Disassembly of an airhead R75/5 BMW final drive

Here is a photo essay of sorts detailing the disassembly of a 1973 BMW Final drive. The same applies to all 1970’s era BMW’s (R90 R100 R75 R80 /5 /6 /7 S etc). The seals were shot inside causing leaks. The splines are decent so no need to weld on a new set of splines. I plan to have the drive blasted and powder coated to match the rest of the bike for an upcoming build.

The image descriptions have notes on the disassembly.

Took Apart and Cleaned the BING carbs

These are the 64/32/9 and 64/32/10 BING carburetors. They were standard issue on the R75/5 BMWs back in the early 70’s. From what Siebenrock says, they will work just fine with their power/performance upgrade piston/cylinder kit. They may need different jetting but I’ll cross that bridge when I mount them to an assembled engine.

I finally got around to looking into them. Holy crap, they are used and abused. I have two sets, one from the parts bike and another from our ‘main’ bike.
One set has been modified by a previous owner who added vacuum ports on them. The ports look bad, and who knows if they are completely sealed so i’m going to shy away from using that set of carbs. Too risky.

The other set is nasty but can be rebuilt. The diaphragms are OK but everything else leads me to believe they have never been rebuilt before. The only drawback to using these carbs is that the BING nameplates are trashed and replacing them is a tricky job that requires precisely drilling out each metal nail. It may require many hours and machine work that I’m uncertain about. I’m thinking that I will polish the crap out of those plates instead.

Here is the carb, dirty and ready for a new life:

Here goes:

Choke removed, needles coming out:

Everything apart (an unofficial diagram):

The second carb had a slight issue… One of the butterfly screw heads turned to putty and stripped when I tried to remove it. I had to drill it out, and bastardize some parts from the second set of carbs to replace the pivot arm that the butterfly attaches to.

I was lucky enough to receive an Ultrasonic cleaner as a gift earlier in the year. I LOVE the idea of NOT using a highly chemical, messy, toxic carb cleaner kit. So I poured a bit of simple green in the tank, a small dab of laundry soap and some hot water.

After about 20 minutes of vibrations, the carbs came out nice and clean.


Next steps:
Time to buy the complete BING rebuild kit for $210. And maybe blast these carbs for an even cleaner surface.

Rotor Removal

Once again, I had to return to the old donor engine block for some spare parts. The rotor on the engine to be rebuilt was a rusted mess. The rotor on the donor engine looks brand new. I will test it with an voltmeter to be safe, but being that it is so clean, i am not too worried.

To remove a Rotor, you need to have the rotor puller bolt.
BMW sells it, Huckey BMW sells one, Motoelekt.com sells one… it is between $5-$10. Or, you can search online on how to make your own. (click the photo to enlarge it)

Once you removed the alternator,
To remove the rotor, all you will need is a 6mm hex bit and a 13mm socket.

Loosen the rotor bolt with the 6mm hex bit and remove it completely. You may need to block the engine from rotating.

Then screw in the puller bolt.

Begin slowly tightening it with the socket until the rotor pops off the crank. That’s it. Done.

POR 15 Gas Tank cleaner and coating

I’ve used this product before with reasonable success on about 3 or 4 motorcycles now. In one instance, it started to peel from the roof of the gas tank and large chunks would float in my gas. Once i removed them, everything was fine.
Something must have gone wrong with the prep or paint process. I have a feeling it was from the tank drying upside down and the POR15 dried in a puddle that eventually broke loose.

I have a bunch of tanks sitting around and an extra kit of POR 15. Unfortunately, this process takes nearly an entire day.

Follow the directions and you should be good to go.
Try to do it on a sunny day.
It really helps to have a heat gun to speed up the process of drying out the tanks.
Prepare to get wet or splashed.
TAKE CARE TO PROTECT THE PAINT OF YOUR TANK. If any of the 3 step process gets on the tank, it may leave a streak and damage the paint.
Wear Gloves. (see below)


The vicitms... ready for a day in the sun.

The first two processes require lots of sloshing liquids inside the tank. Then drying. A heat gun helps speed up the dry time. I hair dryer may work too but not as effectively.



This photo was taken after the first step, 'Marine Clean' was used. The Marine Clean removed a ton of the rust inside. When i drained it, the water came out Rust-Red.

It looks like a previous owner used the tank cream or something similar. These chunks, and many more came out after the second process 'Metal Ready'. I ended up using a long screwdriver and tried to scrape off as much of this crap as i could. It was a previous tank coating gone wrong. There is still remains of it in the tank. I hope it doesn't cause problems later on. The POR 15 is such a good sealant, i think it won't be a problem.



Tanks sealed. This process is a messy one. It would help to have an old set of petcocks to use while you slosh the sealant around. then remove them to drain the sealant and dry.



Out on the porch to dry. They were originally in the garage but they stunk up the whole house.

I went Michael Jackson style and only wore one glove. I got the sealant on my other hand and it sure doesn't look like it is coming off anytime soon. I used paint thinner and lots of scrubbing and... well... the wife isn't happy a day later... opps!

Reassembly of transmission and final drive

I’m definitely not going to take on these daunting tasks myself.  Just look at all those parts!

Someday when i have lots of extra time and extra transmissions to experiment on, i’ll try to figure this stuff out myself.

But for now, they are going to the hands of  an expert.  Luckily everything fit in the saddlebags of my R100s.

Somehow, those parts are going to fit into that shiny housing.

Final drive guts

Trany guts. Looks like a clock exploded.

BMW parts inside of BMW bags.

Testing engine paint

My motorcycle lift has become a test center. I plan on fuel injecting my 1972 BMW 2002 someday… so i have some car parts to test high heat engine paint on before i apply it to the motorcycle bits. Last time i had great success rubbing paint on. I may use the same paint/technique… however the paint i want to use looks best when i spray it… oh decisions…

A plethora of BMW aluminum.


I love bare steel.
I got a call from the powder coat shop yesterday. My parts are blasted. I went down there today to go over what needs to be masked and not coated. I spent about an hour roughly taping things off. I can’t imaging how long it will take somebody to mask it all properly.

hubs, fork sliders, center stand, final drive.

Frame (and somebody's car rims).

Little bits. Final Drive.

Even smaller bits.

Some Stop Tech brakes.

More Stop Tech calipers. After they get coated, there is a crew of women who screen print the logos on them.

Men at work... plugging and masking.

Men at work... spraying.