B50 Decompressor

We all know about Oil Leaks on British Motorcycles. One area that can be a problem on the BSA B40, B44 & B50 models is the Valve Spring Decompressor. On these larger capacity single cylinder engines a mechanism was fitted to allow the rider to decompress the exhaust valve when he had turned the engine to the top of its compression stroke. This allows the rider to give a good kick down to start the engine. Believe me on a 500cc single with 10:1 compression it is needed. The problem comes from the fact that the decompressor spindle which is made of steel, turns in a housing made of aluminium and while it is turning it has a side thrust applied to it by the pressure of the exhaust valve. This leads to the hole in the aluminium becoming elongated or oval which eventually allows oil to seep out.

To overcome this problem, you can fit a brass or phosphor bronze bush into the rocker cover which not only restores the hole back to its original size, but also provides a harder wearing surface for the decompressor shaft to turn in and this should prevent the problem from reoccurring for quite a while.

Step 1. The Bush

The bush has an internal diameter of ¼" with 10-15thou clearance for the shaft of the decompressor. It is ¼" long and 7/16" outside diameter, but this can depend on the drill/reamer you use. (Check the diameter of the B40/B44 shaft, but it is likely the same part.)

Note: Make the outside diameter of the bush 12mm if you can not get an imperial reamer.

Bronze Oilite Bush (QT Type) Distributor

Step 2. Remove the rocker cover.

Remove the petrol tank, rocker oil supply, head steady and the nuts holding down the rocker cover. On the B50 you will have to remove the engine as the rocker cover will not come off in situ, but on the B40 & B44 the engine can stay in the frame.

Step 3. Drill out the hole

Drill out the hole to suit the bush outside diameter. I used a reamer to get a more accurate finish. To accomplish this I mounted the rocker cover on a pedestal drill and checked both the vertical & horizontal alignment.

Step 4. Fit the new bush

The bush can now be pulled into place. First clean both the housing and the bush. Apply the appropriate Loctite to the joint faces and then with a nut, bolt and washers pull the bush into the housing until flush with the outside face.

Step 5. Refit the decompressor

Let the loctite set overnight then rebuild the unit using a new o-ring washer, having oiled the bush & shaft.