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Lag/play between input and output shaft on M97 transmission


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I purchased a used transmission online from a '98 R1100RT with only 18k miles to replace the ailing transmission in my '95 R1100RS.


I'm attempting to bench test the used tranny according to the method proposed by Duane Ashermand (http://w6rec.com/duane/bmw/trans/). While turning the input shaft by hand using an old clutch, I can shift between all gears, although not very smoothly or succinctly. Sometimes misses or shifts late. (Kind of like if you tried to shift through all the gears while stopped) I can sometimes go through all gears on the 1st try.


My question: There is some lag between the input and output shafts if I say hold the output shaft with one hand, and turn the input shaft back and forth with the other hand. It's around 20 degrees rotational movement from stop to stop. Note there is no side to side or axial play, and no seal leakage. Just rotational lag.


Is this normal? Or should the input and output shafts have no lag?


Also, how true to life is this testing procedure?

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Evening SnakePliskin


That rotational play between the input shaft & output shaft is normal on the BMW transmission. You have a spring loaded dampener in the input shaft, then there is some slop in the actual gear tooth mesh between the internal shafts, then you have the place with the most play & that is the shift dogs to their splines & to the gear slots. Those shift dog spurs need a fair amount of slop in their respective slots or making a shift between different speed shafts would next to impossible.


That missed shift or late shift is due to the shift dogs not instantly sliding into the matching gear slots (difficult to tell if that is normal of not without feeling it myself)--More than likely normal but until you install the trans, fill it with gear oil, & actually ride & shift the trans then you won't really know.


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Thanks D.R. for the technical explanation! Glad to hear that it is normal. 'Slop' is the word I was trying to remember.


Since my post, I was able to remove the swing arm housing from my bike and expose the drive shaft (before popping it off the output spline). I put the bike in first and grabbed the end of the drive shaft and rotated back and forth and it seems to have the same amount of slop as the used RT tranny.


Note that when I first bought my bike 8,000 miles ago, I loved the 2nd gear torque, and would shift cleanly, but quickly, from 1st to second and take off hard in second gear. I used the clutch properly and didn't think I was hurting the bike, but noticed after a little while, there was slightly more 'slop' and clunk like say when moving from coasting to accelerating in first gear. I stopped doing the hard second shifts and it's been fine for 5000 miles. I wonder if this extra play developed in the tranny, and this used tranny had the same treatment??


Note that I just checked, and my driveshaft and rear drive, measured separately, have minimal play. So the clunk I felt when riding must be in the tranny. I had previously assumed it was farther back.


And just to be clear, I'm replacing the tranny because of the 'reshift' problem in 5th gear you helped me diagnose earlier.

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  • 2 weeks later...

I'm happy to report that the transmission swap went off without a hitch (just a bit of sweat) and it shifts wonderfully, a little smoother than before, and without the 5th gear 'reshift' problem. :clap: Thanks again dirtrider for the reassurance!


I used the following guides primarily:


A very helpful step-by-step. I made a spreadsheet of this so I could tick off my progress, rearrange a few steps, and add notes.


There is a little language in Chris' video but it is very helpful in walking you through and alerting you to several pitfalls. I had it playing in the background the whole time. He was generous to produce this and put it up.


I had the Haynes manual but did not use it much.

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