" limit it will self destruct. I believe this ability for the crank to float is the biggest design flaw in the motor, not the fact that the timing side has a plain bush.

So, how to help this problem. You can fit a ball bearing on the drive side, once the cush nut is done up there is no end float. The problem here is that a ball bearing has about 40% less loading capabilty than a roller bearing, I believe that in a standard low compression engine a ball bearing is quite suitable. It will not last as long as a roller bearing but will last as long as the timing bush. There are quite a few who have used ball bearings in their engines quite successfully, me being one. My plunger A10 which has a 356 cam and runs 7.25-1 compression did 47,000 miles on a ball bearing main, it was the white metal timing bush that finally gave up, the bush didnt wear much but started to break up. The late BSA Bill, who most would have heard of, here in Aus used ball bearing mains in his A65 which did very high mileage.

Is there a solution ? I believe there is. What we need is a bearing that has the radial load capability of a roller but that holds the crank in place by not allowing end float. I have spent many, many hours trying to find such a bearing and finally have. Norton owners will probably have already guessed where I am going with this. The bearing I am going to use is commonly known as a "superblend" bearing. A single row spherical roller. These bearings have a higher radial loading rating than a cylindrical roller which is the bearing used in the A series engines, they also have a axial (sideways) loading ability, not as much as a ball bearing but I believe enough to cope with the forces involved in the bottom end of our engines.

The bearing in question is a German made FAG 20206T. I have purchased one of these and intend to fit it to my 61' Flash. This engine runs a 357 cam and has 8.5-1 pistons in it. My intention is to have a bottom end that will safely cover 50,000 miles without excessive wear. Hopefully I will have the bearing fitted in about two months time, its still reasonable weather here so I dont want to pull the motor down until winter. Once its in the engine I will add to this post to let everyone know how its going as the miles add up. I am currently doing about 10,000 miles a year on this bike but will make the effort to use it more than the others so I can get more miles up. Once I have done about 10,000 miles I will take the primary cover off and feel the crank for any movement.

Like all these things someone has to have a go to see what happens.
Title: Re: Drive side main bearing
Post by: alanp on 14.04. 2011 08:29
Excellent post Brian. I've heard that on Nortons this was a great update but didn't think about its application for us.
Title: Re: Drive side main bearing
Post by: a10gf on 14.04. 2011 09:13
What we need is a bearing that has the radial load capability of a roller but that holds the crank in place by not allowing end float.

Looking forward to the road test reports. Thanks for an excellent post.
Title: Re: Drive side main bearing
Post by: Ron B on 14.04. 2011 15:58
Wasn't this type of bearing used on the later unit Triumphs? Just wondering
Title: Re: Drive side main bearing
Post by: trevinoz on 14.04. 2011 22:45
Interesting, Brian.
                             The Norton problem was in the Combat engines when the crank flexed, the square ended rollers dug into the race and destroyed the bearing.
The "superblend" is a bearing with slightly barrell shaped rollers which shouldn't dig in with flex.
When I rebuilt my Atlas engine I used NJ306E rollers on both sides, replacing the ball bearing on the timing side with a roller. "E" denoting barrell rollers.
With this set up end float becomes important as the NJ bearings have the inner race on the shaft and the rollers and outer race in the case, the opposite to normal A7/10 practice.
This engine is an extremely responsive unit and has had no problems in 15 years.
Your bearing appears to be a 1 piece item so should hold the crank as long as there is a good fit in the case.
It would probably be a real good job if there was a circlip to retain the bearing.
Go for it, Mate!

Title: Re: Drive side main bearing
Post by: Brian on 15.04. 2011 00:22
G'day Trev,
               the bearing I am going to use is a one piece set up, its the bearing in the picture. It has no lateral movement, how the hell they assemble it I have no idea ! It has taken me months to find this bearing, there are many manufacturers who make dozens of different barrell roller bearings but this is the only one that I think is suitable. The biggest hurdle was finding a trapped, single row in the width we need, 16mm.

I intend to still shim the crank to around .001"-.002" theoretical end float so at the very worst I wont be any worse off than the original roller set up. It will place more stress on the outer race/crankcase fit but as I ran a ball bearing in my plunger model successfully for so long I dont think this will be a issue. I dont believe the lateral forces involved are excessive, just a constant pressure up against the face of the bush which over time wears and increases end float. When the end float gets excessive, .010" or more is when the problems can start. Thats what I believe anyway, I may well be wrong.

This is not a radical modification and does not require any machining etc, just a different bearing that I believe will enhance the life of the bottom end.

Unfortunately I will need about two years to tell if this is any good.
Title: Re: Drive side main bearing
Post by: MG on 15.04. 2011 00:35
That's how they do it, Brian:

The balls/rollers come in first, then the races are centered, the rolling elements are evenly distributed and the cage comes in last. I bet many have been wondering how they get them balls in there, I have been, until I saw it!

Cheers, Markus

P.S.: Good idea with the bearing, it is definitely an improvement over the cylindrical rollers since it will allow some angular misalignment (flexing crank and/or cases, misalignment to t/side bush, etc.)
Title: Re: Drive side main bearing
Post by: Brian on 15.04. 2011 00:52
Thanks Markus,
                     I wondered how they put them together.

I'm not one for radical modifications, I dont think I would ever consider a needle roller conversion or anything like that but I do like to make what I consider to be sensible improvements that enhance the life of our bikes. I am hoping this is one of them. This is not going to make the bike any more reliable or perform any better etc but will hopefully give the crank a easier life so it wont wear so quickly and lessen the risk of a major failure.
Title: Re: Drive side main bearing
Post by: Nourish on 30.01. 2014 11:48
Well - How has it worked out?
Title: Re: Drive side main bearing
Post by: Briz on 30.01. 2014 13:00
Yes, it'd be interesting to hear a follow-up.
It should be mentioned that this is not the same type of bearing as the 'superblend' bearing used in Nortons. The Norton one did allow endfloat.
Title: Re: Drive side main bearing
Post by: Brian on 30.01. 2014 21:44
To cut a long story short Nourish, it didnt !

When I wrote the original article I had spent many hours at my local bearing company sourcing the "correct" bearing. I had lots of e-mails back and forth and was assured by the bearing company and the manufacturer that the bearing I ultimately ordered would have no end (lateral) float. Unfortunately when the bearing arrived it turned out to have about .010" end float so in essence was no advantage over a std roller as it would have needed shimming. The whole point of the exercise was to have a bearing that would not need shimming.

At that time I had my plunger engine on the bench so I ended up fitting a std ball bearing, it has only done about 10,000 miles since I put it together with the ball bearing main.

I am still a firm believer that end float or lateral movement of the crank is the biggest design flaw with the A series engine. I still think for a std engine a ball bearing is a better option than a roller. With a ball the crank is held to the drive side of the engine, no end float. As I said in my original post my other plunger A10 did 47,000 miles on a ball bearing and it wasnt showing any sign of wear when I replaced it.

I think the BSA engineers also thought this as on the later A65's they fitted a thrust washer on the timing side to try to take some of the lateral load. Unfortunately the A65's were worse than A10's and suffered many engine failures. Why BSA stayed with the timing bush on the A65 series is one of lifes great mysteries.

So my thoughts are, in a std engine, one with up to about 7.5-1 compression and a 356 cam I would fit a ball bearing. In any of the more "sporty" engines, Road Rockets etc I would fit  a roller bearing.

One thing I do is I only ever fit one shim behind the bearing. I make a shim the correct thickness.
Title: Re: Drive side main bearing
Post by: wilko on 30.01. 2014 22:31
I suppose if you can find a heavy duty ballbearing with extra balls and deep grooved it would last even longer.