Gearboxes on the aircooled boxer motors have really been given a bad repution, and unecasary so because they are so simple in design and are incredably robust.
Discussing a few problems that is the reason for their dimise, is firstly that it is importand to learn how to shift smoothly.
Even a very clunky box can be shifted with almost no resistance or niose if you keep the revs in the right place and with light presure on the gear shifter that will allow it to drop right into gear when the dogs are aligned.
Next, service the oil in the box when you do so for the motor witch should be at 4500 miles or 7500km, or not more than 6 months.
Changing more regularly will also reduce the amount of water colecting in the box.
Now the reason why water enter the box is by ways of the speedo cable.
When you look at the right side of the bike where the speedo cable enter the gearbox you will find a cone shapen rubber sealing of the cable as it enters the box.
This rubber is the problem as they start to grow hard and crack with age.
It cost approxematly 2$ and will take 10 min to replace. All you need to do is to use a 10mm spanner to undo the small bolt holding the earth cable from the battery. This bolt also hold the speedo cable in position. when it is completly removed you can pull out the speedo cable with the rubber crommet and a cylindrical plastic holder. By tugging hard you can pull the rubber grommet of, in most cases it will tear of. Fit the new one using some grease to help as it fits very tight.
Do not forget to replace the earth cable when refiting and make sure the recess in the plastic holder is turned toward the thread of the bolt.
The next will be the single biggest problem with these gearboxes. In 1982 BMW enginears decided to omit a small circlip on the front of the output shaft, directly opposite the drive shaft. This circlip has the very important job of holding a bearing in place on the shaft directly next to the helical 5th gear. Because of the helical design of this gear it exercise a lateral force onto the inner race of the bearing and ball tipe roller bearings are not desiged to take lateral forces, the bearing is bound to collapse after high milage. This causes tremendous damage to the fith gear, intermediate shaft and sometimes the output shaft and selector forks. These items being the most expensive parts in the gearbox.
It was only in 1996 after these models were allready taken of the market that BMW started to sell the output shaft in conjunction with the circlip.
To rectify the broblem you need to have the gearbox overhauled of coarse, sometimes you might be lucky and there is a croove in the outputshaft for the circlip but in most cases an engineering shop will have to cut one.
When the new bearing get fitted it is very importand to make sure the one from BMW, part no. 23 12 1 338 795, is fitted. This bearing is recently introduced has been changed to accomadate the circlip without a small wire clip, witch have a beter fit.( I have seen where even the circlip have been forced out of its groove by this presure from the fith gear.)
The BMW part is also more refined to give a longer live than stock bearings.
To check if your box is due for an overhaul you can place it on the centre stand and with the bike in gear with someone holding the clutch you can feel any roughness in the bearings by turning the rear wheel.
Bigger than normal metal particles on the magnetic drian plug is also a good indication of a worn bearing.
Last but not least, do not atempt this job yourself if you do not have all the special tools and a good manual, Haynes will not do as they do not explain how to set the endfloat of the shafts to well.
Check with your dealer that the right bearing go on the output shaft and most important the groove with its circlip behind it.
If this is done together with regular sevices you should do more than a 100 000 miles before the next overhaul.
It is important to change all seals and other bearings at the same time.