tech31

A50/65 Crankshafts and Bearings
Lynn Bennett

There has been a lot of chatter of late, both vocal and electronic
about the "bad" crankshaft design of the BSA A50 and A65. I am partly
to blame as I frequently point out to anyone who will listen about the
very poor crankshaft bearing design of those motors. Unfortunately, not
all talking or writing has it quite right, so here is a short tutorial
on British crankshaft designs, at least as much as I know.

Intro:

The component choices a designer has for crankshafts bearings are as
follows: plain bushings (either two piece shell or full circular), ball
bearings, roller bearings (of which needle bearing are a special case),
or a very special barrel shaped roller bearings (as used on some
Nortons).

Plain bushings are the prevalent ones on modern motorcycles (and
autos). They rely on copious quantities of highly filtered oil. The
shaft actually rides on the oil not the bearing itself except at
startup when the oil pressure is virtually zero. They are made of
bronze, aluminum, brass, and similar other metals and coatings,
sometimes backed by a steel base. With clean oil and adequate oil
pressure they last for virtually the life of the motorcycle.

Ball bearings are made up of several round steel balls riding on inner
and outer steel races. They are so designed as to make the balls
captive and to keep the balls separated to avoid crowding all to one
side under a load via a cage. They handle both axial and radial loads
meaning both the load of the moving piston and the end to end motion
cause by bevel drive oil pumps and out of alignment primary drives. The
application of the load for a ball bearing is the sum of the individual
point contacts of all of the balls on the races.

Roller bearing are usually made up of several cylindrical rollers on
inner and outer steel races. Some applications have the rollers running
directly on the hardened shaft without the inner race. They are so
designed as to make the rollers captive and to keep the rollers
separated to avid crowding all to one side under a load via a cage.
They handle only radial loads meaning the load of the moving piston but
not any end to end axial loads. The application of the load for a
roller bearing is the sum of the individual line contacts of all of the
rollers on the races, which for a given size makes the roller bearing
much more capable of handling radial loads than the equivalent sized
ball bearing. But they have zero capability for end to end loads, axial
loads. Tapered roller bearing, not used on crankshafts but in steering
crowns, have the tapered cylindrical rollers mounted at an angle to get
load caring capability in both axial and radial directions. Needle
bearings are just roller bearings with long skinny rollers, usually
designed to run directly on the shaft and in the bore, sans races but
with a cage, so to speak. Needle bearings used in transmissions usually
run directly on the shaft but have a thin sheet metal outer race
without a cage.

There is also a special roller bearing that uses barrel shaped rollers
that allow the shaft some ability to not be absolutely true to the
bearing. This happens when the centrifugal loads of a heavy crankshaft
at higher RPMs cause the crank to flex, in effect bend. In standard
roller bearings this would cause the rollers to bind on one end making
the uniform line load on each roller be heavier on one end than the
other. This can cause the bearing to drag on its rim of the race that
keep the rollers confined. These bearing are difficult to find and very
expensive.

Combination bearing designs for crankshafts:

Several combinations of the above bearings have been used over the
years. The most expensive but the one requiring the least lubrication
is all roller/ball. Most two strokes and some older big singles use
this approach. The big end of the rod is a roller while one main crank
bearing is a roller and the other end is one or more ball bearings.
Minimal lubrication is required and is usually done by splash (oil in
the gas for two strokes, which have the air/fuel mixture temporarily
stored in the crankcase). This, of course, requires that the crankshaft
be built up rather than a one piece forging.

The earlier British twins used a ball bearing on the primary side with
a plain bushing on the rod and on the timing side. This approach allows
the pressurized oil to be fed through radial holes in the bushing and
through matching holes in the end cylindrical surface of the
crankshaft. The crankshaft has drillways that route the oil through a
centrifugal sludge trap and on to the rod journals which are plain
bushings. This system works because the main crank bush gets full oil
pressure oil all the time as do the rod big end bearings. The axial
loads are handled by the ball bearing on the primary side. The
crankshaft is mechanically restrained to eliminate all endplay of the
crankshaft assembly. The inner ball bearing race is captured between
the cheek of the crank and the primary sprocket (the alternator rotor
and associated washers are just spacers), hence no end play.

Later British bikes including Triumphs use a roller bearing on the
primary side and a ball bearing on the timing side. The radial loads
are handled by the roller and ball bearing while the axial loads are
handled by the ball bearing. Again, the inner ball bearing race is
captured between the cheek of the crank and the timing gear, hence no
end play. The oil for the plain bushings on the rods is fed axially
through the timing side end of the crankshaft. A small cavity on that
case side, fed by the oil pump and confined by an oil seal, provides
the transport mechanism for the oil into the crank, hence termed end
feed. The roller and ball bearings are splash lubricated by excess oil
thrown from the rod big end plain bearing and sometimes from drilled
holes in the rod big end that "jet" the oil to a needed point inside
the motor.

But the BSA A50/A65 motor, except for the very first years which used
the ball/plain bushing method, uses a roller on the primary side and a
plain bush on the timing side. It can be seen that there is no
allowance made for axial end to end loads. BSA left the crank to kind
of float back and forth. To limit the float they used shims between the
rollers inner race and the crankshaft cheek. To make matters worse
those shims are paper thin and can be torn, leaving the crank to wonder
back and forth outside of the 0.000 to 0.002 inch design limits. Such
banging back and forth causes excessive wear on the vertical face of
the plain bushing further increasing the clearance for end play. The
end result is that the oil transfer holes in the crankshaft and the
main plain bushing get masked and the oil supply to the rods is
reduced. This leads to rod failures. It has been said that most BSA
A50/A65 rod failures are caused by a worn out, too loose, main plain
bush but I don't believe that is true. The end play problem is probably
the real culprit since other motors that use the ball/plain bush
approach don't suffer the A50/A65 high rod failure reputation. The best
approach I have heard so far, without resorting to the expensive SRM
roller/ball conversion, is to use what I believe is termed "arbor"
shims instead of the pack of super thin BSA shims. BSA's shim is a pack
made up of a combination of several thin shims to get to the total shim
thickness required. The arbor shim is a single thicker shim determined
by trial and error assembly of the bottom end. This approach probably
precludes the shims from tearing and falling out. And the end float or
play needs to be as close to 0.000 inches as possible without being
negative. That is to say absolute minimal end play. Perhaps the reason
the thin shims tear is that the nut, that holds the rotor of the
alternator on, gets loose so it is most important to insure that the
alternator nut is tight, and stays tight.

Conclusion:

Each bearing type has its strengths and weakness. The trick is to
combine them to make the crankshaft design both strong and durable.
Several crankshaft bearing designs have been used through the years on
British bikes. For downright strength the all ball/roller is the best
approach. For the regular street bikes the ball/plain bush approach is
adequate. The least desirable is the approach BSA used in the A50/A65.
In original all BSA parts form, it is a disaster waiting to happen but
the single shim modification probably removes the disaster part. If
more A50/A65's were totally rebuilt before being put into vintage
service using the above arbor method or reverting to the original ball
bearing and plain bush of the earlier bikes, I am pretty sure that the
bad reputation of those motors would go away. This bad reputation can
cost a BSA seller up to $1000 compared to an equivalent Triumph. I hope
this helps more people understand the A50/65 crankshaft/bearing
designs.

Postscript:

When replacing the plain bushing on any motor so equip it is imperative
to get the shaft to bush tolerance correct. Too tight and the bushing
will seize to the shaft and spin in the case bore causing the oil holes
in the bush to misalign, and too loose and the oil pressure leaks out.
Full circle press-in bushes undergo "crush". That is, their inside
diameter reduces after they are press fit into a bore. To regain the
clearances needed the bush has to be line reamed to size. Line reaming
uses a reamer installed on a tooling fixture that uses the bearing
(ball or roller) on the other side of the case as a pilot or guide.
Using a worn out ball or roller bearing in the other side case will
compromise the quality of the line reaming job. In searching for the
answer of how to do a BSA A50/A65 plain main bush replacement over the
last five years I found a complete lack of knowledge at many of the
vintage dealers in the greater LA area and a reluctance to even do the
job. That is, until I talked with my friend Keith Moore at Moore Cycle
Center. He has the answers. (714)447-4402