tech29

British Lubrication Systems Explained
by Lynn Bennett

For the period that includes the 1950's, 60's, 70's and 80's most
British bikes used a similar system, only different in the details, the
dry sump lubrication system. The basic concept is that the engine's
crankcase, the sump, is constantly keep empty of lubrication oil. The
way that is accomplished is with two oil pumps: one supplies oil under
pressure to the system of bearings, and another larger pump returns the
oil back to a storage container. The storage container can be an oil
tank mounted on the frame somewhere or closed cavities within the frame
its' self, like the Rickmans and the "oil-in-the-frame" 1970's BSA's
and Triumphs.

The pumps used by BSA and Triumph were of completely different types.
Most Triumphs used a plunger pump where a tightly fitting small piston
moves back and forth helped by miniature ball valves controlling intake
and exhaust of the oil. There were actually two pumps in one body
(pressure pump and return pump). The pumps were driven off an eccentric
pin on one of the cam drive gears. The supply pump was located low in
the crank case, well below the bottom of the oil supply vessel, the oil
tank or frame. At engine off times leakage through the pumps was
minimize by the miniature ball control valves. BSA designs used rotary
pumps that used tightly toleranced rotors that moved the oil from the
intake ports to the output ports much like an American automobile oil
pump. The drive for BSA pumps was directly from the crankshaft end on
the timing side of the motor, using a helical gear system. The BSA
approach required check valves to assure that the oil did not back flow
from the oil tank and fill the crankcase when the engine was off. A
motor that fails to keep the oil from back flowing is said to be "wet
sumping" and , unfortunately, is common regardless of the check valves.

Between Triumph and BSA over the years various means of supplying the
oil to the crankshaft and eventually the big end of the rods, were
used. Two ways dominate: plain bushings crankshaft main bearings, and
end feeding of the crankshaft axially. The oldest way, plain bushings
crankshaft main bearings, was used on early Triumphs, small
displacement BSA singles, and BSA twins. The better way, end feeding of
the crankshaft axially, was used on late Triumphs and most large
displacement BSA singles. This latter way usually means ball and/or
roller bearings on both ends of the crankshaft for main bearings.

A circuit description:

The oil flowed by gravity from the oil tank to the intake port of the
oil pump where the pump (either type) moved it under pressure to the
pressure regulator. Various styles of pressure regulator were used over
the years but the concept of operation was the same: oil pushes a
piston or ball against a calibrated spring until the moving piston or
ball uncovers an escape port. That amount of pressure to get to that
point was the system operating pressure maximum. Excess oil was drained
back to the sump bottom, while the working oil was routed to the
crankshaft. For the plain main bushing motors the pressurized oil was
routed in the engine castings through a radial set of holes in the
crankshaft main bearing bushing to a set of holes on the bushing
surface of the crankshaft. For the ball/roller crankshafts the oil was
routed in the engine case castings to the end of the crankshaft. There,
an oil sealed cavity, that the crankshaft end poked into, was
pressurized allowing oil to flow into the crankshaft axially drilled
end. The crankshaft included drilled passageways that routed the oil
first to a cavity in the rod throw on the crankshaft. In that cavity
was a tube, drilled such that centrifugal force separated the sludge
part of the oil from the rest of the oil. This device was the sludge
trap which should be cleaned at every engine rebuild. The drilled
passages lead to each crankshaft throw and consequently to the big end
rod bearings, which could be plain bushings or roller bearings. The
pressure was such that it leaked from the rod ends, creating a mist or
fog, covering the cylinder walls and other exposed surfaces in the sump
area. Later BSA twins included a drilled passage in the big end of the
left rod that squirted oil onto the cylinder wall. All of this left the
once pressurize oil in the bottom of the sump where the pick tube of
the return pump was positioned.

The return pump began by sucking the oil from the sump cavity and
delivering it back to the oil tank. From the pipe leading from the
outlet of the return pump, a tap was taken to lubricate the engine's
top end. The tap was so sized and positioned such that a majority of
the oil flowed back to the tank rather than to the top end. This bypass
pipe delivered oil to the head on BSA twins or directly to the rocker
arm shafts on Triumphs and BSA singles. Somewhere in the head or rocker
shafts passageways some sort of restriction was situated so as to limit
the quantity of oil delivered to the rockers. This oil supply was also
routed to the tips of the rockers through drillways to provide
lubrication for the push rods and the valve tips. The escaping oil
flowed into the head cavity, collected, and then flowed down to the
tappets and eventually to the camshaft lobes (by drip). The oil was
collected there in some models by a trough where the cam lobes dipped
as they rotated. As the oil was thrown off the cams it collected in the
sump to be returned to the oil tank by the return pump.

Crankcase Ventilation:

Since the pistons move up and down changing the volume under the
pistons and in the crankcase (sump), pressure can build. The solution
is to vent the sump. A simple vent has the problem of pulling air in
when the pistons rise, along with dust, bugs, and anything else in the
vicinity of the vent tube. The first British solution was to block the
crankcase vent with a plate attached to the end of the cam timed such
that the vent is only un-blocked as the pistons fall (increased
pressure). Later, post 1970, BSA and Triumph used a different approach.
Holes were drilled at the appropriate height in the crankcase wall
separating the primary chain case from the crankshaft cavity. The
primary used the engine oil supply (after an initial filling at engine
oil change) and the holes allowed the under piston pressure to bleed
into the large primary chain case cavity. The primary was fitted with
an open vent including an oil separator so that only fumes left the
engine. The change in pressure in the combined crankcase cavity and
primary chain case cavity was enough to move the fumes out but not to
pull contaminant laden air back in. The hose for this vent was routed
all the way to the rear of the bike to further increase the volume.

Problems:

Several problems with this lubrication system have been found through
the years. On plain bearing crankshafts the bearings must be tight and
the endplay sufficiently small as to preclude masking of the oil
transfer holes in the crankshaft. The system lacked effective
filtering. Some models did include a filter element (cartridge type)
installed in the oil return line but most did not. This mandates
changing the oil regularly to get out any oil contaminates generated by
operation of the engine. You cannot overdue the changing of the oil. I
like to do mine every 1000 miles or 6 months, whichever comes first.
Kits are available to retrofit these filters (some are based on the
Norton filter) but unless installed right a motor can be damaged, as
per Internet problems discussions that I have seen.

Oils:

Oil type is always an issue. I believe these motors were designed to
use 40 wt. oil in the cold climate of England. In Southern California
our weather year round is more like their summers. Consequently, I
prefer to use 50 wt. oil, or at least multigrade oils that go up to 50
wt. I also believe high detergency oil is detrimental without an
adequate oil filter to collect the sludge rather than simply circulate
it. Racing oil tends to have lower levels of detergent and have not
been modified to EPA standards of "energy conservation", which have
been said to do less than an adequate job of lubrication in air cooled
motorcycles.

Conclusion:

Well, there you have it. Keep your plain bushing motors tight and
change your oil frequently.