There are two different types of BSA oil pumps, type 12T and type 13T.
12T refers to the number of teeth on the pump gears. These have a
different tooth form to the 13T type and should not be
If you haven't taken the pump apart yet, simply remove the two small hexagon headed bolts. They hold the assembly together.
It will then readily come apart and you will be left with the steel base plate (with captive ball),the pump body, the alloy drive end plate and the four pump gears. Those are retained into the pump body only by the end plates.
You will see there are two narrow gears and two wide ones. The narrower gears are the feed gears and the wider ones the return gears. The latter have a larger pumping capacity and ensure the crankcase is scavenged.
Firstly the spindles on the gears that fit into the pump body (return gears) and the spindles that go through the alloy end plate (feed gears) should be a free fit and not restrict the gears rotation.
If they are tight ease the holes carefully with a half round needle file or a piece of fine emery paper wrapped around a suitable piece of rod.
Try each gear individually. When they are all moving freely clean everything off, reassemble the pump and tighten the two bolts fully.
The pump should be free to turn by hand. If it is tight strip it again and remove all the gears.
Tightness is caused by deterioration of the Mazac pump housing and end plate. When Mazac deteriorates it tends to expand putting pressure on the gears when the bolts are tightened.
First place the alloy end plate on some fine emery, on a flat surface and lightly emery it to remove any high spots.
Next take the feed gear that has the pump drive spindle on it, apply some fine grinding paste to the lower face and lap the gear into the pump body in its normal position until the recess in the body has a flat surface.
Repeat the process with the other three gear recesses. When that is complete thoroughly clean all the parts and reassemble the pump fully. Try turning it again. If it is still tight repeat the lapping process, looking for 'witness marks' caused by the gears contacting the pump body to indicate high spots.
When the pump turns freely with the bolts tightened next fit two 1/4" bolts through the holes that the pump mounting bolts fit through. Put nuts on the other end and tighten both nuts. This simulates bolting the pump into the engine.
Check the pump again for free rotation. If it now goes tight you will have to do some more lapping!. If it turns freely it is ready for use.
When you build an engine, after heating the cases to fit bearings etc. the next job is to fit the pump and pump drive shaft. This allows you to check the pump is free by turning the upper end of the drive shaft with your fingers.
A tight pump will lead to premature wear of the oil pump drive spindle and crankshaft pinion gears so should be avoided.
I have yet to have a BSA pump that doesn't pump. That is never an issue, but having lapped in scores of them I just regard it as another patience test each time!