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Too much oil in sump


Can anyone help me with this please:

My 57 Dominator 99 seems to have too much oil in the sump.

The return to the tank is a constant heavy flow and the force is enough to blow oil out of the filler if the cap is removed and even though a new cork seal has been fitted to the cap it leaks badly.

The cylinder bores are good and have been lightly honed and new rings fitted.

i seem to lose a fair amount of oil out if the crankcase breather.

all this seems to indicate that there is too much oil getting to the sump

Any ideas 



Is it a new event on a bike that used to be OK, or a new bike? Or has some major work just been completed? It sounds (to me) on first reading like an assembly issue.

Hi David

I bought the bike a year ago and understand that it had been stood for a very long time. I did a fair amount of work to re-commission it such as draining the oil, removing and cleaning the oil tank, replaced oil pipe hoses, plugs leads etc etc. It wet sumped after just a few hours of standing and so I fitted a manual valve to the oil feed line.

When I got the bike there was a long piece of plastic tube fitted to the breather tube which almost touched the ground I'm guessing that the previous owner had problems. I covered about 500 miles last year and the bike ran reasonably well but did blow out a fair amount of oil from the breather. On a fairly fast run on a warm day it began to nip up and I had to pull over and let it cool.

it got me home and a few days later I removed the head and barrels.      The LH piston was scuffed and the rings were stuck in the grooves.

I also found that one of the valve guides was very loose in the head. The head has now had new valves & guides fitted.

the cylinder bores were in reasonably good condition and have been lightly honed to glaze bust them. I managed through the forum to find a NOS set of rings to fit the Hepolite pistons.

On dismantling I've found that this engine has been breathed on it has Atlas inlet valves fitted, polished rockers, gas flowed ports, hi-comp pistons and looking at the cam I think that is a bit special.

I also took the opportunity to remove the timing cover and overhaul the oil pump. When I removed the pump there wasn't a gasket fitted between the pump and the crankcase. (There is now). I also replaced the pump to outer case seal. I also removed the oil over pressure valveand cleaned/inspected it.

Does that throw any more light on the issue?

Many Thanks

Terry Guy









What you are describing is an over-abundance of oil.  Yet the partial seizure, scuffed bore and seized rings point to a lack of oil.  I'm wondering if the crank is full of sludge, and the oil is having to go everywhere else. 

Your motor seems to have been modified a lot.  What oil pump and drive gears does it have?  If it has the later 6-start pump gear, has the rest of the system been modified along with that?  The feed holes in the cases need to be larger, plain rocker spindles, etc etc



The oiling system on these motors is a bit of a fine balance. The head and drive side piston often run a bit short of oil.If the rebore was less than 4 thou clearance  they can nip up ,although in theory they should be fine. The oiling system is sometimes modified to "solve" things and can make it worse in other ways. The oil return to the tank should become intermittent after a mile or two when the crankcase is cleared. If this is not so then something is wrong ,Could be large gear pump with high speed drive fitted but original (or restricted) return piping, Too much oil diverted to head with a Jubilee tank fitting,(oil going in circles). Oil tank needs a clear breather and not to be filled too much as it can get pressurised by the return. In normal classic use my motor barely warms the tank, not  enough to stop condensation so I run with a low tank.A motor can take a couple of thousand miles to bed in and stop over blowing thro the breather(some never run in!). Weak mixture or poor timing can overheat a piston. High comp pistons,need backed off ignition  and harder plugs, A bit of good detective work needed by someone who knows all the wrinkles!!.

Hi Paul,

Thanks for your reply. The blocked crank could be a possibility. I was hoping to avoid a total strip but it may be necessary .

I'm new to Nortons and didn't pay attention to the pump drive I also didn't replace the crank/timing cover seal ( it looked OK and I hadn't purchased a replacement ). I'll have the cover off again check the pump gear and replace the seal. Your analogy does make sense it could well be a blocked crank.

regards Terry


If too much is being pumped back then too much must be being delivered. If the crank were blocked there would be less oil flowing in. I think you are correct and the timing case lip seal is not working. The other possibility is the pressure relief valve is stuck open or has the wrong spring. That happened to me once and there was oil and smoke all over the place! 

Hi Robert,

Thanks for you response. I am convinced that there is too much oil getting in the sump and possibly not going where it should. I'm thinking that it might be a good idea to test the oil pressure. a low reading may indicate that it's taking an easy route which would probably necessitate a full strip down anyway. I don't know where the best place to a reading from is though?

I did weaken off the fuel mix by dropping the needle a notch. There wasn't a choke fitted to the bike but it would start and run in cold weather easily which didn't seem right. I put the seizure down to the very loose valve guide allowing air to be drawn in and weakening the mixture, a hot day and me giving it too much welly.

You might be right about the timing, I haven't touched this but the bike does have the propensity to kick back if I don't swing the pedal with conviction. It also runs a bit "heavy" at low revs but certainly flys when opened up.

I'm sure that the oil tank is not pressurising it's all been off and thoroughly cleaned and the breather hose renewed. However the oil does return with some force.

best regards

Terry Guy


Hi David

Thanks for your reply.

yes I agree that it's being delivered but it may be escaping via a route of less resistance in which case a greater volume may be pulled through. The pressure relief valve is also another possibility. I have removed this and cleaned it thoroughly. Initially I thought there may be   Spacers missing from it because there was no resistance when the domed nut was fitted but on reading the forums I understand that this is correct i.e: "the assembly can be heard to rattle when shaken".

please note that despite my issues the bike does not smoke unless of course I don't turn off the manual anti drain valve and start the bike after it's stood for a few days.

many thanks 

terry guy



Oil is not compressable so  whatever the pump  flows will be delivered ,no more, no less.a faulty seal will not allow more oil to flow,it will just go to the wrong place. As the scavenge side of the pump has greater capacity than the delivery side ,the return should contain air eventually. Norton cranks don't block easily . And they usually manage to suck oil through with the centrifugal action even if the pumped supply is not good.They have been known to survive high revs with no pressure at all.


Robert... of course.. you are correct that the path it takes is ultimately irrelevant!  Return must exceed delivery after the wet sump has cleared.  Is it just wet sumping and will sort itself out after 5 minutes or so?

Surely it's not possible to fit the pipes so the return ends up delivering, and vice versa? Ending up with more delivery than return? 


Yes, do an oil pressure test with the engine and oil properly warm.  Bear in mind that pressure can be lost if the timing cover lip seal is faulty, the rubber button between pump and timing cover is split or has insufficient compression, or the pressure relief valve is malfunctioning.  It could be stuck open or closed....

Do not be concerned if the oil pressure is low at idle, it may be around 5lb on a good engine, rising to 40ish at a fast idle.  The first Model 7s had a tank-top oil pressure gauge, but owners got upset about low idle pressures while hot, so the factory solved the problem by deleting the gauge.

Does anyone have a spare Dominator oil tank return union?    My bike had insufficient oil up stairs, and the previous owner did several bodges.  I fitted a blanking plate to give more rocker oil feed.  I want to experiment with the oil feed rate



The return side of the pump is normally double the size of the feedside, this is achieved by the gears being wider. So as already mentioned the return side should be a mixture of air and oil once the drain back to the sump is cleared. There are 4 potential reasons for the return flow to be constant, there could be more.

1. The return side is also being used to feed the rockers and if too much oil is going to the rockers it drains to the sump to be returned again removing the effective over capacity.

2. The oil is being aerated inside the engine from possibly blowby so effectively increasing the oil volume and again removing the over capacity. The return oil flow will be constant but have micro bubbles in it and look milky, sometimes the oil level in the tank seems to increase.

3. There is a short circuit in the feed side eg a seal is missing and the feed oil is going direct to sump with no restriction to reduce the flow which you would get if the oil went to the proper place ie the big ends which create a restriction. With this the oil pressure will be low on the feedside.

4. The sludge trap is blocked, the pressure increases and the pressure release valve is fully open and you get a short circuit to the sump. Less flow than with 3 but the oil pressure will be high.

Hi John,

Thanks for your reply, I've now got lots of things to look at.

pressure testing is the No1 and so I've ordered a pressure testing kit from RGM along with a timing cover/crankshaft oil seal and a timing disc.

I'll report back my findings.

Thanks everyone 

Terry Guy


Not sure if this will help but i have just rebuilt a Commando engine which had stood idle for 40 odd years. It had covered 16k miles. i decided to do a full strip down. The results were piston rings jammed in the slots, sorted by a rebore and new pistons.  I took the crank to pieces to sort out the sludge trap and found barely any sludge at all.

You haven't over filled the tank by any chance?


Hi John,

Thanks for your reply. I'll check that but from memory the level is about 75mm down from the top.

Oil pressure gauge test kit, timing cover seal various gaskets etc all arrived yesterday from RGM after being ordered on the net Sunday evening. Over the past year I've purchased a lot of parts from this company and am extremely impressed with the general quality and the quick turn-around. Hopefully I'll get a chance this W/E to carry out further investigations.

Best Regards


Terry Guy

Hi Again Robert,

I understand that the standard ignition timing for a 99 is 32 degrees BTDC.

as mentioned earlier my machine is fitted with Hi-comp pistons, however I don't know what compression ratio they give. Would you have an idea of how far I should retard the ignition?

Are we looking at around 30 degrees BTDC.


Thanks again

Terry Guy




Yes ,possibly.  You can judge the timing by looking closely at the spark plugs earth strap once you have it running cleanly, Normally you will see a faint band of colour across the strap  where the heat has burned off deposits ,should be somewhere close to the bend on the long part. But this only works if you have the correct plug .


Hi again Guys & Gals,

Just had another thought:- As I mentioned earlier the top end on this bike has been rebuilt and new valves, guides etc fitted. The gasket face of the head was also ground on a sheet of plate glass and carborundum paste

On its first run I covered about 25 miles only to get home and find that it was making an unusual cracking noise. On closer inspection I could see that I had a lot of oil leaking from the rear of the RH head gasket (oil drain hole area) and it was blowing at each combustion. On checking the head bolts they were all extremely loose.

I'm an Engineer by trade and I know they were fully tightened when I put it together, I'd even purchased the special slim sockets to fit down in recesses.

 I went around the head fastenings several times again until all were tight and on starting up the blowing and the oil leak had ceased. In the back of my mind I'm now wondering if I should have removed the head and replaced the gasket? I also remember that this gasket wasn't a very good fit and had to be eased over barrel spigots.

Could my oil problem be a blowing head gasket? 


Terry Guy




You could do a compression check. Steel head bolts need re=doing a couple of times. Stainless ones can be weak and stretch ,I don't like them.


Unlikely that the loose bolts explain oiling problems. But the bolts going loose does sound like bad news - the only time that happened to me was when the central rear underside nut would not tighten up because its thread was stripped.  A helicoil fixed it.

Going back to the oil pumping.  If it pumps continuously into the tank from the large gear wheel side of the pump, it must empty the sump eventually because the delivery side is smaller.  It makes no difference which way the oil travels inside the engine.  If oil is coming out twice as quickly as it is going in, it must empty itself in a few minutes even if the sump is completely full of oil.  Some Commandos apparently wet sump when running at high revs because the scavenge side of the pump is not collecting the oil.  But in that case the symptom is too little being returned to the oil tank, not too much.

Are you certain this goes on all the time, and is not just for a few minutes until the wet sump is clear?

The only other option I can think is that the outlet nozzle into the tank is too small or partly blocked, so the oil backs up and the air inside it gets compressed and it squirts out in a much thinner stream then usual.  The usual jet is perhaps about 4 or 5 mm across and continuous if the bike has wet sumped, and then after a few minutes it becomes a bit more sporadic as air bubbles come through with the oil.  But maybe if the exit nozzle right inside the tank is too small, it might come out in a much narrower continuous stream and, although it still carries air, the air would be much less noticeable because it would be compressed by the geared pump?

If your main issue is oil coming out of the tank and all over your trousers - they all do that if over filled.  Remember not to fill it up before starting up, because it will end up grossly over-filled if it has wet sumped.  It cannot come out of the tank filler unless it is well over filled (unless it has frothed up grossly).

I apologise if all this wet sumping business is well known to you!  But all comments seem to be clutching at straws!  My bet is that it is simply over-filled (although that does not explain a continuous returning oil stream after a few minutes to clear the sump).


Hi David,

Yes of course you are correct its not possible to constantly pump more out than is going in.

The problem is that it seems be doing this whenever I stop and take a look. It may be that when I ride the bike the return is working correctly or maybe not returning enough. 

I'll carry out a pressure test over the weekend and maybe ride out to the country where I can race the engine and not upset neighbours and observe the flow back at higher revs.

Can anyone advise where the oil level should be, I've got it about 3" below the filler neck?

With regard to the tank outlet nozzle:- It certainly doesn't appear small, I'd say the jet of oil which comes out is at least 3mm dia.


All the Best

Terry Guy


Hi Robert,

The bolts are steel. What really surprised me was just how loose they had become after 20 miles or so.

Reading back through the forum I've now found a number of articles regarding poor quality composite gaskets. I have to say I wasn't impressed with the quality of the one I fitted and may go to a solid copper item.

I'll re check the fixings over this W/E.

Thanks again Robert


Hi Guys,

The best laid plans of mice & men etc.

I didn't get as far as hoped investigating my oiling issue this weekend. However I did manage to hook up an oil pressure gauge to the tapping just beneath the Pressure relief valve.

I half expected a low pressure reading but was surprised to see 70PSI at tickover. As I increased the engine speed it rose to 80 PSI and from then on stayed at that reading.

I've temporarily secured the gauge with cable ties just below the Fuel tank so that I can look down in see the readings whilst I'm riding. Unfortunately the weather conspired against my plans and it threw it down with rain so no road test.

Now I've already stripped and cleaned the pressure relief valve and found nothing untoward. When you put the cap on it does not compress the spring (If you shake it when assembled it rattles) hence why I thought I might see a low pressure reading.

Note: that the pressure readings are with cool oil as I couldn't give it a run..

Also during this brief "on the centre stand" run I could see the engine was breathing quite heavily out of the crankcase breather pipe and there was some oil loss from here.

Could high oil pressure throw the oil around inside the crankcase more than normal and allow it an easier passage out of the breather?

Any more thoughts?











The spring rattling is correct, normal routine is to remove shims until it just starts to rattle, however with cold oil the pressure is also set by the size of the oil hole the excess oil uses when its being dumped to the sump. Hence why you get a higher oil pressure on cold oil than on hot as the cold oil cannot get away fast enough through the small hole. The oil pressure figures you are seeing are good but a hot test will be needed to confirm, you are looking for a minimum of 10 psi per 1k revs with a max of 50 to 60 psi but would not complain at 70 psi on a hot engine.

If the high pressure inverts the seal in the timing cover then the oil gets dumped into the sump instead of feeding the crank and will give excess oil in the cases, but then the pressure also drops as the oil has an unrestricted route to sump giving no back pressure so this seems doubtful based on your pressures.


Seems on the high side. The INOA tech digest says that cold oil about 60 idle, .45 approx. warm. Did you measure the length of the relief valve spring? I found a too long spring recently. Just to get better understanding. What grade oil and what temperature do you have? Do you have head oil from the return side and scrolled rocker spindles? Have you checked amount of oil in the sump after a run? And a a day later to check how much it wet sumps?

Hi Michael,

I need to take the bike out on a run to get the oil hot unfortunately work and weather has prevented since I fitted the pressure gauge over the last W/E.

Oil is Castrol classic 20/50 and yes I have scrolled rocker spindles with the oil feed taken from the return line of the pump.

I have a manual valve fitted in the tank to pump line which I close after each run but I will look at removing the sump plug after a run just to see how much is sitting in the sump during normal running.

Does anyone know how much I should expect to get out straight after a run?   

I haven't measured the relief valve spring length and will do but I suppose it might pay me to replace the whole assembly just to be sure. Who knows maybe in the past someone lost the original spring and found one which fitted but has heavier gauge wire etc. I keep discovering a lot of performance upgrades to this machine so who knows what else has been done to it.

It originally belonged to the 3 times IOM TT winner Harold Daniell for the first year of its life and then most of the rest of its life (55 years) it belonged to a Mr Bevan who came from the London/Dartford area. Unfortunately the dealer I purchased this bike from had obtained it from the deceased Mr Bevans estate and its mechanical history has been lost so I have a non-standard machine which I'm attempting to get to grips with .

Many Thanks





It was common for the dealer to put his name in the book , so I would not read anything into that. My 99 has Harold as the First name in the book too. 

Hi Robert,


What year/month is your 99?. Mines early January '57. The second owners entry isn't until February '58 so it sat in the shop for a long time.

When is the second owner registered in your machine?



Hi Terry, My 99 was built just after 3/9/59 and registered  as a 60 model. First by Daniel on 15th feb 1960 and then by the first owner /Rider on 11th March 60. .He kept and used the bike up to around 1970 when it was written off . It was fitted with Avon handlebar fairing,combined fiberglasslegshield/engine enclosure and alloy paniers and suitcase top box. Machines built after August were usually coded for the following year. Mine was the 86th made of this model (a 99 DL) out of about 1250 in total. My bike was used by a photographer to travel Europe  following the  GP races. I have met the first owner and received photos from that period. I would be interested in any history of my first DL an 88 reg VAP807. was in full touring trim Avonaire fairing ,Craven panniers etc. I think its now described as an 88SS in black and chrome!.


Hi Terry 

How are you getting on?

If you decide to strip the engine I think there is a club CD that you can buy for yours. I used the Commando one and it was invaluable giving full details on stripping and reassembly with some clever savings.


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