To remove the pump from the engine, first disconnect the three oil lines and the pump control cable. To reduce the chance of damaging the threads in the pump body, I suggest that you just pull the oil lines off leaving the banjo unions and check valves undisturbed. (Remember you will have to clamp or plug the end of the inlet line to avoid draining the oil tank contents).
If you decide to unscrew the union bolts, first check which of three types of pump union you have. Early 175 pumps have plain alloy banjo unions on the outlet ports. Take care when unscrewing these as they may have loose ball and spring check valves inside the union bolt, which are easy to lose. Later 175 pumps and all 200 pumps had the check valves contained in large, one piece brass unions which come off complete after the bolts are removed. Note that these brass check valve union assemblies can be fitted to the earlier 175 pumps and are probably better idea, but make sure you discard the loose ball and springs in the union bolts if you do this. Two check valves in an the output line will seriously restrict the oil flow.
Remove the two 5mm screws holding the pump body to the pump gearcase, and then remove the four 6mm screws holding the gearcase to the disc valve cover (you will need to turn the pump body to one side for access to two of these screws). Lift the oil pump and gearcase assembly away from the engine, taking care not to lose the two 6mm locating dowels or damage the joint gasket. Be prepared for a small amount of oil spillage as the gearcase joint is broken.
Turn the pump over and remove the 10mm nut in the centre of the pump drive gear. The drive gear can be held from turning by a flat blade carefully wedged between a gear tooth and the gearcase. Pull off the drive gear and remove the Woodruff key from its keyway in the drive shaft. The oil pump gearcase can then be lifted away.
Below the Drive Shaft cover on the front face of the pump is a small bolt with a 10mm hex head - this is the plunger stop pin. Remove this (Note:- it is a LEFT HAND thread), and then unscrew and remove the large brass hexagonal cap on top of the pump. Take care not to lose or damage the spring and plunger inside the cap.
Remove the triangular plate on the bottom of the pump (three 4mm screws) to expose the plunger operating cam. Before proceeding further, take careful note of the position of the operating cam lobe in relation to the pump operating lever. The cam can be installed in either of two positions 180 degrees apart, and the wrong position gives maximum oil flow at idle, decreasing as the throttle is opened - a sure recipe for oiled plugs followed by a seized engine. If necessary, mark the cam to ensure that it will be positioned correctly on reassembly.
Drive out the roll pin in the centre of the cam using a nail or the shank of a small pop rivet as a drift, then remove the pump control lever retaining nut and lift away the lever, complete with its return spring. On the 175. there is a single "e" clip holding the camshaft into the pump, located against the inside face of the pump body casting. Hook out this “e” clip and then pull the camshaft out of the pump and lift out the cam. On the 200, the pump cam shaft is located by a tang on the bottom cover, which you have already removed, so the shaft will again just pull straight out.
On the front face of the 175 pump, is a diamond shaped cover held by two 4mm screws. Remove the retaining screws and you can push the drive shaft and cover out of the pump body towards the front, and then push the main pump plunger out through the top of the pump body.
The 200 pump doesn’t have the removable cover over the end of the drive shaft, and I can’t see any way to remove the drive shaft and pump plunger without the risk of damage, so this is far as you can go with the 200 pump.
Check for damage or scoring on the pump plunger and on the internal bore of the pump body. Also check the plunger stop pin for wear on its tip, where it bears against the cam form on the base of the plunger.
Examine the four "O"-ring seals - one on the brass top cap, one on the drive shaft cover, one under the triangular bottom plate, and one on the cam shaft. The latter is the one that most often gives trouble, and a smear of silicone RTV gasket sealant here on reassembly is a good idea.
If you are attempting to build one good pump from two damaged ones, most ancillary components can be swapped around quite freely, but beware when swapping the main plunger and its driveshaft. The gear ratio between these affects the speed of the pump and therefore its oil output rate.
Factory parts lists show different numbers for the externally identical BS175 and BS200 oil pumps, which I suspect have different driveshaft/plunger gearing. To be on the safe side, only use a matched plunger and driveshaft set that you know originated from the correct BS model.
Before reassembly, ensure that all internal pump components are scrupulously clean and primed with fresh 2 T oil. Assembly is simply the reverse of the stripdown, but be careful not to overtighten any of the screws or bolts that thread into the pump body. The oil pump body is cast in a soft zinc alloy and threads are easily stripped.
Replace the plunger stop pin, and then refit main plunger from the top of the pump body. Place the cam into the bottom of the pump with the cam form itself on the control lever side of the pump. Slide the camshaft back into the pump and through the centre of the cam. Replace the camshaft retaining “e” clip, then refit the pump control lever and its return spring and firmly tighten the lever retaining nut.
With the pump control lever in the closed throttle position, turn the cam so that the roll pin holes in the cam and camshaft line up and the high point of the cam form is pointing towards the top of the pump, thus lifting the main pump plunger. (This will provide minimum plunger stroke at idle, increasing as the throttle is opened). When you are sure that the cam is in the right position, replace the roll pin, driving in so that it protrudes equally on either side of the cam shoulder. Check that the pump control lever moves freely through its full range and returns to the throttle closed position when released.
Replace the pump driveshaft and its cover followed by the brass top cap with its plunger and spring. Then turn the pump upside down, fill the lower chamber with oil and refit the triangular bottom cover. Slide the pump gearcase casting back onto the pump spigot and replace the woodruff key into its slot in the driveshaft. Refit the pump drive gear and tighten the retaining nut, using a flat blade again to stop the gear from turning. Refit the pump assembly to the engine and firmly tighten all the retaining screws.
Prime both outlet lines with fresh oil from a pressure can and then reconnect the inlet and outlet oil lines to the pump unions. Re-attach the pump control cable, not forgetting to bend over the small tag holding the cable end to the pump control lever (you don't want it to come adrift in use), and adjust the pump cable so that there is 3 mm gap between the pump control lever and its stop pin at full throttle.
If you have an early model BS175 with a level plug in the oil pump gearcase, remove the level plug and the upper gearcase fitting screw. Fill the case with 10W/40 oil via the gearcase fitting screw hole until it runs out of the level plug hole. Later engines splash feed the pump drive gears via oilways from the gearbox and have no level plug in the oil pump gearcase. It is, however, still a good idea to prime the gearcase chamber in the same manner before running the engine to prevent the drive gears running dry on initial start up.
Finally, start the engine and allow it to idle. Hold the pump control lever fully open and check that oil is being pumped through both outlet lines. (This task is easier if you replace the original black outlet lines with transparent tubing.) You should see signs of oil and air bubbles moving in both lines within a few seconds. If no pumping action is apparent, stop the engine immediately and investigate the cause. Otherwise, continue to bleed the system by holding the pump control fully open with the engine running at a fast idle until the outlet lines are free of air bubbles.
Once the system has been bled, take the machine for a short test run, inspect the oil pump for any obvious oil leaks and re-check the oil outlet lines for air bubbles. Any air pockets remaining after the pump has been re assembled and bled should have cleared after a few miles of normal running. If bubbles continue to appear in the oil lines, it is likely that you have an air leak in the system somewhere. A minor leak may not be enough to allow oil to escape when the bike is parked, but it can allow air to be drawn into the system when the pump is operating. The union bolts need to be firmly tightened - but don’t overdo it and strip the threads. If this doesn't cure the problem, examine the oil “banjo” unions, washers and fitting bolts for cracks or other damage, and replace any that are suspect. The oil lines also must be free from visible splits and a good fit on the banjo union spigots. If in doubt, fit new oil lines and add a smear of silicone gasket around the spigots - but make sure none gets inside the oil lines or you may clog the oilways and wreck your engine.
