CptKelly wrote:The oil leak appears to be coming from the crank oil seal, and I just replaced it.
Mike

I had a leak from the crank seal on my pre-MkIII 850. I'm running a dry belt so it was obvious where the leak was coming from. I replaced the seal twice with no luck. They were the rubber coated type seals. Jim recommended using the steel bodied seal with a light coat of JB Weld. It worked.

Whilst you recon it's a crank seal leak, you may have oil escaping from the oil seal on the gearbox shaft/back of chain case. I believe the Mk3s have to have matching gear box and engine numbers (machined as a unit) to ensure the gearbox shaft is central in the oil seal. Fitting another gearbox may compromise this. Only way to see this is with the clutch and primary drive removed (major hassle) Run the bike stationary with a large mirror under it. saves jamming your head under the exhaust pipes.

Your crankcase is being pressurized by compression blowby, which is forcing oil through the main seal or around the too limited seating for the seal.
A leakdown test will confirm this diagnosis. See: http://www.harborfreight.com/catalogsea ... own+tester $39.95
Prior to the development of the molded lip seal, engines relied on a tight compression ring seal and gravity to retain crankcase oil.
And it worked.

Frank Forster

Your crankcase is being pressurized by compression blowby, which is forcing oil through the main seal or around the too limited seating for the seal.

Not necessarily. A Commando without any check valve in the breather will have a pressurized crankcase. Although, piston ring blowby will create crankcase pressure, too. I've read here about a lot of work being done to stop oil leaks, special joint sealant, special seals, special "O" rings and the like. While these are all worthwhile, you may still end up with oil leaks if you have a pressurized crankcase. Guys, don't fight it. The Commando does definitely benefit from a reed type check valve in the breather circuit. They are cheap, the Mike's XS 650 PCV valve is $18. I see CNW is now selling the version that mounts directly to the crankcase. More better! I've tried the Motormite vacuum check valve. It's little more than a placebo. If it works at all it's for a very short time.

Donlon39 wrote:Your crankcase is being pressurized by compression blowby, which is forcing oil through the main seal or around the too limited seating for the seal.
A leakdown test will confirm this diagnosis. See: http://www.harborfreight.com/catalogsea ... own+tester $39.95
Prior to the development of the molded lip seal, engines relied on a tight compression ring seal and gravity to retain crankcase oil.
And it worked.

Frank Forster

Yeah, might be, simpler to just stick a reed valve on it, since you want one anyway, even on an engine that has zero blow by. I mean, we have 750 or 850 or even 900ish ccs of pumping pressure on every down stroke, the same on the up stroke of course, lets net a vacuum out of it. Lemonade from lemons.

xbacksideslider wrote:

Donlon39 wrote:Your crankcase is being pressurized by compression blowby, which is forcing oil through the main seal or around the too limited seating for the seal.
A leakdown test will confirm this diagnosis. See: http://www.harborfreight.com/catalogsea ... own+tester $39.95
Prior to the development of the molded lip seal, engines relied on a tight compression ring seal and gravity to retain crankcase oil.
And it worked.

Frank Forster

Yeah, might be, simpler to just stick a reed valve on it, since you want one anyway, even on an engine that has zero blow by. I mean, we have 750 or 850 or even 900ish ccs of pumping pressure on every down stroke, the same on the up stroke of course, lets net a vacuum out of it. Lemonade from lemons.

Totally agreed, Slider! If you are chasing oil leaks isn't it better to start out with an unpressurized crankcase? If you still have leaks at least you know it's not coming from crankcase pressure.