1973 BMW R75/5 vintage race bike
(how many errors can you find on this picture? I know of four.)
This is the current condition of my race bike project. Notable work has been:
- Lots of new parts from Battley Cycles (listed further down the page)
- 336° race cam installed by Tom Cutter
- Woodcraft clip-on handlebars modified by Accu-Products (to fit 22mm BMW controls)
- Flywheel lightened by Randy Long (pop-up | URL)
- Frame, tank and fenders repainted
- /6 front disk brake with K75 master cylinder (not enough braking power yet)
- Mikuni carbs and Hagon shocks from Rocky Point Cycle
- Removed starter, alternator, lights, accessories, etc.
- Drilled oil feed to 7/16"
- Fabricated minimal wiring harness (pop-up | URL)
- Raask rearset footpegs from Omar's Cafe (apparently closed; was Omar's Fiberglass for a while).
Note: the only 336° cam still offered by BMW for the /5 has the large seal area, while the /5 engine cover has the small seal. I had to swap for a later engine cover with the large seal. There are large-seal engine covers available without the tachometer cable cutout which would be even better. Additionally, the 336 cam comes with a new inner oil pump rotor, since the cam takes a 2-flat mount and the old rotor is keyed. My rotor was VERY difficult to mount on the camshaft end, and needed some filing.
Still to do:
- Reshape seat
- Improve front brake (smaller master cylinder and/or larger caliper piston)
- Raise compression or at least get properly-fitting pistons
- Work on suspension and ride height
- Add tachometer and neutral light
Here's a picture of me in the Airheads race at Talladega Grand Prix.
Lessons learned so far:
The one-piece pushrods don't clear the pushrod tubes with the 336 cam. I had one of those pushrods and three of the three-piece ones, and the inside tip broke off while running on the track.
With the engine in the stock position, and good tires, cylinder head clearance is the limiting factor for cornering no matter how much I hang off. Raising the engine is necessary.
History:
|
2/8/03 I took delivery of this bike, a '73 R75/5. Originally silver with black pinstripe, it had been purchased in Germany and brought to the US, taken back to Germany, sold to the second owner, and then brought back to the US again. It's on its 3rd speedometer but the mileage is probably about 140,000. The original large tank was exchanged at one point for a smaller one when the /6 front end (with disc brake) was fittted, and the whole bike (including larger Acerbis fenders) was painted in a desert tan as part of a GS-type conversion, of which the dual-sport tires and wide handlebar remained at the time I took possession. A bottom-up engine rebuild was completed about 5~10k miles before. The paint on the tank is stained from gas leakage. |
 |
| The transmission had a problem where the kick-start pivot shaft loosened (probably by me trying to start it) and fell into the box as I was trying to fix it from the outside. It's been repaired by Tom Cutter. This picture reflects the start of the cleaning efforts on the engine and frame (right side) and you can see the oil weeping around the end of the pivot rod (directly under the pushrod, slightly hidden behind the frame crossmember). The oil on the swingarm, at the top of the picture, is probably due to leakage past the swingarm boot. |  |
The bike was in mechanically rough condition when I received
it. In short, I found:
New parts include:
|
 |
| Here's the clean engine block, showing the inner crank thrust bearing. You can see the two locating pins that keep it from rotating; if the crank moves forward the bearing can come off the locating pins and rotate out of alignment. If this isn't detected, then when the crank is pushed pack into position (as the front bearing carrier is bolted on) the pins will push into the bearing material and possibly damage the crank and the block. Although my bearing had become misaligned at some point, the crank and block were mercifully undamaged and there was only a slight punch-mark in the flywheel where the pin had been pushed backward (the same pins locate the thrust bearing on the other side of the engine partition, where the flywheel is). |  |
| Here's the complete short block ready to be reassembled. It's so clean! |
 |
| Since the rotor forms part of the timing cover seal, I needed to find another way to seal the engine with the rotor gone. A 47x22x4 seal fit perfectly and seals directly against the crank taper. |  |
Just for yuks I weighed some of the parts that came off:
| Part removed | Weight (g) |
|---|---|
| Alternator & rotor | 2053 |
| Choke assembly | 279 |
| Starter | 4550 |
| Taillight ass'y | 1057 |
| Centerstand | 2075 |
| Bars and grips | 2400 |
| Headlight and wiring | ~3000 |
| Misc electrical | 700 |
| frame trimmings | 310 |
The flywheel weighed 11 pounds!!! Now it's less than five (including removal of the ring gear). Still no idea what the bike weighs.
Oh, by the way I am getting fond of these bikes and now have another /5 for street use.
Links
Nick Greear's /5 wiring diagrams
MARRC (and racing school)
R75/5
){kind=link}
