Overfill chaincase with oil blow oil seals ?

[Strummer10]

Seen this on a FB thread but wondered whether anyone on here knows the true facts as to whether overfilling your chaincase with gearbox oil would blow an oils seal OR have any other detrimental effects on the engine ?

[dave411]

I am sure that it will blow them.The crankcase is designed to run with a specific amount of oil in it,give or take a few Italian cc's.It will cause more pressure because there is less air volume,and the breather wont be able to express the pressure fast enough.So the pressure has to go somewhere,thus popping a seal.Simples

[Monty]

canceled post

[Monty]

I'm not sure that that's correct. My logic says the chain case is open to the atmosphere. The primary side is suck blow hence sucking oil if the seal is no good. If the chain case was sealed and overfilled I could see it, I will wait for an adult to come along no offence intended Dave411

[johnduke]

hi, agree with monty john

[rosscla]

Me too

[rainbowrunner]

the excess oil will come out the breather making a mess off your engine etc, the breather hole is,nt a pin prick ! it will p155 out

[Meds]

I suppose it depends on how high you fill the chain case.
Submerge any seal in oil and it will have a static head pressure, add in some movement to it and you will increase the pressure acting on that seal to the head pressure + a bit more depending on the movement and how deep it is in the fluid

The only seals you are likey to blow are the rear hub seal and the kick start seals if you fill above those seals.
Looking at a crank case this shouldn't be possible if you fill using the correct method and remove the upper plug doesn't cover those seals.

[coaster]

I'm in the 'no it won't' camp, I doubt the difference in head pressure of a few centimetres would have any effect. If the breather should become blocked it might be a different story though. There was a chap selling a hi capacity breather which had a large bore plastic pipe that runs up above the rear mudguard. I bought one from him mainly because it looked 'trick' tbh, I think Camlam sell them as well.

[rosscla]

There's plenty of bad breathers on sale, which can cause issues. Usually they're the stainless or chromed variety. The wire wool inside will also often be missing.

If the breather becomes blocked then that can cause seal failure.

I have seen gearboxes filled to the top before.