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Oil Pump


The oil delivery into the oil tank at a fast tickover on my 1956 Dominator 99 is a continuous full stream. The feed to the engine is fitted with an oil tap so there should be minimal oil in the sump on start up but the oil return to the tank is instantaneous and is a continuous jet. Does this indicate that the pump has been upgraded to a higher delivery type ? I understood that the standard pump gives a dribble return after start up.

I have been having problems with oil leaking passed the oil tank cap during a run and whilst I have replaced the cork washers/gasket I am beginning to wonder if the high volume oil return is contributing to the leakage. 




Interesting George.  As the return gears are larger than the supply the return to the tank would normally contain bubbles once any wet sumping has been cleared.  I suppose its possible that if the return side of the pump was very badly damaged or worn from eating engine debris  then the supply and return would be more matched and the return would only just manage to keep pace,hence  no bubbles.  I do like a detective story!. I think you should investigate . If only to satisfy my curiosity.


There should never be a continous stream of oil being returned to the tank except for a certain time after starting a bike that has been standing for a couple of days or more.

After clearing the accumulated oil in the sump, the oil return to the tank should be in spurts and dribbles.



If the flow and return rates are similar then its also likely that you could get a build up of oil in the engine which would make for a messy bike as excess oil will find a way out through the breather and leaks.

Thanks for the responses . Am I correct in thinking that if the pump is an uprated unit then the larger gear on the engine feed will obviously feed  oil at an increased rate to the engine and therefore in turn a greater amount will drain back into the sump resulting in the necessity for the return feed to work overtime so to speak to keep pace with the increased amount of oil returning to the sump ?

Or am I missing a trick here ?  I guess that I should remove the pump and check it out.









An uprated pump both feeds and returns more oil.  Usually too much for the motor unless other modifications are done. The double speed driving gear set does much the same to an old pump and will also wear it out twice as fast. Not a lot wrong with the original set up if in good order. The exceptions are the engine breather that can make a bit of a mess if the engine is not in good order ,wet sumping if out of regular use and sometimes  not enough supply to the rockers (which can be improved with a little work to the tank union.


A few other points of interest worth a mention. If an larger capacity oil pump and double speed gears have been fitted to a 99 motor then the system is actually over-pressurised.  On the smaller 1.50" crank journal engines a larger capacity oil pump or double speed gears is fine but not both. You only really need the full up-rated set-up for crankshafts with the larger journals and conrods with cylinder barrel oiling holes.

If the full up-rated gear is in place then the pressure release valve will be firing off and flooding the timing cover with oil. This excess is then constantly draining back into the sump before being returned to the tank. In between, worn return gears or a scored chamber in the pump will not allowing a high enough flow to completely clear the sump.

Another possibility is the fitting of an oil filter in the return side of the system. The tubing can expand with pressure pulses and this can even out the return flow to the tank.


I have removed the oil pump and it looks to be a standard pump. I have stripped it and can find nothing wrong . There is 0.003" end clearance on the face of the gears relative to the pump body and perhaps this needs to be reduced by lapping the pump body face although perhaps it is not too excessive. 

I can find no reason as to why after running the engine for two or three minutes the oil return does not reduce to a trickle into the oil tank.



The pump should have a stiffness in it after the clearances have been removed but free up quickly if oil is introduced. I run the pump up on a lathe with oil fed to the open ports for a few minutes and they free up really quickly so not really needed but I still do it.


I'm told that oil feed to the head is taken from the return line on earlier Dommies. If too much goes up instead of to the tank, maybe the sump will never be clear of oil so that the return pump starts sucking air.

The 3 thou axial pump axle play seems to me being degrading pump capacity, it will be interesting to know if fixing it solves your problem.


If someone has fitted a jubilee tank fitting  to "improve head oiling " this can send too much oil back to the engine .The fitting can be easily removed without draining the tank and drilled out bigger to nearly the original dommy size  to achieve a balance  , try drilling out in small steps.


The tank fitting is screwed into the oil tank through the rocker feed banjo and takes the rubber oil return pipe from the engine.As it screws into the tank internal return  the oil cannot drain from the tank if the fitting is removed .The jubilee tank fitting looks identical to a dommy one but has a smaller internal bore ,thus forcing more oil up the rocker feed banjo.  

I have been suffering from slight over oiling to the head and we suspected that maybe a Jubilee fitting (oil return adaptor) had been used on my oil tank - so I ordered the correct new part no. 19380 and did a comparison.  The correct new part has exactly the same internal diameter as my old one but the cross drilling (for the banjo head feed) is a very slightly larger bore and therefore will increase the flow to the head, not decrease it as I desire.  So I believe my oil tank adaptor is original.

I have considered your advice Robert about drilling out the main return bore to increase the flow back to the tank rather than the head, but the internal bore of the return adaptor is much larger than the hole in the return pipe inside the tank.  That makes the hole in the tank the restrictor and not the adaptor itself.   Now, if only I could get a drill inside the oil tank.



You are probably right as I have not found a Jubilee fitting but modified my Dommy one to measurements I have been given . I then had to experiment with the sizes to balance the oil flow. It is also possible that either of your 2 fittings was made  (pattern) by contractors and  are meant to be the same!.  Its also possible that the return pipe in your tank has a restriction it should not have ,bit of solder,kink or flattened at the top outlet.


Back in the 60s when pressure feed to both camshaft and rockers became a good idea that was initially used by the race boys, they were all hit by over-oiling of the cylinder head. This being pre-plain rocker spindle times. The solution was to fit modified carb main jets into the oil feed system to control the flow rate..


Maybe just a silly thought, but, was the engine run, until warm, before topping up the oil tank?  If not, wet sumping may have already occurred, and, there will be too much oil in the system.


Not all of us!  I am quite happy, with a 65 year old Dominator.  A few minor mods over the years,but, if I could afford it, I would return it to it's original spec.  


Right at the beginning of this diatribe was the observation that oil was leaking past  the oil tank cap-maybe as inferred this was coming from under the cap but I have seen a great number of oil tank caps with a small breather hole. This shouldn't be as the tank has it own breather and of course you don't want oil coming out the top anyway.

I have reduced the oil leak from the cap by making a new gasket and ensuring that the top of the tank is now absolutely flat. There is no breather hole in the cap. However, I am still scratching my head as to why the rate of the oil return flow into the tank is high even after the bike has warmed up. 

It has been suggested to me that I should investigate the oil pressure relief valve as if for some unlikely reason it is failing then excessive oil would find its way into the sump. I will check this. 




... that that would be the case. The amount of oil that gets into the crankcase is the same whether it goes round the big ends, main bearings etc or bypasses them via the pressure release valve. It all ends up there eventually and is merely a function of the amount passed by the feed side of the pump.


Do not over think this one, the one and only way the pump can continually pump oil back to the tank is if the feed side of the pump is directly and solely connected to the return side. It can't do anything else. Even if the oil pump feed seal was missing, the return side turning at the same speed would soon return the excess volume back to the tank. 

If the PRV was jammed open, the pump would still deliver a considerable amount of oil to keep the engine safe. 

It is not that time consuming to drain all the oil, remove the timing cover, check everything and start again. It is the quickest and easiest way of resolving this problem and understanding what is happening. 

Thank you Ashley and others. This evening I am going to remove the timing cover and check the pressure relief valve. I have already replaced the oil pump compression seal. If all is ok then I will keep my eye on oil levels etc and use the bike. 

Yesterday the run to and from the the Crich Tramway museum and Classic bike show of only approx 20 miles was  trouble free but on my return home the oil was returning a plentiful supply of oil back into the tank.




The flow rate provided by the small feed gears to the engine and by the large return gears to the pump is and must be identical.  The only difference is that the return flow has air bubbles in it.  the size of the bubbles will presumably depend at least partly on the thickness of the oil.  The return flow should be a continuous stream (albeit with bubbles creating short staccato gaps in the flow), and it is very definitely NOT a 'dribble'.  There should be a 'plentiful supply of oil back to the tank', and the return flow should start almost immediately since there will always be some oil in the sump after stopping the engine. 

If you cannot see the air bubbles, I wonder if the outlet nozzle within the tank is a bit damaged, so the return is a bit restricted, and the air is more nearly atomised into a continuous frothy flow?  Have you had a look inside someone else's oil tank to see how theirs behaves?


I suspect yours is perfectly normal.

On stripping  the pressure relief valve I found that the actual valve poppet was partly jammed in the bore and it took a lot of fiddling and ferreting to remove it. After polishing it and making sure that it was a sliding fit it should now operate as normal assuming that the spring is the correct one.

I will send off for a new spring assuming that I can get the correct one for the 1956 Dominator 99 from the usual suppliers.

However, after changing the oil and firing the bike up this morning I did not detect any discernible change in the flow rate return but there are air bubbles  on the oil surface. The flow is continuous and not staccato. So perhaps after all I have been chasing hares all along ! 

I am on a steep learning curve with my 99 and thank you all again for your constructive comments.



I quote from the original Norton manual (88 & 99 1951-62) :-

Engine Oil Pump.

This is of the gear type.  The pump contains two pairs of gears, one on the feed side and the other on the return side.

The gears on the return side are twice the width of those on the feed, having twice the pumping capacity.  This ensures that the crankcase is free from oil when the engine is running.

To check the return of oil to the tank, remove the oil filler cap.  The oil return pipe can then be seen.   After the engine has been running a few minutes, the oil return flow will be spasmodic, due to the greater capacity of the return gears.

Oil Level.

The oil level in the oil tank should not be above three-quarters and not below half. 

If the level is above the three-quarter mark, when the engine is running, the pressure built up in the oil tank, by the oil return side of the pump, may force the surplus oil through the air release pipe on to the road.

Always run engine for a few minutes before checking oil level.  It is possible, when an engine has been idle for any length of time, for the oil to syphon through the return gears to the sump.

When this happens, all the oil is returned to the tank in the first few minutes the engine is running.

When the oil level is below the half full mark there is such a small quantity of oil that it tends to overheat.


I appreciate that many bikes do not have oil level marks, on the tank, and, others may have modified breather systems.  However, the above is how Norton designed the system to work.  

If there is a constant flow, from the return to the oil tank, the pump cannot be emptying the sump.



Surely the function of the return side of the pump is to return oil to the tank and keep the level in the crankcase low. If there is a problem with the return oil will accumulate in the engine and the level in the tank will fall. The easiest way to test is measure the level in the tank after a run, then measure it again after another run. If it is fairly constant (consistent with the magnitude of the leaks.....) then surely all is well. If the level falls, and there is a lot in the crankcase immediately after a run, there is a problem.

I have 2 heavy twins, one spurts and the other is a more consistent flow but both maintain the level in the oil tank at a consistent level.

At least if oil is coming back after a run you know some is going in...…...


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