Another Crank Rebuild Question

[2XSive]

So I'm rebuilding a spare motor. 1981. Checking the connecting rods big end side clearance I'm getting closer to .58mm, and the max spec is .40mm. Side to side at small end is within spec. Does that mean the crank has begun to separate? Big end bearings seem tight otherwise. I'm a firm believer if you're going to do something, may as well do it right. Should I just run it, or am I better off sending this out for rebuild or for those DIY-ers, how difficult to split, re-press, and true up the crank? Already split the cases to put a 5th gear overdrive in (that part is done). I do have a press, but have read a lot of threads on the ins/outs of rebuilding a crank on your own and truing up the crank is the tricky part. Just looking for advice from those more experienced. Never done a crank before....

 

[TwoManyXS1Bs]

From this thread (post #92)

http://www.xs650.com/threads/1980-x...-the-rootbeer-bobber.22952/page-5#post-397515

You'll want to start at post #60 in that thread.

Might be something with the big-end shims. They could be worn down a little, or:

Early in '70, the cranks were modified to use shims p/n 256-11685-00, with side clearance spec of 0.012" - 0.024" (0.3mm - 0.6mm).

Then, somewhere around '76-'77, the rod shim part number was changed to 90209-26014-00, with side clearance spec of 0.006" - 0.016" (0.15mm - 0.4mm).

This new shim superceded the old shim, so it shows in the revised parts manuals for the older models. There's no info I can find about the specifics of the change, wondering if they're the same thickness. Note that the rod side clearance specs tightened up about the same time as this shim change.

What you'll want to do is get the rest of your crank dims, particularly the wheel face spacings.

An old Bob B. pic.

 

[2XSive]

Thanks 2M! I'll take further measurements....stay tuned and appreciate your help!

 

[2XSive]

So I was able to take a couple of more measurements. The crank width measures within the spec of 66mm both sides, as does the center pin interior measurement of 54mm. The free play on the small end of each connecting rod measures .025", which I thought was quite good. In the pic above it shows acceptable con-rod side clearance of up to .6mm. I'm at .58mm, but the manual for an 81 indicates acceptable limit of .4mm.

The only other concern I have is with crank runout at each end. I'm assuming the correct approach to measuring this is by putting into a truing stand. When I take one measurement on the rotor side, there are sections that measure .025 and other areas that measure .035. Gonna pull the crank and weld up a truing stand to get better measurements on runout. If I send this into Hugh's, I might just as well consider re-phasing this motor to a 277. Not that much more to get it done, except for the ignition.

 

[TwoManyXS1Bs]

So far, it sounds like the crank hasn't spread. A good sign...

 

[2XSive]

It does seem like the shims are the issue. If they are in fact worn to the point of being beyond the .40mm spec, does that warrant rebuilding the crank? When measuring crank runout at the point of where the seal seats on the crank on rotor side, i get a consistent .0015 which is within spec. When i move dial gauge outboard to farthest point of taper just beyond woodruff key location, i get .0025.

 

[TwoManyXS1Bs]

Measuring runout at the seal contact area is correct.

I have no info on the thicknesses of the 2 versions of the conrod big-end shims. The relaxed nature of crank and crank parts interchanges between '74 to '83 "447" cranks implies that the side clearance may not be that critical. The manuals do indeed make specific clearance call-outs, changing with the new shims in the 76-77 timeframe. And, since crankwheels, conrods, and crankpins didn't change, the only way to change the side clearance is by different thickness shims. This is a somewhat grey area.

These cranks are "big end regulated", hence the specific call-outs for big end clearances.
Post #9 of this thread contains a pic describing "conrod regulation".

http://www.xs650.com/threads/correlo-rod-oil-feed-to-bigend.39332/#post-394748

My gut tells me that Yamaha may have slipped-in this "tightening-up" of the big-end clearance in response to some unknowns. Maybe they found that the tighter clearance was okey-dokey, and would lead to longer conrod life. Maybe they found that their older spec was cautiously unnecessarily loose. Dunno. I haven't seen any bulletins covering this. There *was* the infamous transmission gear/spline change in that same timeframe. May have been part of an engine upgrade program back then...

 

[TwoManyXS1Bs]

..Already split the cases to put a 5th gear overdrive in (that part is done)...

This other issue. Did you check the OD 5th gear dog pockets for proper undercutting?

Glennpm's epic thread:

http://www.xs650.com/threads/overdrive-5th-gear-dropping-slipping-out-to-neutral.41539/

 

[2XSive]

Wow, thanks for that advice! I'll read through the thread you referenced. I did replace both the gears together to be on safe side, but pinion probably wasn't necessary from what I've read.

 

[member3480]

This is who I have used for 35 years Good People

 

[dps650rider]

I just bought a '75 crank for ebay mainly looking for good OEM rods. Bearings all appear to be good, rod top ends look great, rod small end side to side play is good. However, when I checked the wheel widths one of them is 66mm and the other is 66.1mm. Does it sound like this is spreading?