AJS & Matchless Owners Club

Please excuse my poor English.
I am riding a 16MS (1955) motorcycle.
Oil sprays out of the oil vent pipe quite a bit. Is there any way to reduce the amount of oil that sprays out?
Also, would drilling a hole in the TAPPRET INSPECTION COVER and attaching a hose to it to make a breather have a good effect?

There are two vent pipes one on the rear of the engine on the drive side and one on the oil tank.

Either could blow out if "wet Sumping" has occurred (this is when oil has seeps past the oil pump into the crankcase while the bike has been unused over a period of time) and the oil tank has been toped up to compensate for the apparent loss.

Another cause could be a worn piston/cylinder bore that allows pressure into the crankcase, do you have good compression?

Has the engine been rebuilt recently and the breather disc omitted from the breather body?

A modified/after market tappet/pushrod inspection cover is often seen on the competition engines but should be unnecessary on normal road bikes

Which pipe are you referring to? If the vent pipe from the oil tank then try putting less oil in the tank, mine tends to blow out oil if there is too much I'm the tank.

If you mean the engine breather pipe then the problem is probably blow by past the rings. I would not expect a breather above the piston would help because the oil will be blown out by pressure build up below the decending piston made worse by pressure blowing past the rings on the driving stroke.

I appreciate everyone's answers.
The oil vent pipes I was concerned about are the oil vent pipes from the side of the engine (next to the main bearing).
The engine has not been run much since the rebuild and the piston gap is 0.12mm and the ring gap is 0.64mm.
The pistons are made by GANDINI and the compression is considered to be good, but I am concerned that the ring gap is too wide. The oil vent unit on the engine side has just been disassembled and cleaned. I was also concerned about the oil falling out of the oil tank, so I installed an Anti Wet Sump Oil Pipe Tap With Switch - Magneto between the oil tank and engine at the time of rebuild, and turned it off except when starting the engine.

keishis wrote: ↑Sun Apr 28, 2024 11:24 am
The oil vent pipes I was concerned about are the oil vent pipes from the side of the engine (next to the main bearing).

If you are referring to this one.......
This is a possibility............

I appreciate your answer.
Is it an oil pump problem...I had a problem with the oil pump about 10 years ago and replaced the crankcase and pump,
but the oil return is poor. I will check the oil pump again.

The oil return on the singles is poor at the best of times, although sufficient to keep them running. Before you start delving into the oil pump especially if there are no signs of oil leakage from the end plates I think we should look further at your piston gaps.
You say your piston has 0.12 mm gap to the bore. That converts to 4.75 thou of an inch. I am not familiar with the Gandini piston but believe they need more clearance, perhaps someone else can comment. However 0.64mm ring gap which is 25 thou sounds too much. The rule of thumb (courtesy of Spriddler on search) is 1.5 thou per inch of bore plus 1/2 thou. Bore of a 16ms is around 2.75 inches which give a ring gap of about 5 thou. Yours is 25 tho which would I think allow significant blow by. In addition the rings need to be set with the gaps 120 degrees from each other, if closer the blow by will be worse.
My experience of these problems comes from an overheating seizure on my 500 combo which sealed part of the rings into the ring gaps. Blow by caused so much oil vapour to be pushed out of the breather, stationary and only on tickover, you could see the vapour rising as smoke from behind the primary chaincase.
Perhaps Spriddler will check I have got my figures right and comment.

i was thinking that you could determine between oil level in crankcase and back pressure, by running engine until its coming out of breather, then stopping engine and quickly draining crankcase. If the return is doing its job, there shouldnt be a crankcase full of oil? (like on a twin where you can see the crancase is empty by the change in flow of return, you wouldnt expect to get a lot).
I've no experience on singles (yet) so just thinking aloud...

Thank you all so much for your advice.
I could not find any trouble in the oil pump section, so I will try to change to the stock cylinder and JP pistons.
By the way, the piston ring gap is less than half of what it used to be (it seems to be 0.012in).

keishis wrote: ↑Sun Apr 28, 2024 11:24 am
The engine has not been run much since the rebuild and the piston gap is 0.12mm and the ring gap is 0.012".

Does/did the engine smoke when running? Is the sparkling plug becoming oiled and black?

Without being within sight and smelling distance of the engine it is only possible to theorise and offer opinion but here are my thoughts which may be of some help............


Your 0.12mm piston gap is 0.00472" which is more or less spot on with the general rule of thumb of the recommended clearance of 1.5 thou per inch of bore dia + 0.5 thou".
The standard bore of your 350 is 2.7137" therefore 2.7187" x 0.0015 + 0.0005 is a recommended clearance of 0.00460" (vs your 0.00472") which suggests that it is only a negligible 0.00012" over the nominal ideal clearance.

The pistons are made by GANDINI and the compression is considered to be good,

Since you have good compression and a good fit of piston to bore I would look elsewhere for the cause rather than reboring with a replacement piston.

Your ring gap of 0.012" (12thou) is within tolerance. Just for guidance the Army Technical Standards manual gives a designed gap when new of 6 to 10 thou, acceptable at 12 thou and condemned at 30 thou.
You say the engine has been rebuilt and not run much. From memory all the piston compression rings that I've fitted have had a tapered outside face so it is possible that one or more of the compression rings hasn't been fitted the correct way up. The upper face is normally marked.