Got my R75/6 engine apart and am measuring up all the bearing journals and examining the bearings. Front main bearing has a bit of scoring and a small wear patch on one side. See photos. This wear patch is at about 3 o'clock looking at it from the front of the bike. Almost looks like there was a high point on the original bearing that has gotten worn down or that the front and rear main bearings were not in perfect alignment. Interesting. Would like to hear some ideas as to why. Big-end bearings are absolutely perfect with no detectable wear so finding wear on these main bearings was a surprise to me. Bottom end of engine has never been apart on this bike as far as I can tell. 76K miles. When I drag my fingernail across the score marks, I can barely detect them. Rear main bearing looks fine. All journals on the crankshaft are right in spec and are perfectly smooth. Is this front bearing something you would replace? If I replace it, should I replace both fore and aft? Any and all advice greatly appreciated!
Tobin
Several thoughts....
► First of all, that's not wear. Wear would show up as the copper under-cladding showing though. You have all the Babbitt intact. Those little irregularities are trash in the oil, which due to their placement had to be after the oil filter, as you follow the flow of oil. Probably trash in the filter itself. And when we say "trash", we're talking about microscopic particles.
► I suggest you replace all the plain bearings. Not becasue of wear, but because of embedded material. The Babbitt is so soft that metal debris gets embedded in it, so in a sense it's acting like a filter. After many thousands of miles the amount of material embedded in the Babbitt starts to wear the crank journals. When I went to engine school in the 1970's that number was "about" 50K miles.
► In a plain bearing cap, like the rod big end, we use a product called Plastigauge. This product is a piece of precisely made plastic "wire" which is laid on the journal and crushed between the crank journal and the bearing cap during a dry test assembly. The width that the Plastigauge flattens out to will tell you the bearing clearance. That "clearance" is actually the oil film thickness.
The mains are not assembled in the same manner as "halves", so I have no idea how the proper clearance is gauged. Probably by the spin of the crank before rods are added.
► Obviously, your work must be done in a super clean environment to replace plain bearings. Not only that, but the new main journals must be pressed in square with their blocks, and not cocked to one side. Then the faces of the front main holder must also be exceptionally clean so that it can bolt-up in an aligned position. Until the oil pan and cylinders go back on to seal the engine, your entire shop must be as scrupulously clean as an operating theater. And your tools need to be as clean as the parts they assemble.
I need not mention part cleanliness. You'll need access to lots of hot soapy water, compressed air, and clean shop rags. All the work is in the unbelievable number of hours done in preparation. Assembling an engine is fun; doing all the preparation to assemble an engine is a royal PIA.
► There are specialty "assembly lubes" used during final "wet" assembly. You'll also need some LocTite thread locker for the various journal fasteners. You'll also want a product called "P80" for oil seal installation. Maybe a sealant for unions like the oil pressure gauge. There I suggest LocTite 510 gasket maker.
Best of luck.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Thanks again Wobbly! Great tips there. Will plan on replacing both bearings and will endeavour to make the process as antiseptic as possible!!
Thanks again Wobbly! Great tips there. Will plan on replacing both bearings and will endeavor to make the process as antiseptic as possible !!
"Both bearings" ? I meant to suggest ALL plain bearings, including the rod big end inserts.
I mean, as long as you have the money gun out and are happily blasting away !! 👿
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Even if the rod bearings look perfect? I have never seen rod bearings look so clean with no visible wear. Guess I could measure them to make sure. On the other hand, they are not that expensive and if I replace everything I'll have a brand new Airhead!!
Even if the rod bearings look perfect? I have never seen rod bearings look so clean with no visible wear.
You haven't given us very much to go on. No photos, no measurements, only a 76,000 mile odometer reading.
Allow me to remind you my friend, you are about 1/10th of the way through this undertaking, and spring is coming.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Will get you some photos soon! Don't have any of that Plastigauge stuff for measuring. Maybe I need to get some!
This is complete restoration number 8 for me (British and Guzzi up to now) so I have a pretty good idea what I am up against. The BMW part is all new for me though and has been fun. Hoping it will come together quickly once I get all my parts lined up ready to go. Have to make some decisions about electronic ignition or stay with points, new exhaust or no and a few other things like that. Frame is powerdcoated, tinware is heading off to painter soon so I am pretty sure I'll get 'er on the road this summer. In the meantime, I have other bikes that need saddle-time!!
You haven't given us very much to go on. No photos, no measurements, only a 76,000 mile odometer reading.
Rod bearings:
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