Matchless G9 Crankcase oil leak

Leathers · Post by **Leathers** » Sat Jan 15, 2005 9:20 am

I recently bought a 1955 Matchless G9 500cc twin which is great. I have been trying to sort it ready for summer and a trip to the TT's in the Isle of Man in June.

My current big problem is an oil leak when the bike is running, which ends up throwing oil over my right foot and, after ten miles or so makes changing gear a bit difficult.

The leak is coming from the crankcas joint below the dynamo cradle. I think the oil filter is just below that. I am wondering if my oil pressure is too high but I don't have a gauge to check it. The bike recently had a rebore and new pistons. Can anyone offer any suggestion please.

Biscuit · Post by **Biscuit** » Sat Jan 15, 2005 9:27 am

Hi Keith,

This location, together with the narrow joint face between the cylinders is very prone to this, in the main due to previous owners prising the cases apart with inappropriate tools. I cured my G11 by fitting a gasket between the crankcase halves, I also cured a section members oil leaks by facing off the crankcase halves, using a mill as a large lathe.

Leathers · Post by **Leathers** » Sat Jan 15, 2005 4:37 pm

Thanks Alan, I guess that is what I feared. Would you be able to fix mine for a price or do you know anyone who could?

Eric · Post by **Eric** » Sat Jan 15, 2005 4:39 pm

Alan is quite correct in what he says it might not always be due to previous bad treatment. There is supposed to be a thin paper gasket between the crankcase halves just where the filter tunnel is. It looks like a large washer. It is or was available as a separate item from the spares scheme. At this part of the crankcase joint the internal pressure within the filter tunnel can be very high when the engine is cold and once the leak is started it just becomes the normal thing. The new pistons and rings may have just added a bit more to the general crankcase pressure to get things started. I did once contemplate trying to make a gasket as Alan did but put my faith in modern sealants instead. But I still have a minute weep from somwhere around the area even with the tunnel paper gasket. It should be a good opportunity for someone who can and mill a small groove all around one half of the crankcase joint so that a small diameter rubber seal could be fitted.

Eric · Post by **Eric** » Sat Jan 15, 2005 5:14 pm

I also intended to say check all of the bolts and studs that hold the crankcase halves together for correct tightness. Dont be tempted to overdo things for the bolts, they have been known to break leaving the thread part behind to challenge your deep hole broken bolt extraction skills.

Fred-AJS · Post by **Fred-AJS** » Sat Jan 15, 2005 6:52 pm

Hallo Keith

We had the same problem over here ,after a rebuild.
When cold the engine blew the gasket from the filter tunnel joint
Instead of taking the engine apart, I made thin bush and fitted that inside the filter tunnel with loctite pipeseal. that was 4 year a go , and since that day not a drop of oil came out.
Hope this helps Fred

Leathers · Post by **Leathers** » Sun Jan 16, 2005 7:29 am

Thanks very much Fred, that sounds like an excellent idea.

RogerNorman · Post by **RogerNorman** » Sun Jan 16, 2005 10:33 am

The easier thing to check is the gasket between the dynamo and the back of the timing case. If the dynamo has been off recently but the old gasket re-used its probably leaking. If you fit a new one however you will almost certainly have to file it where the stud that pulls through from the timing case passes through the gasket. Don't get over enthusiastic with the file however - you may go straight through the gasket! I don't think I have yet seen a joint that doesn't leak a bit here - its just the degree! Try running a thin smear of silicone around the faces before you fit the gasket - this helps. Its certainly a quicker check than the oil tunnel gasket - 15 minutes as opposed to a full engine strip.

lawrence · Post by **lawrence** » Sun Jan 16, 2005 6:52 pm

Excellent suggestions guys! I am going to check my dynamo gasket as i have a continually oily right boot. It does waterproof the leather well [:D]

Lawrence [}:)]

Leathers · Post by **Leathers** » Sun Jan 16, 2005 7:44 pm

Roger, thanks for you contribution. I did have a major problem in that the oil was seeping into the dynamo but I cured this by fitting the cork gasket and making gaskets for behind the dynamo drive gear and for between the dynamo and the chaincase. I am sure that the leak is not coming from the dynamo and pretty sure that it is coming from the filter housing. I am going to take up Fred's suggestion. Thanks to all of you for your contributions, it was most frustrating not knowing what else to try.