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Odd primary case oil leak.


First off apologies if this is a repeat but I can’t find anything in the archives.

scenario is a 1960 ES2 engine fitted with a Commando gearbox and RGM belt drive, all housed in a slimline frame.

Sadly my Santa-ride for charity was halted due to the clutch not freeing up making  control difficult.

Thinking this probably an oil seal problem I have stripped down the primary side and found one definite problem and possibly two more.

1. The sliding discs that fit to the inner chaincase round the gearbox shaft were not genuine Norton parts. They are too small for the chaincase hole leaving a small gap at one side. I am taking this to mean that oil from the rear chain is being thrown through this gap.

2. The factory manual explains there is a.....timed breather on the mainshaft with exit through a small hole at the bottom of the bearing boss. This lines up with a hole in the stator mount. This would appear to take any oil discharge out between the engine and the back of the inner chaincase.      Is this correct ?????

3. The stator mount fixes to the crankcase with three screws at approx 7, 11 and 3 o’clock. The threaded mount holes at 7 and 11 are blind, being 7/8” depth. The hole at 3 however is 1 3/4” and appears to go into the crankcase.   Is this correct ???  as I suspect this is another oil entry around the screw threads.

Sorry for the ramble but hopefully that explains the issue.  Thank you for looking and any ideas feedback you can give,

.regards John.


Added couple of pics






My '61 ES2 also has two blind holes and one (number 3) that goes through the crankcase - a screwdriver touches the flywheel. Is this the way Norton designed this - surely not!

My ES2 has no timed breather - that is usually fitted to the camshaft.


what is the clutch? I have a Newby on my special, and it gradually gathers surface rust on plain steel plates if not used, and will not free-off if not regularly kicked over with clutch pulled in. Only solution is to take it apart and scrub off the rust.


If the belt is tight or tightens with expansion as things warm up then the clutch gets pulled about and wont free off properly messing up gearchanging and neutral selection.

Yes Richard there is indeed a camshaft breather on my 88 twin...but the ES2 breather on the mainshaft is exactly as described in the Norton factory guide.   No camshaft just individual cams.

Not sure if I should be glad you have the 3 hole through to flywheel...seemingly odd design if this is correct.

thanks for reply.

In reply to by jan_nelder


Jan, it’s the full RGM kit so has Commando clutch. Oil was causing the sticking plates not rust. All dismantled and cleaned out. Drum was pretty messy as well.

thanks for reply.

Robert its not the belt, have taken great care to get that adjusted correctly. Problem was oil in the clutch drum and plates.  Also the PO has fitted a second gearbox adjustment on the drive side so gearbox mainshaft is firmly lined up.

Have you come across this 3 hole going in to flywheel ??????

thanks for reply


John, I had the same hole with the stator screw penetrating the case in that position. It goes through at a thin part of the casting at the breather port. I only discovered it upon disassembly and presume that it came from a too long screw or maybe the tap going too deep. Definitely not intentional. I plugged it with a bit of JB weld. Mine had no stator gasket and screw leaves little space unless the primary case space is allowed for. At any rate I tested after the patch and all is well. I suspect it for one of my many oil leaks.



Are you sure that it is engine oil messing up the clutch? Could be gearbox oil. Never experienced it but there is quite a bit of talk about oil seeping between the main gearbox axle bore and the clutch pushrod.  And there is a number of solutions.


John As to the timed breather on my 1962 ES2 there is a passage in the crank shaft where the hole lines up with a machined disk in the case. The disk opens the hole for about 30 degrees each side of BDC. That is the hole you are refering to behind the primary case. Being open only as the cylinder is begining its upstroke theory has it that it will draw air in briefly and help cool the drive side main. Not meant as a drain.


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