I'll be restoring a '42 M20 later this year and was in the process of tearing everything down when I was fortunate enough to acquire a fairly cheap '44 non-running parts hanger bike. In comparing the oil tanks, the one from the earlier bike has a large square welded area inside the curve where it would go against the frame. Not sure if this is a repair or how it was fabricated. The tank from the later bike does now have this feature but it has a fitting near the top on the side where the battery bracket connects. This had a hose connecting it to one of the oil line vents found on the lower side of the engine. The earlier pattern bike doesn’t have this fitting and the line coming from the engine didn't connect to anything else so was simply serving as a vent and place for a small amount of oil to drip from over time.

Basically want to go with the better part for my restoration which seems to me is the later one with the added fitting to allow oil to be reclaimed. Assuming this doesn’t put too much air pressure from the engine gases into the oil tank and cause other problems.

Can anyone shed any light on these two tanks and if the oil return is a good thing or a bad thing? Both tanks otherwise appear to be totally functional and interchangeable. Both tanks have the vent tube at the bottom but this is only visible in the sideways shot of the earlier tank that I have.

Photos attached in case this didn’t make sense.

email (option): dgordon4@austin.rr.com

Hi Dave, the square plate on the rear of the first tank is a non original repair. The fitting on the second tank is also a later addition and is not present on standard tanks. From you description it sounds like it was connected to the crankcase breather outlet pipe on the drive side/underside of the engine. I guess if this was the case it was done to recirculate any oil blown from the breather thus stopping it leaking on the garage floor or similar!
Personally I would blank it off and return to a standard set up.
The vent pipe under the tank should have a rubber connector similar to the ones on the engine oil pipes and then a length of copper pipe that ends about level with or just below the lower edge of the gearbox mounting plates...Ian

email (option): ian@wright52.plus.com

Appreciate the Info Ian and I think I'll go that route since there's no telling what was done to the tank with the welded repair. Can't imagine how it would have become damaged in such a protected area so it likely was cut open for internal work and then welded back up. Easier to plug the later tank as you mentioned and it won't be visible. Mainly wanted to verify that the oil return wasn’t something officially done to some bikes late in the war.

Sure I'll have other parts to hot swap as I go through the bikes. Wish there was a more clear path for determining early war vs late war vs field mods vs postwar civilian mods to things. The British ACIs and LoC records are really handy in this area for weapons and equipment but don't ever cover vehicles changes.

email (option): dgordon4@austin.rr.com

I did exactly same thing to my oil tank as on your first photo because it was very badly dented.