Now I think that the leak is coming from these plugs (or whatever) that I marked with arrows on the photo.
Am I right? Does anyone had a problem like that and how to fix it properly?
blues-mack: You are correct that your arrows indicate plugs. When the head is produced, there are internal oil passages which make a right-angle turn. Never saw a drill bit work a 90 degree hole. So, the machinist drills a second hole at 90 degrees through the first hole and then plugs the outer end. It is my understanding that these plugs are tapered zinc and they are hammered into place to create the outer seal. Can they loosen and leak and fail? Perhaps.
I had just such a leak on my 98EV while it was still under warranty. I didn't want to disturb the cylinder or sealing so we tried everything we could to stop the leak. Even got permission from the dealer to hammer on it with a pin punch in hopes of expanding it to recreate the seal. No joy. Guzzi eventually gave me a replacement head without question.
Now ignore that. One of the primary leak sources on big block heads is the sealing o-ring under the top/center head bolt cover plug. If you do any head work or torque the heads, then you have to remove that top/center cover inside the rocker chamber to access the unique top/center head bolt. On early round heads, that sealing was with an aluminum crush washer. Square head motors had a top/center cover with a recess to hold an o-ring. It is definitely NOT a lifetime sealing item. Should probably be replaced every time you remove that top/center cover. (at least every other time). The constant heat/cold cycles and potential of high heat on an air cooled head make the rubber o-ring flat and brittle and no longer resilient. I would quickly replace that o-ring on both sides of the engine. Cheap and it eliminates one common source of oil.
Next I would wash and de-grease that head area fastidiously. Spray all around with cheap foot care spray talcum powder. Go for a ride and you should quickly see brown staining emanating at the leak source into the otherwise white powder spray. Wash and repair.
It might be possible to seal the plug leak by adding something external. There are oil-tolerant sealing compounds (such as SEAL-ALL which I have used a lot. Maybe JB-Weld? Maybe JB-Weld specific for high temp exhaust repairs? Holding up just fine on my camper's cat converter.) However, this location is going to get very hot and the oil behind that plug is under pump pressure so might be 60 psi or more. Hard to seal that from the outside unless you are the Dutch boy at the dike.
Please return with a post to explain how you resealed your leak. Last resort, get another head.
Patrick Hayes
Fremont CA
