Modifying a BMW engine to drop into a Dnepr or Ural

[fencermatt]

Howdy y'all,

A while ago, I posted with a couple of questions having to do with my project to drop a BMW engine into a Soviet era Dnepr MT-16 sidecar rig, and I promised to follow up with a post on how I'm doing the conversion. After many fits and starts, I've finally started the process. I'm hoping that, at the end of all of this, I will have the best of both worlds--a strong and reliable engine mated to a tough-as-nails sidecar rig with two-wheel drive and a reverse gear in the transmission.

The engine I chose for this project is out of a 1985 BMW R80. I wanted this engine because I think that the 800cc engine is the best suited to being a Dnepr powerplant, and by 1985, the BMW engines had factory electronic ignitions, nikasil cylinder bores, and functional unleaded valve seats. The downside, of course, is that I had to do more work to the engine so that it would mate up with the Dnepr transmission.

Stock R80 engine

Engine-Before.JPG (162.28 KiB) Viewed 8591 times

Above is the engine as stock. The entire clutch & clutch-carrier needs to be replaced with various parts from the 1960s and 1970s. Luckily, all the parts bolt in easily.

--First, I removed the clutch and the clutch carrier.

--After everything was removed, I first needed to install a flywheel from a 1970s era airhead. Make sure to get one with 11mm bolt holes (post 1972, I think), and if you get one with 93 teeth--which I did, you will also have to replace the stock 8-tooth starter with a 9-tooth starter. While I was in there I also changed out all the leaky BMW rear main seal and oil-pump gasket.

--Next, I installed a 1970s clutch, but with a friction disk from a /2 (1958-1969, I think). This is important, because the /2 friction plate has 10 splines, just like the Dnepr transmission. It is also very important to make sure that the friction disk is lined up. I bought a special clutch-centering tool in order to make sure that everything was in its proper place. Once all of that is installed, the engine looks like this from the rear:

R80 Engine with a /6 clutch & flywheel, and a /2 friction disk

Engine-After.jpg (48.52 KiB) Viewed 8591 times

Now the BMW engine is ready for the Dnepr Transmission, but the transmission is not ready for the engine. The input shaft mates up with the clutch, but the mounting holes are not lined up:

Unmodified Dnepr Transmission

Transmission.jpg (68.57 KiB) Viewed 8591 times

Welding up the transmission case is far beyond my abilities, so right now the engine and the transmission are at my local custom-motorcycle shop being modified to fit. While it's gone, maybe I'll start rebuilding the final drive . . . or not, 'cause it is football season, after all:)

[Airbear]

Football is only a game, Matt. Get on with it.

[hal]

I really look forward to the rest of this story. Good project! Would have been something for the winterseason.

[Major Softie]

Thanks for a great report.

Why do you feel the R80 engine is preferable to the R100? Is it because you think the torque of the R100 might be a bit too much of a strain on the tranny and driveline?

[fencermatt]

Why do you feel the R80 engine is preferable to the R100? Is it because you think the torque of the R100 might be a bit too much of a strain on the tranny and driveline?

That's part of the reason I went with the R80. There are people who've done this with R100 engines and reported no problems, and others who've said it puts too much strain on the driveline. There isn't much shock-absorption in the Dnepr driveline, just a rubber coupler at the transmission output.

The main reason I picked it, though, is because I don't think I could easily use the extra power that the R100 has. Dneprs (and Urals) only have 4 forward gears, and they just aren't geared for speed. Too much horsepower, and I think that, in top gear, I wouldn't be able get the engine up into its powerband before I hit the top speed of the transmission (about 65 mph).

[mattcfish]

Howdy y'all,


Now the BMW engine is ready for the Dnepr Transmission, but the transmission is not ready for the engine. The input shaft mates up with the clutch, but the mounting holes are not lined up:

Transmission.jpg

Welding up the transmission case is far beyond my abilities, so right now the engine and the transmission are at my local custom-motorcycle shop being modified to fit. While it's gone, maybe I'll start rebuilding the final drive . . . or not, 'cause it is football season, after all:)

Is the Ural Tranny bolt pattern the same as the Dnepr? I've got a friend with A Ural that has a crapped out motor. This could be the conversion for him.

[Jean]

Response to Major softie's comment about "why an R80"...The R80 is "supposed' to be the best of the airhead engines as far as longivity and smooth running. I'm not too sure of the electronics, but the mechanical parts are reputed to be a lot sturdier than the R100 FOR THE DISPLACEMENT.
Around here, making an "RS" out of an R80 was considered the "cats pajamas" and there are/were quite a lot of them runnning around the Atlanta (GA) area in the 1980's.

[fencermatt]

Is the Ural Tranny bolt pattern the same as the Dnepr? I've got a friend with A Ural that has a crapped out motor. This could be the conversion for him.

No, it isn't. I'm having a welder move the bolt holes on the transmission to match the BMW's pattern, but some other people have made adapter plates. With the adapter plate you don't need to mess with welding up the transmission case, but you do need to lengthen the input shaft and shorten the drive-shaft. I'm not planning on switching back to the Dnepr engine, so I decided to modify the tranny.

[arbalest]

Matt,

Just a piece of advice. Do not rely on the transmission mounting holes to center the transmission input shaft to the center of the crankshaft. I use a centering ring that fits over the alignment tabs on the transmission and locates the transmission to the inside of the BMW bellhousing. Another word of warning. If you modify your clutch pushrod to work with the BMW clutch, the pushrod will eventually break. Where the rubber sleeve fits in the reduced diameter area of the pushrod, there is no radius at the diameter change. This is where the pushrod will break. This is for Dneprs only. On the Ural, the pushrod is considerably shorter than the Dnepr pushrod and thus doesn't flex as much. Another problem area is the throwout bearing. The balls wear against a flat surface on the "top hat" and on the clutch "piston". The Russian metallurgy is not that good. The considerable added pressure from the BMW diaphragm spring will wear grooves on both parts. The ultimate answer is to use a BMW throwout bearing and accompanying race. This means you will have have a new "top hat" made to accomodate the additional length of the race. On the "piston side", you will have to find a hardened washer, and then machine off the face of the piston to accommodated the length of the washer. This will end the throwout bearing problem. If you are using a Dnepr transmission, you need to pay particular attention to the "Dneprmatic" adjustment. This is the little rod that disengages the clutch at the extreme throw of the shift lever. Adjust so that when depressing the shift lever, you are not exerting excessive force on the clutch pushrod/throwout bearing assembly. Also, be prepared to develop a Popeye arm from the force of the BMW diaphragm spring. You may also have some problems with the clutch actuating arm flexing. If you have any other questions regarding this swap, you can drop me a note at arbalest04062 AT gmail DOT com.

Mike Lydon

[mattcfish]

Is the Ural Tranny bolt pattern the same as the Dnepr? I've got a friend with A Ural that has a crapped out motor. This could be the conversion for him.

No, it isn't. I'm having a welder move the bolt holes on the transmission to match the BMW's pattern, but some other people have made adapter plates. With the adapter plate you don't need to mess with welding up the transmission case, but you do need to lengthen the input shaft and shorten the drive-shaft. I'm not planning on switching back to the Dnepr engine, so I decided to modify the tranny.

Why can't the Airhead tranny be used with a modified or custom built drive shaft?