tech24

A65/A50 Crankshaft Problems and Solutions
by Lynn Bennett and edited by Mike King

Over the last few years there has been a lot of mis-information about
the A65/A50 main bearings published and on the Internet. Some of it
from me, unfortunately. The problem is with the terrible design that
BSA did on the A65 crankshaft that can lead to major destruction of the
engine cases if the crank endplay is not maintained to within the
factory specs. After a recent series of emails on one of the Internet
sites I am convinced that I understand the original design concept, the
failures that have occurred and why they occurred, and perhaps a
solution to part of the problem.

First let’s discuss which years/models are the concern. In the original
A65/A50 design BSA used a ball bearing on the primary side of the
crankshaft and a plain bushing on the timing side. This is not a bad concept. The ball bearing absorbs the vertical (radial) loads of the
rotating crank and also absorbs any horizontal (axial) loads created by
the primary chain misalignment and the rotary pump’s worm gears. It
also locates the crank left to right reducing the endplay to essentially zero. The bush provides an easy solution to getting the oil supply to the rods by passing it through radial holes in the bush and into the timing side of the crankshaft, like all automobiles motors do. In 1966 the ball bearing on the primary side was replaced with a roller bearing to increase that member’s vertical loading capacity but reducing the horizontal load capacity when the crank moves to the right to zero. To absorb the horizontal loading towards the timing side, a thrust washer was placed between the right crank cheek and the right side of the cases. Movement to the left, the primary side, was limited by the edge of the rollers running into a lip on the roller bearing’s outer race. While this sounds kind of OK, that thrust washer was a bronze material and wears over time. With that wear the end float of the crank can increase. When the end float gets too excessive premature wear of the bush is experienced as the crank shucks back and forth rapidly wearing out the bushing. Once the bushing clearance increases to about .003, the oil pressure falls off dramatically and starves the rods for oil. The result is a seized rod bearing and a rod through the
cases. Regular monitoring of the endplay so that it never exceeds 0.003
inches will preclude total destruction. Another common problem is that
the alternator rotor retaining nut gets loose allowing the thin shims to be torn loose. These shims located between the inner race and the crankshaft cheek centralize the crank and set the end clearance between .0015 and .003. The lack of correct shimming increases the endplay with the same results as high mileage on the thrust washer. Recently an internet user found that his old A65, which ran fine before teardown had nearly 0.020 inches (almost seven times the factory specification) of end play with 0.016 inches of wear on the right side thrust washer. and the bushing had .0033 clearance.

So the shims tear and fall out because the alternator rotor was allowed
to loosen at some point in time. The thrust washer on the right side wears due to use but that wear is accelerated by ever increasing endplay that should be monitored by regular checks using a dial indicator. And major damage occurs due to a failure to monitor endplay periodically. This all leads you to believe that the problem is associated mostly with high mileage machines, which is the only kind currently available on the market place. After all, the youngest of this machine is at least 30 years old.

Both the Triumph 650 and the Triumph 500 unit twins (after 1968) solved
the problem by end feeding the oil into the crank and using a roller on
the primary side and a ball on the timing side. The roller bearing gives the high vertical load capacity at the primary side where it is needed and the ball gives adequate vertical loading for the timing side as well as horizontal loading capacity. The whole crankshaft is tightened up against the ball bearing’s inner race, locating the crankshaft left to right and reducing the endplay to zero. This is the perfect solution used by virtually every bike since the 1960’s. But BSA never updated to the better approach so what can we do to help?

We can check the endplay periodically, keep the alternator rotor tight,
replace the shim pack with a single correctly sized spacer or shim, and/or revert to the original ball bearing on the primary side. The expensive solution is to do the SRM modification where the right side of the cases and crankshaft are modified to accept a special combination ball/roller bearing and the crank is end fed its’ oil. Reverting to the original ball/bush approach requires a little extra work as trial assembly is required to determine the correct replacement shim size for the shim pack. In fact, the single shim pack replacement shim will probably have to be made by a machine shop, as it probably should be ground to the correct thickness. The ball bearings inner hole may have to be honed every so slightly to make assembly easier. To do that honing the races and ball area would have to be masked off to avoid getting any of the grinding residues into the ball bearing. That residue is death to any bearing. An alternative approach is to heat the bearings and cases to reduce the interference fit of the ball bearing to crank, allowing minimal force to separate the case/bearing from the crankshaft. The right side thrust washer would ultimately be thrown away after a new one is used to determine the shimming to be used on the primary side (part of trial assembly and used to centralize the crank properly). In this approach the shim on the primary side gets captured between the left crank cheek and the ball bearing’s inner race and all is tighten down with the rotor nut reducing the endplay to zero. The endplay stays at zero, as there is nothing to wear out to increase it other than the ball bearing itself, which normally takes forever. What is the drawback? The vertical load capacity of the ball bearing is 40% lower than the roller bearing it would replace. Worried? No, as this exact same approach was used for several years through 1965 with apparent success. We don’t ride these bikes anymore with the total abandon that we might have when we were young so there should probably not be a problem. I believe BSA changed from the roller to the ball as a shotgun approach to solving an erratic (maverick) spark problem that took years to uncover. Many other changes were made to keep the earlier bikes from seizing, and throwing rods from overheating which was really caused by the maverick spark thanks to a bad ignition cam from Lucas that made a spark during the intake stroke.

I have yet to try this approach of reverting to the earlier ball bearing on the primary side but have every confidence that it will work. Several on the Internet have used it, all claiming great success. I intend to try it on an A65 some time in the future, so tune in for the results. In the interim you can help your A65/A50 survive by monitoring the endplay of the crank periodically and not riding it when that endplay exceeds the 0.003 inches factory specification. Also check your rotor nut for tightness so that the shims cannot fall out.

Balancing;

I believe that you balance for 70% by putting weights on the crank throws that are 70% of the sum of the following items: piston, rings, pins clips and the weight of the small end of the rod. For this last item I have seen them use half of the rods weight complete rod with caps and bolts/nuts or to place the small end of the rod on a scale while the rod is held parallel to the floor. I'll bet you get different numbers between those two methods. BSA used to supply the "correct" weights to put onto the throws based
on using their replacement pistons, rings, etc..

Indeed this is static balancing and does not take into consideration that the crude casting or forging of the crank leads to different weights between the different ends of the crank. In fact in later years BSA took to fully machining the cranks to minimize the dynamic imbalances associated with their old approach.

Static balancing on a big twin beyond that done at the factory is
controversial. Since the Brit twin has no better balance than a big single it can never be perfect without balance shafts (separate shafts turning at engine speed or half engine speed or reverse direction, etc.). If you rebalance the crank shaft to some other factor than 70% you move the major vibration up or down the RPM scale. A change in factor may put the vibration zone right where you ride most of the time, like down the hiway, say at 65 MPH. It is all predicatable with math but few balancer shops have the knowledge to do it. The "correct" factor is usually determind by
experience on the particular motor to be balanced. So you see a auto balance shop can indeed do the 70% stock factor and the static balance will probably be no better than doing nothing. The dynamic balance certainly would be better as it eliminates left to right imbalance as well as radial imbalance. If you are going to bother to balance at all go for the dynamic balance with the stock 70% factor and the vibes will only get better. If there is a particular RPM range that you want to minimize find a BSA specialist who does dynamic balancing and have him do the job. So, I think you have several choices: no balance, dynamic by a BSA twin specialist (good luck finding one), static re-balance to make you feel good
or to compenste for the new pistons having a different weight than the originals. To make sure your bike will be the same as before the rebuild compare the weights of the complete new and old pistons to see if there is any substantial change. If there is at least go for the static balance. Do nothing if the weights are the same. Go for the dynamic balance if you want it better.

Here's some other points I learned while rebuilding another A65 motor with a roller/bushing setup.

(1) Always align bore the bushing using the opposite side bore as the reference. Do not hone it or ream it to fit the crank. This can be done on a jig-boring machine or with a skilled setup on milling machine. It is the only way that the crank will run true. If you don't, you cannot get the right clearance and still have the crank turn freely unless by luck the original case was perfectly true. The result will be rapid wear of the bushing causing loss of oil pressure... And then it is down hill from there.

(2) When measuring endplay of a roller bearing/bushing set up, you must tighten the cases securely and tighten the crank nut that sandwiches the shims and cup between the inner bearing race and the crank cheek. You will find that tightening the crank nut with the spacer, sprocket and rotor in place will change the endplay by several thousandths because when the bearing, shim cup and shims are loose, you cannot get a repeatable accurate end play reading with your dial indicator. The motor runs with the cranknut tight so that is how it should be measured.

(3) As the engine heats up, the aluminum cases expand about twice as fast as the cast iron crank, so any clearance set cold will increase once the motor reaches operating temperature. Note: I actually measured the clearance cold and then heated the assembled cases up and noted that there was about a .0015 increase in the clearance. When I first did it, I saw .003 increase, but that was because the case heats up much more rapidly than the crank in my makeshift oven (hotplate and bar-b-q grill cover). I do not know the temperature, but it was warm enough that you could not hold your hand on there indefinitely.

This knowledge reinforces my trust in a ball bearing set up. You just don't have to worry about that crank moving back and forth or go through that time consuming set up. I took the crankcase halves apart at least 6 times before I was satisfied I had the end clearance right. As long as your not punishing the engine the ball bearing set up is much simpler and easier to get "right"

Finally, I think that with all BSA A65 bottom end problems, the culprit is low oil pressure, and that low oil pressure is the result of that bushing being improperly set up by mechanics but also the original factory setup.
I have seen cases where the bores are misaligned from the factory.
It becomes very obvious whether the bores are in line when you set it up in the jig-boring machine. The reason I think it is all low pressure is this: Once oil pressure drops off, the left side cylinder does not get oiled. The right side does OK because all the oil that is squishing out of the bushing gets splashed up into the cylinder and bearings. That is why they put those spit holes in the left side rod. The rod bearings should see about the same oil pressure as they are fed out of the sludge trap that is a reservoir with
holes drilled to the rod journals. Now if the sludge trap is filled up with sludge, the left side bearing could see the oil pressure drop sooner. Plain bearing motors cannot run at high speed with low oil pressure. They just will not last.

Finally, filter oil well!

Addendum:

The BSA crankshaft problems, that I have pointed out over the last few
years, has turned out to be very controversial. Some here who race the
A65 claim if done correctly the plain bush motors are very durable and
rugged. Others who have high mileage version make the same claim.
Others have reported massive rod failures (an indicator of low or no
oil pressure presumably caused by a loose plain bush on the timing
side) with destroyed cases. Still another racer claims the problem is
incorrectly drilled passages in the pressure regulator part of the oil
system. I don’t really know anything except that from an engineering
point of view and for home engine rebuilding, there is a lot to be said
for an engine with a roller bearing on the primary side and a ball
bearing on the timing side. And add to that end feeding the oil into
the crank shaft. The Triumph 650 has that and the BSA does not. Which
is better????? The SRM style modification attempts to fix the BSA A65
but it is expensive and it pretty well assures the problem, if there is
even a problem at all, will be put to rest. Need the peace of mind then
do the mod. Don’t care, then have fun finding a competent, experienced
BSA machinist to do the plain bush on the timing side as that is what
it will take to get it correct.

Some here assume I believe all A65’s are exploders or are near
explosion. That is not the case as I ride an A65 with the plain bush
and it is fine. I like peace of mind and knowing that the oil pressure
may be low or not breaks that peace of mind for me. I also find it hard
to recommend the A65 to a newby when I know he has no way to determine if the oil pressure is too low or not. I suppose the Triumph could have the same problem but it is less likely,, at least in my mind. So I
hedge on recommending an A65 to anyone except those who just have to
have one because they want one and are willing to fix whatever breaks.
To them I usually recommend they consider the problem and consider the
SRM modification, factoring the SRM modification cost into what they
are willing to pay for a vintage bike. I think being informed is important. There is no better way to kill one’s new found Brit bike hobby then to have the motor to fail catastrophically, especially when no one warned you of the danger, which excluded any pre-emptive action.