There have been many posts about this problem, much discussion about sealants and in some cases people get leaks fixed. In other cases the leaks come back. It seems strange that so many different people, some with good wrenching skills, still get the problem.
So I have the opportunity as I've always promised to get my '97 perfectly oil tight. Most of us who've 'been there' know that BMW rely on a bead of silicon gasket sealant placed in the right places to make the seal. It is vitally important that the sealant is applied to a grease free surface and in most cases the sealant is allowed sufficient time to cure - which could be up to 48 hours. When this work is done at the front, with oil in the bike and a motor in the frame, getting oil free mating surfaces can be very difficult. In fact on my own '97 KRS, most of the oem sealant was pushed away from the cover and block surfaces with very little actually sticking.
I was once puzzled when a previous champion of timing cover leaks posted that there was a lot of oil behind the cover which puzzled me a while since you would only expect splash oil feed at low volume. you might get the kind of leak that stains the case at the edges, but reported bad oil leaks surprised me.
Before I started my rebuild, I acquired some 20thou Nitrile rubber sheet, and some high performance glass fibre based 10 thou gasket material. I aim to use the nitrile sheet to make a re-useable gasket on the timing cover. The glass fibre material is for use elsewhere.
Now I've had a big shock which might be the real cause of all the timing cover leaks. The main oil feed gallery from the oil filter carries oil at very high pressure. It passes into the timing chamber and then connects via a milled land to oil lines cast in the timing cover. These meter oil at a lower pressure to the timing chain tensioner and valve gear. With the motor horizontal, oil returns to the sump via large holes in the block at the far end opposite to the crank. Too much oil around the top end will aggravate the smoking problem at start up as more oil will hang around the valve stems.
If the main oil gallery connection to the timing cover leaks, The timing cover will get filled up with a much larger volume of oil than normal, since this is the high pressure side before metering. I hate to dwell on this scenario, because it has the potential for reducing oil pressure where it's needed most - at the crank bearings.
I've posted 3 photos. 2 photos show the large round part of the casting and the oil hole. Note on the block photo how the hole is off centre in the land.
The first photo shows the result of a 'print' after my timing cover was coated with engineers blue and mated to the block. You can see how the oil gallery misaligns badly and in fact the hole centres in the timing cover and the block are misaligned a whopping 3mm. You can see in the third photo that the area of contact to achieve an oil seal is very poor at the top edge.
I don't know how many other bikes are like this, but the next time you remove a timing cover, just check where those hole centres sit. I'm not enamoured by the fix either, which is probably to fit a threaded ali plug, drill a new hole and re-face the surface.
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