The lubrication system is of the dry sump type and is operated by a double gear pump situated in the bottom of the crankcase on the right-hand side. The oil tank capacity is four pints and oil is drawn from the oil tank to the supply pump (top set of gears). It is then pumped past the non-return valve (A), and along the hollow main shaft to the big-end.

After lubricating the engine the oil flows down through a filter to the bottom of the crankcase from which it is drawn by the return pump (lower set of gears) past the non-return oil valve (c), and delivered up the return pipe to the tank. At the junction of the return pipe to the tank a by-pass pipe leads a supply of oil to the rockers, push-rods end, etc.

The valve (A) prevents oil transfer from the tank to the crankcase while the machine is standing, and together with the sludge trap (F), does not require attention until such time as the engine is completely dismantled.

A by-pass valve (D) ensures a constant pressure in the system. Surplus quantities of oil are discharged back into the crankcase.

If the ball valve (C) should be stuck in its seating there will be no return of oil to the tank. In this event remove the cover plate (B) below the pump, insert a piece of wire into the valve orifice and lift the ball off its seating to free it.

On models after 1966 the feed and return pipes are swapped over. The feed pipe is on the left-side and the return is on the right-side. (Opposite to the above diagram)

C15 Crankcase Breather

The breather is mechanically timed and takes the form of a hollow camshaft with a radial drilling which, at the appropriate piston position, is brought in line with a drilled port in the inner timing cover, this port has its outlet inside the outer timing cover. Pressure is then released through a small radial cut-away at the rear end of the outer cover joint face.

C15 Changing the Oil

This should preferably be done immediately after running, so that the oil is warm and will, therefore, flow more freely. Disconnect the oil pipe union nut (A), at the base of the tank and collect the old oil in a suitable receptacle.

Filters

Remove the oil tank and crankcase filters for cleaning at regular intervals, this can be carried out in conjunction with the change of oil. After releasing the oil pipe at (A), unscrew the hexagon plug (B), which carries the filter in the tank, and wash thoroughly in petrol. Make sure that all the petrol has evaporated before replacing. Refill with the correct grade of oil.

The pump filter can be withdrawn after removing the crankcase cover plate and should be thoroughly washed with petrol. dried and replaced. The oil pump is extremely reliable and it is most unlikely that it will give trouble therefore it should not be disturbed unnecessarily. The pump is held in position by three bolts. The two other bolts hold the sections of the pump together.

External Filter

BSA fitted an external cartridge filter on the later B25. You can fit a disposable canister type into the return line which will prolong the condition of the oil and the engine. These are available from most Classic Bike Suppliers.

Sump Magnet

A small magnet placed in the bottom on the sump, which will adhere to the square sump plate, will collect any stray metal particles and help prevent damage. Obtainable from most suppliers or use the one out of an old magnetic cupboard door catch.

Primary Drive

This is lubricated separately from the engine and has its own drain and level screws. To refill, remove the level screw and pour oil in through the clutch adjustment cover until it issues from the level screw hole.

   
Primary Drive                                               Gearbox

Gearbox

The level is gauged by a level tube. To refill or top-up the bottom level screw is removed and oil poured into the filler until it issues from the bottom level screw opening.

Click on picture to enlarge

Oil Types and Capacities

The original oils used were standard engine oils of a fixed viscosity.

• Primary chain oil type:-  SAE 20 (1/4 pint)
• Gearbox oil type:-         SAE 40 (3/4 pint)
• Engine (tank) oil type:-  SAE 40 (4 pints)

These can be replaced by good modern oils as follows:

• Primary chain oil type:-  SAE 20/40   (1/4 pint)
• Gearbox oil type:-         SAE 20/40   (3/4 pint)
• Engine (tank) oil type:-  SAE 20/40    (4 pints)

On the latter models with side points, (after 1965), the Gearbox oil was changed to EP90 and was 1/2 pint. You can use EP90 on pre 65 models but there is the possibility of it seeping along the gear-change and kick-start shafts allowing it to mix with the engine oil and you would consequently need to change your engine oil more regularly.

The EP90 is a gear oil and so is preferable in the gear box, subject to the above comments about oil mixing.

Tip from Jim Downey: When you have rebuilt your engine and start it for the first time, place your finger over the oil return hole in the oil tank for 10 - 20 seconds. This forces some extra oil up to the top end to get it lubricating.