Skip to main content
000000 000001 000002 000003 000004 000005 000006 000007 000008 000009 000010 000011 000012 000013 000014 000015 000016 000017 000018 000019 000020 000021 000022 000023 000024 000025 000026 000027 000028 000029 000030 000031 000032 000033 000034 000035 000036 000037 000038 000039 000040 000041 000042 000043 000044 000045 000046 000047 000048 000049 000050 000051 000052 000053 000054 000055 000056 000057 000058 000059 000060 000061 000062 000063 000064 000065 000066 000067 000068 000069 000070 000071 000072 000075 000078 000081 000084 000087 000090 000093 000096 000099 000102 000105 000108 000111 000114 000117 000120 000123 000126 000129 000132 000135 000138 000141 000144 000147 000150 000153 000156 000159 000162 000165 000168 000171 000174 000177 000180 000183 000186 000189 000192 000195 000198 000201 000204 000207 000210 000213 000216 000219 000222 000225 000228 000231 000234 000237 000240 000243 000246 000249 000252 000255 000258 000261 000264 000267 000270 000273 000276 000279 000282 000285 000288 000291 000294 000297 000300 000303 000306 000309 000312 000318 000321 000324 000327 000330 000333 000336 000339 000342 000345 000348 000351 000354 000357 000360 000363 000366 000369 000372 000375 000378 000381 000384 000387 000390 000393 000396 000399 000402 000405 000408 000411 000414 000417 000420 000423 000426 000429 000432 000435 000438 000441 000444 000447 000450 000453 000456 000459 000462 000465 000468 000471 000474 000477 000480 000483 000486 000489 000492 000495 000498 000501 000504 000507 000510 000513 000516 000519 000522 000525 000528 000531 000534 000537 000540 000543 000546 000549 000552 000555 000558 000561 000564 000567 000570 000573 000576 000579 000582 000585 000588 000591 000594 000597 000600 000603 000606 000609 000612 000615 000618 000621 000624 000627 000630 000633 000636 000639 000642 000645 000648 000651 000654 000657 000660 000663 000666 000669 000672 000675 000678 000681 000684 000687 000690 000693 000696 000699 000702 000705 000708 000711 000714 000717 000720 000723 000726 000729 000732 000735 000738 000741 000744 000747 000750 000753 000756 000759 000762 000765 000768 000771 000774 000777 000780 000783 000786 000789 000792 000795 000798 000801 000804 000807 000810 000813 000816 000819 000822 000825 000828 000831 000834 000837 000840 000843 000846 000849 000852 000855 000858 000861 000864 000867 000870 000873 000876 000879 000882 000883 1.slide1 2.slide2 3.slide3 4.slide4 5.slide5
English French German Italian Spanish

Oil not returning to tank


Hi. Sorry if this is long post!

After buying my 71 Commando in September i rode it about in sun and rain for a few months doing about 5 or 600 miles with no problems before salt started appearing on roads and she was put away for winter.I think the previous owner had monograde 40 oil in it.

I changed oil over when i put it away and changed over to castrol 20/50(don't know if this could be of any relevance?guessing probably not!) when i came to use again in Feb i drained the excess oil from sump and put back in tank and she fired 2nd kick.Seemed to run fine for  about 150 miles over several weeks before springing an oil leak from the points cover a few miles from home.

I had a couple of attempts at replacing the oil seal. The first leaked in less than 10 miles the 2nd attempt i thought had worked after about twice the distance but on taking cover off there was a tiny amount at bottom of ignition.I have been too busy to go near bike for about 6 weeks till the other day and decided to take it out in case the tiny bit of oil i saw was just from assembly.

I had to drain a couple of litres out sump which i am guessing now is not normal wet sumping over 6 weeks? put new spark plugs in and she sounded and ran great,stopped after 6 miles and no sign of oil on inspection at the little drain hole.did a few more miles then returned home going a bit quicker.when i got home i found it was leaking at same point,also oil pushing out at tacho cable.Loads of oil in sump(1.7L) so i assume the oil is not returning to tank and probably the oil leak is a symptom rather than the problem! 

Still pretty ignorant of these kind of things having never had an old bike with dry sump before.Any suggestions what to look at.

Do pumps suddenly stop? and presumably for the oil to get down into sump like that it must be working partially atleast. how can i check it,is it easy to remove without touching much else?

I wondered if the pressure relief valve might be at fault,i have removed it and although not able to get it apart i can push piston and seems freeand springs back(i will probably replace anyway to be sure). i notice it has an o ring instead of the copper washer between this and timing cove,could this slight difference in clearance cause a problem with operation of valve?

Please Help!




This is the seal on front of pump between that and timing cover? i did replace that also but its possible it dropped of at wrong moment when fitting cover i suppose. Will take a look. Thanks Robert




Hi Oliver,

Providing your engine has the large sump plug, remove the sump plug and check for 'debris' blocking the filter in the sump plug. The oils return journey starts here. Also worth checking if any 'sealant' has made its way into the drilling going up to the pump.

Before stripping down, can I suggest that with the oil back in the tank, you try removing the oil pipe that goes from the engine to the oil filter at the filter end.  With a tray under the bike, briefly run the engine and see if any oil comes from it. foam and intermittent spurts are ok. If oil gets there, then refit that hose and remove the other  one from the filter. Does oil still flow? If not then the issue is with the filter.

If you get as far as removing the timing cover to check the seal (this shouldn't stop oil returning, just not feeding the crank), then I would remove the oil pump and blow through the galleries behind with an airline to ensure that they aren't blocked.

If you are feeling really keen, do a search on here for 'oil pump servicing'. Straightforward to do, and may give you a clue as to where the problem lies. Prime the pump prior to refitting with oil (immerse in oil and rotate drive cog a couple of turns) will also help.




Did you use the correct tool that screws onto the end of the cam shaft and guides the points oil seal smoothly onto the cam shaft end when you refitted the timing cover, if not it will leak. Rev counter leak at the engine can be cured by changing the whole drive to a Mk3 version, buy it from Andover Norton and ask if it will fit a 750 engine, mine was an 850 MK1. 

Good luck I'm sure you can sort it.


You didn’t mention physically checking if oil is returning to oil tank with engine running. That would be the first thing to do as it’s the easiest job. Remove seat , remove oil tank cap with attached dipstick and watch for oil returning from return pipe entering tank near top of tank.

Thanks George,

It doesn't have the large sump plug unfortunately!  good suggestion about checking either side of filter will give this a go first. i was wondering if it could be something like a dodgy filter as problems started fairly soon after changing.(in mileage anyway)

Is removing pump easy?  do i need any special tools to hold everything else in place or can i just unbolt it and remove

Thanks John

Yes,i used the correct guide tool. my thinking is(and obviously i could be completely wrong!) that too much oil in crankcase is causing high pressure and forcing it out where it can. Seems to be little in tank and a lot in crankcase now after a run.possibly ,although i hadn't realised that might have been the case when it first leaked.And i treated a symptom rather than cause!


Hi Oliver,

Regarding the filter,  the housing should have arrows for in and out on it. If the pipes are fitted the wrong way to the housing, then if there is a non-return or anti drain valve in the filter (rubber seal inside filter between small holes and element, visible through the small holes - some filters have them, some don't) it will do a very good job of restricting the flow. The flow direction through the filter should be through the small holes, returning via the centre threaded boss. 

Do hope your issue is something simple.



I've just looked at the AN site and it appears that their improved housing with oil seal is the only version offered for all relevant models. Looking at the page for 1972 models all that is offered is:


part-no: 06.7254

£19.95 + VAT

Likewise, the old plain drive gear is now replaced by the items also shown on the Mk III page:

TACHOMETER DRIVE GEAR with Groove for O Ring (06.1089)

part-no: 06.5201



part-no: 06.1089/S

Both are a heady £39.95 + VAT. It's not clear to me (but perhaps a Mk III owner will explain) whether both of these are compatible with the housing 06.7254.

I assume that the new housing is compatible with the old-type plain gear that relies on a push-on O-ring. 



Can't help you on that side call AN they will give you the answer you need. I tried the double oil seal fix which did not work very well, mine being an 850 the replacement housing was the proper fix.


Oliver when I changed the oil on mine, filled with oil and started the bike with the oil filler cap off, the oil took a long frightening time to return, to the point where I nearly stopped the engine to check.


.. a long time to fill the filter hence the delay.


Thanks All,

So i put the pressure relief valve back in and then unbolted the filter housing and jubilee clip from return of tank and dropped filter to check as suggested by George. I didn't get as far as taking pipe off inlet to filter but started bike and it was obvious from outlet from filter that it was now returning oil!  Put it all back together and checked again at tank then went for a little ride stopping after every few miles at first to check,after a few stops went a bit faster home and oil level stable and oil seen returning.I also see a little bit of oil and air spitting back from breather on top of oil tank,is that normal and expected?

Now although i said PRV didn't seem stuck in my original post, there was a little bit of resistance when i first pushed it but certainly not totally seized but now i think it must have just been enough to stop it operating.I am going to order a new PRV and install with proper copper washer and probably do a few more test miles to make sure all ok before worrying about sorting the little leaks.

Hopefully i am learning and will remember some lessons from this.I think i should have known that the leak i had from points cover (which didn't cover the whole of bike!) wasn't enough for oil level to drop so much,i remember being shocked,as when it went i only did about 7 miles total trip including getting home.Keen to fix i looked at drawings and identified the parts needed and ordered the 2 seals and gasket and tried to get sorted.Should have investigated and thought more rather than jumping to the easiest conclusion!


... is normal as the return side of the pump has double the capacity of the feed. So it sucks up all the oil from the crankcase then continues to pump whatever it finds in there which as you've seen is bubbly oil.

Glad things seem to be sorted out now. One thing to remember is not to leave the bike for several weeks, look in the oil tank and see it's low then top it up. Most Nortons wet sump to some extent or another so the oil will have migrated to the crankcase.


… an extent which may surprise and alarm the innocent.

A few days ago I measured the amount of oil that came out of the crankcase after my bike had been standing for exactly seven days — 500 cc !


They do seem to vary wildly however, from those like yours to those which can be left for months.


The majority of oil in the sump in the first 24 hours comes from the timing cover, so this will distort any levels taken after the first 24 hours, the amount collected over the 2nd and 3rd day / week is what is actually passing. 

With the Combat engine known for failure, I know of two occasions when the debris has blocked the return oilway to the pump. One owner even checked with a pipe cleaner and though because the it stopped from both directions it must be clear, but he was hitting just blockage at the change in direction. Always prove oil ways with oil, not wire. 

The lipped oil seal in the modified tacho housing will work on a MK3, but what some owners do on the MK3 is not fit the O ring on the tacho shaft, then try and fit the shaft through the housing turning the lipped seal inside out. Fit the O ring as normal and with both parts in your hand put them together carefully while you can see it go through the seal ensuring it does not turn the seal inside out and then drop both parts together into place. 


Norton Owners Club Website by White-Hot Design

Privacy Policy