Re: Project "walrus" Yam XS 750
Shamelessly quoting senlisian on the yam triples forum. Lots of meat regarding crank bearing sizes in this thread:
http://yamaha-triples.org/forums/thread-view.asp?tid=14331&mid=122807#M122807
 Posted 2013-05-21 9:45 PM (#122807 - in reply to #98881)
Subject: RE: Conrod & main bearings  Offline
Regular
Posts: 144

Location: FranceHi guys, I have found the answer to your questions & frustrations...
It's very simple in the end - the XS750s have 2 types of crankshafts with different tolerances.
The first type has a set of 5 possible bearing colours going from blue (thickest) to yellow(thinnest). On this crankshaft there are no markings on the face of the primary gear.
The second type has a set of 5 possible bearing colours going from RED (thickest), then to blue, then on to green (thinnest) - yellow doesn't apply to these cranks. All the other colours in between are the same for both cranks. On this crankshaft there is an 'X' stamped on the face of the primary gear.
The calculation used for the first type of crank is case number - crank number = bearing code.
The calculation used for the second type of crank is case number - crank number - 1 = bearing code (0=RED). This calc is used ONLY for the main bearings, not for big-ends.
The plastigage clearances quoted in previous postings on this thread for 'early' & 'later' cranks are correct.
The reason for this change is that there are mods to the oil passages between the main & big-end bearings to provide more reliable lubrication to the big end bearings at higher revs.
The internal passages for the main bearings aren't drilled through both sides (1 hole instead of 2 - see picture below) as they were for the early cranks. This was done to reduce oil loss due to higher centrifugal forces that could induce aeration of the oil. Hence the revised tolerances for the later cranks.
If anyone is interested in the actual tolerances that the stamped figures on the case, crank, and con rods represent then I can post these as well.
Concerning the 850s I have no definitive info other than the main bearing size changed to be the same as the XS1100 so an educated guess would be to go with XS1100 values (if you can find them
).
PS. Credit where it's due; it all came from the 'revue technique'.
Edited by Senlisian 2013-05-21 9:47 PM