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Old 10-16-2012, 12:18 PM   #29
MJ Gurunathan OP
Adventurer
 
Joined: Apr 2008
Location: Singapore
Oddometer: 10
Wink Update: Repair procedure on cracked gearbox

Good day all.

I would like to share my experiences with all of you, so that I can help anyone who faces the same issues that I did. Sorry this update took so long, but my health hasn't been too good. Finally got the job done though!

Synopsis:

Motorcycle Model: 1994 R1100GS, M94 transmission.

1) Whilst removing the RHS pivot pin for the swingarm from the gearbox, excess force was applied to a seized pin. This caused the housing to crack as in this picture (cracked portion is circled below):


DSC03288-1 by MRF3343, on Flickr

Tips:
a) Use the correct threadlocker when installing the pin.
b) Heat the gearbox around the pin prior to removing it. The heat will soften the threadlocker and make it easier to remove the pin.
c) If the pin seizes, DO NOT FORCE THE PIN! Drill it out and carry out a thread repair using a wirethread insert.
d) Oversize holes can be repaired using a solid insert. Cracked holes must be welded, the case crack tested and then retapped.

2) The crack was caused by excess force that was applied to the swingarm pivot pin. The gearbox was stripped down to facilitate repair. Here are some pictures of the internals of the M94 gearbox:


IMG848 by MRF3343, on Flickr

And again:


IMG861 by MRF3343, on Flickr

Tip:
a) Take pictures as you carry out each step in the disassembly process.
b) Put all parts in plastic bags. Label them with the BMW part number.
c) Match spacers to shafts, bearings to races. These parts are already bedded in.

3) After the gearbox was stripped down, it was cleaned in a solvent bath and then plastic-blasted to remove all traces of the original coating. The gearbox cases were then placed in an oven set to 140 deg. C for about an hour. This was done to drive out any oil or solvents that remained in the porous aluminum. Once I was satisfied that the gearbox was clean and oil free, I handed it to the welder to start the welding process. Here are some pictures of the gearbox after cleaning and plastic blasting:


IMG925 by MRF3343, on Flickr

And another view:


IMG946 by MRF3343, on Flickr

Tips:
a) The paint on BMW gearboxes is very tough. To save time when blasting, soak painted parts in paint stripper to loosen the paint. You can then blast the paint off with some ease.
b) Use kerosene to clean the gearbox, then a solvent degreaser.
c) Heat parts in an oven to drive out oil and gases from the porous aluminum.
d) Plastic bead blast the gearbox casings. Plastic blasting is gentle and will not harm the parent metal. Accidentally blasting a bearing seat or gasket surface may damage it if you're using highly abrasive media like alu-oxide.

4) The gearbox is made of an aluminum alloy. Some form of inert-gas welding must be used when welding aluminum. In my case, I TIG (tungsten inert gas) welded the gearbox. The material was built up in small sections around the cracked area. Once the crack was built up, the threaded hole was built up to repair sections of thread that had chipped off. Buildup was continued until the hole diameter was less than 24mm. This is because the pin has an M27X1.5 thread; a 27.0mm-1.5mm = 25.5mm hole must be bored to allow the thread to be created. Below are pictures of the gearbox during the welding process.


IMG926 by MRF3343, on Flickr

And again:


IMG927 by MRF3343, on Flickr

After blending:


IMG944 by MRF3343, on Flickr

Tips:
1) Make sure the affected case is clean and free of oil, dirt and grease before welding. It is important to heat the part in an oven to drive out anything that may 'gas out' during the welding process. This can cause weld porosity.
2) It is a good idea to build up more material than is necessary. You can always machine the part to size later.
3) If possible, leave a small amount of undamaged thread to act as a hole center reference and as a guide when tapping the hole.
4) If possible, do not weld completely the flat surface next to the pivot pin hole. You will need a small portion of undamaged material to act as a datum to machine the face flat.

5) Once welding was complete, I started planning the machining of the affected area. To set the workpiece up correctly, I needed two datums; one to set the gearbox parallel to my machine table, the other to set the gearbox perpendicular to the table. I set the gearbox up on a milling machine as in the picture below:


IMG1177 by MRF3343, on Flickr

Here again is the setup:


IMG1178 by MRF3343, on Flickr

And another view:


IMG1181 by MRF3343, on Flickr

The centre of the hole was conveniently found to be related to the distance between centres of the footpeg mounting holes. A pin of the correct size was held in the machine spindle and the table adjusted so that the pin could slide easily into the footpeg mounting hole. I then used an edge finder to set the workpiece zero. This hole centre was confirmed by running a threaded rod into the pivot hole on the other side of the case and seeing if it would clamp neatly in the chuck of the milling machine. I also confirmed the hole centre distance from the mating face with the angle plate by comparing it with a measurement (previously taken) of the hole centre distance from datum of the pivot pin hole on the LHS of the gearbox.

I then bored the hole to 25.4mm ID (0.1mm allowance) by pocket milling. You can see the machine in action below:


IMG1183 by MRF3343, on Flickr

Sadly I had to de-rig the job as I had to start on another project. Not good practice because this meant that I lost the hole centre and would have to setup the job again (clocking the angle plate, finding hole centre etc) in order to accurately tap the hole.

Tips:
1) Again, make sure you leave some portion of the original hole undisturbed. This will help you find the hole centre much more easily.
2) Assemble all your tools before you start. It is best to machine and tap at one go. This will help you save time on workpiece setup later.
3) Check and recheck dimensions before starting machining to avoid expensive mistakes.

6) Once I had some free time, I headed down to the shop to start tapping the hole. I used a dial gauge to indicate hole center and then tapped the hole. The picture below shows how the tap was set up:


IMG1216 by MRF3343, on Flickr

Tips:
a) Make sure you have your hole center correct. Any misalignment might cause the swingarm to shift or the swingarm bearings to wear and will affect the driveshaft alignment as well.
b) Tap slowly. Make sure the tap is perpendicular to the workpiece.

That's it! Job done. I'll powder-coat the cases and then reassemble the gearbox when I have more time. I shall try to get bearing sizes and seal dimensions so that you won't have to go back to BMW to buy bearings and seals. Saves a great deal of money.

MJ.
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