Gregory Bender

Engine oil leaks from the bell housing

Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models



// //

Leaks from the small weep hole in the bell housing immediately above the engine drain plug seem to be more common than we would like them to be. Fortunately, there are good fixes for these leaks.

First, make sure you know what is leaking. Is it engine oil or gear lube from the transmission? Touch it, feel it, smell it, compare it to what is on your dipstick, etc. Make sure you know what is leaking.

Easy fixes: There are really only two easy-to-fix problems that can cause a leak at the weep hole above the engine drain plug:

  1. The engine oil drain plug is not sealing well - wind could whip it back up and make it appear that it is coming from the weep hole in the bell housing. Fix, of course, is to fit a new or better sealing washer.
  2. The hoses that connect the breather to the pipes sticking out of the top of the bell housing are cracked or not secured properly with hose clamps - oil escapes here and runs down into the bell housing. Fix is to tighten clamps and/or replace hoses.

More in-depth fixes: If fixing these problems doesn't solve the oil leak, then the only thing left to do is pull the transmission and then remove the clutch assembly and flywheel and fix a number of possible problems inside the bell housing.

Visual inspection: With the transmission and flywheel removed, and before I clean up any oil, I carefully examine the inside of the bell housing. Sometimes it is possible to determine the location of the leak. Many times, however, it is an oily mess all over inside there and next to impossible to determine the leak's origin from a visual inspection.

Soapy water test: I then take the time to apply very soapy water to the inside of the bell housing and I lightly pressurize the sump with my air compressor. Of course, I must plug the breather-to-atmosphere belch tube so that the air doesn't escape from that location (a 38 inch ratchet extension works very well to plug this hose). I pressurize the sump through the oil fill hole. Sometimes, but not always, I can see bubbles escaping from the leaky location. This technique is especially good for identifying leaks from the cam plug. Do keep in mind that we are not trying to inflate the crankcase, merely pressurizing it a bit. Some people get all up in arms about doing this, but it really isn't a big deal. You are not going to push out the front main seal as there is a physical barrier to prevent such an occurrence. And, you'll be watching the rear main seal. In my experience, you can easily apply 25 - 30 PSI of air pressure without any worry whatsoever (the oil pump routinely pumps out 60 PSI or more and the oil seals are not dislodged nor damaged).

Holistic approach: Even after my visual inspection and soapy water test, I never fix only one possible problem. I always take an all-or-none approach and address everything at once. The cost of all the fixes is not very expensive and certainly not worth the effort of removing the transmission a second time.

  1. Cam Plug can leak oil at the seam where it is pressed into the engine case - Clean it thoroughly and J-B WELD OR silicone the seam.
  2. Breather inlet pipe gasket may leak - Replace with a new gasket. I'm not a fan of using gasket goo on much of anything. However, this is one location where I do use a light coating of Permatex Super 300 Form-A-Gasket Sealant (not silicone).
  3. Breather return pipe crush washers may leak - Replace with new aluminum crush washers. Never reuse old crush washers in this location.
  4. Rear main bearing flange gasket may leak - Replace with a new gasket. I'm not a fan of using gasket goo on much of anything. However, this is one location where I do use a light coating of Permatex Super 300 Form-A-Gasket Sealant (not silicone).
  5. Bottom two bolts on the rear main bearing flange may not be sealing - These threaded holes run through to the inside of the engine case. If not sealed, they will leak oil. I apply a light coating of ThreeBond 1184 semidrying liquid gasket to the threads. In fact, I apply the ThreeBond 1184 to all of the threads, more to serve as a very light thread locking compound than anything else.
  6. I use new bolts and new thick wave washers to secure the flange in place.
  7. I use a block of wood and a hammer to fully seat the flange into the case.
  8. Rear main seal may leak - Replace with a new seal. I use oil seals made from Viton.
  9. The case itself may be porous. I consider this cause very rare and I've not run across it myself. But, I have heard tales of porous castings that will leak oil. I know of one person who tried everything to stop a leak from the bell housing and it still leaked. So, he thoroughly cleaned the inside of the bell housing and applied a layer of silicone to everything. Problem solved. I've never done this fix and I do not recommend this fix until all other possible leak causes have been fixed and the oil leak persists. In other words, please don't get a butter knife from the kitchen drawer and start applying silicone to the inside of your bell housing unless you've tried everything else and it still leaks. This is not a preventive fix that I recommend...and I've never done this fix.