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.
I had a wonderful opportunity of installing a Kat Dash /5 LED kit on my customized California blue 1973 R100/5 (yes, it is a /5 with a R100 engine) .
I had the headlight apart already to tighten up the custom key assembly that started turning inside the headlight bucket when I turned the key.
A nice small, tidy box containing the kit arrived in the mail.
The bulbs are labeled nicely in baggies with stickers that indicate the bulb color. I wasn’t sure what the wiring was for and then read the directions and figured it out.
Side note – This bike has the custom key seen here (not a typical /5 nail style key) and LED turn signals that required I wire in resistors to create more load to activate the turn signal relay. And, I decided to adapt all the wiring to a /6 style relay board. It all makes sense to me…
But here is what the headlight bucket looked like when I opened it up (spaghetti anybody?).
The gold things are resistors I had that were wired along with the LED turn signals.
One lead of the resister was wired with the positive lead of the turn signal and the other lead of the resistor was wired with the same ground as the turn signal.
R75/5 headlight wiring with /6 Relay board
The Kat Dash kit is pretty easy.
Follow the directions.
BE CAREFUL not to twist the LED’s in the bulb housings too much. They will come ripped out of the bulb base if you do.
A few things to realize – There are additional wires with resistors added to allow the Generator/Voltage light to activate with the LED bulb.
The same goes for the Turn Signal indicator Light.
I got half way through my install when I hit a major problem – one of the LED lights popped out INTO my speedometer! Yikes! This was not any fault of the LEDs. It had to do with the bulb base being worn/bent too much to hold the bulb in place correctly. I had to take everything apart, remove the speedometer, and spend about 10 minutes with a tiny magnet until I was able to fish the light out of the speedometer’s rear bulb housing area.
If the bulb sits crooked in the wired base, FIX it before you install it into the speedometer. With my 4yr old’s fingers to help me with the photo, I was able to fix it with a dental pick and bend that lip out to catch the bulb better, and hold it straight in the housing… and most importantly, not eject the light into the speedometer housing!
I continued my assembly, and to my surprise, after I wired in the relay to allow for the new LED turn signal indicator bulb to work, I no longer needed my big gold resistors in my wiring scheme.
Kat had sold me an electronic turn signal relay that works with LED bulbs and I ended up not needing it.
Here is the bike off:
Bike with the LED running light on:
Bike with the LED lights on – So bright the gamut of the camera could not capture them:
I can’t remember the condition of this final drive when I assembled my 77 R100s back in 2003. When I had my rear tire replaced recently, I noticed how terrible the splines are. The splines on the wheel are better then the final drive but they will need replacing someday too.
I found this drive with the same ratio and cleaned it up and replaced the spline. Now it is time to swap the drives.
While it is apart, I am taking the opportunity to pull the tranny back and lube the clutch splines as well.
Ahhh… the phrases I am seeing in the ads when people are selling a complete hunk of crap. I bought the last restoration candidate knowing I was going to do a full rebuild but the owner’s ad and conversations were trying to sell me on the fact that the bike could run with an afternoon’s worth of work.
WRONG. I found all sorts of surprises when rebuilding that engine (and elsewhere throughout the bike).
The bike was an R75/5.
One issue was a deep in the engine when taking out the cam shaft. The lobes were severely pitted rendering the cam useless. The bike still would run but doing a complete rebuild was necessary to make this bike ‘right’. I have a stash of spare parts and happened to have an extra cam shaft lying around. Here are some photos of the bad cam. If you ever get this deep into the engine block (or at least have the cylinders off), take a look at the cam lobes. If they are pitted, the lifters probably need replacing too.
Here is a simple oil filter change fault.
If you don’t know the history of the bike, it is good to do a complete fluid change as soon as possible. When replacing the oil filter, make sure that the entire old oil filter comes out.
In this case, the gasket and end of an old oil filter were jammed into the rear of the oil filter galley. I’m assuming that an old filter got removed, those parts were left inside, and new filters were installed on top of them, thus jamming them in there even further. It took me nearly 20 minutes to devise chopstick like tools, with hooks on the end, strong enough to pull this stuff out.
Just another reason that the used motorcycle market is overpriced on 30-40 year old machines. Unless you know the bike has ZERO issues like this deep down inside, the cost of repairing these motorcycles is much too high compared to the asking prices these days ($4000 for an old Honda? why?).
In addition to the engine needing a rebuild, the steering bearings were rusted (which is dangerous), the splines and driving dog were almost gone on the final drive and rear wheel, and LOTS of other repairs were necessary. Again, I planned on doing all the repairs anyway but beware of the “Just needs a battery and a tune up” ads.
I am glad I am doing a complete rebuild of this bike. The steering was a bit stiff and not smooth. Once I got the neck out of the frame, I realized why.
Rusted Steering bearings.
Since the frame is going to be powder coated, the races have to come out anyway. I could see somebody buying the bike and not getting this deep into the restoration and it would be dangerous to ride with bearings like that.
These went straight to the trash.
These piston and cylinder kits seem to be the rage right now. I used one on the last build and had very little problems aside from the pushrod seals being extremely difficult to install.
The pistons are made to mate with the R75/5 heads and you can use the same carbs (might need to change jetting).
Here is a quick look at the piston and how it compares to a stock R75/5 piston that i have lying around.
It is a lighter, yet wider piston. The piston on the right has the rings on it still. The siebenrock doesn’t have the rings on it.
I didn’t measure heights when i snapped this photo but they seem close.
The piston in the cylinder. The cylinder wall is very thin compared to a R75/5 cylinder.
I haven’t heard of any issues with these yet. Supposedly they give you a 20% gain in power… so that makes it around 900cc.
I’m pretty sure they are cheaper then replacement BMW parts too.