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.
