I have my R75/6 gearbox apart and have been checking out wear on the gears, dogs, bearings, shift forks etc. Everything looks good except the large ball bearing on the clutch end of the output shaft (part number 23 12 1 338 795). All except that one run very smoothly with very little side-to-side play. In other words, no detectable wear, at least to my eye. The large output shaft bearing in my gearbox is a little more notchy and has quite a bit more side-to-side play than the others. From my reading, it appears common for this bearing to wear before the others. My original plan was to replace just this large output shaft bearing and call it good. However, in reading up on the subject, it appears that many Airhead riders replace all the bearings as a matter of course while they have the gearbox apart. I see that that the 5 bearing kit (minus the roller) is not hugely expensive ($130) but I suspect that many Airhead gearboxes are getting rebuilt at much higher mileages compared to my bike (76K miles). So my question is: Should I replace all the bearings except the roller or just the one that appears to have some wear? I used to replace a lot of parts as a matter of course during these restorations but have started thinking that this might be unnecessary and wasteful. I am also wondering if the bearings currently in the box might be of higher quality than the replacements available. Any and all advice greatly appreciated!
Tobin
Holy cow! Forty six views in a week and no opinions??? There never seems to be a shortage of opinions (good and bad) on the Guzzi site!!
Having never built a BMW gearbox, I wanted to allow the "experts" to comment, but will volunteer this much....
• I build or repair a classic or vintage motorcycle gearbox on a weekly basis. Some of them are easier to get at than the BMW, but the principles of design and repair are the same. I have never replaced g/b bearings "just to replace" or as a "standing rule". I wash each bearing, and thoroughly inspect. If the bearing is noisy or loose it gets replaced. For example, I rebuilt a gearbox on a 1948 Norton International this week. I did not replace the bearings. The bearings were marked "Made in England" and may have been OEM.
• The engineering vantage point may offer another view of things. Most all designs are trade offs between the available physical space, cost, and component availability. Maybe the engineer wanted the next larger bearing, rated for 1 million hours in this application. But it would mean ordering a new component for $25 each. So the boss tells him to use the next smaller size bearing, a component they are already purchasing in volume for $1.23 and it delivers 200,000 hours. This looks good on paper, becasue who in their right mind is going to be riding around on a 40 year old motorcycle ? Nobody, right ?! And together they reach final agreement, if the engineer can "CYA" by specifying a better than normal g/b oil.
Be aware that while "tight" engines burn less oil and generally run better, that is not always the case with gearboxes. Gearboxes like a little slack. So while replacing all the bearings may sound good from a preventative standpoint, there may be negative and unforeseen side-effects... such as the shifting may be stiff for the next 3 years. Or the highly prevalent Chinese replacement bearings may only last 2 years !
So my advice is... 1) if the bearings are in spec, then they are good enough for new manufacture, and 2) it's always better to deal with the devil you know.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Thanks Wobbly! That is great information. I too wonder if the replacement bearings I would buy are equivalent in quality to those currently in the gearbox!!
Fafnir, SKF, Timken made anywhere are names you can trust. But you know, they estimate that something like 40% of the items on the market today are worthless knock-offs. And if you think that only applies to designer handbags and Rolex watches then you need to think again.
The Japanese bearings, especially NSK are very good, but I have seen those go bad. Reminds me of one experience while working at a Honda dealership in the mid-70's. I had a stock Honda K5 750/4 come in with a whining gearbox. One NSK bearing was singing so loudly you could hear it over the wind and engine at 60 MPH ! I inspected all the bearings and they all tested good. Remember, on the K5 the complete engine had to be pulled and placed upside-down on the bench to get to the gearbox. Not a fun 6 hours, so I was in no mood to do it twice. I cured that bike by waiting until a K5 with straight pipes came in with a blown gearbox. I simply swapped over every bearing between the 2 bikes.
Both customers and my back were delighted !
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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