Friday, 1 July 2022

BMW engine conversion thoughts

 Briefly reviving this blog to answer a question that comes up every few months on various commie bike forums/pages. This is just so it saves me having to write the same answer every time. Other than that, I got bored of spending ages writing drivel no one seems to read...

So: in 2016-18, I rebuilt a Dnepr MT of some kind with a BMW engine, completely new electrics, and everything else reworked to iron out the terrible build quality, and literally criminal levels of incompetence of a "restorer". 4 years and 5000 miles later, I think I've ironed out most of the mistakes, "temporary" bodges and got a pretty decent outfit from it.

The spec is:

Some kind of Dnepr MT chassis

1982 BMW R80 engine, light flywheel, electronic ignition, all standard BMW.

Dnepr gearbox and final drive

Front disc brake, hydraulic morris minor drum on the side wheel, standard rear brake (inc parking brake)

12v electrics

Twin fuel tap tank

It has done the Dragon rally twice, several long motorway trips (150-200 miles each way), and probably annoyed everyone at NABD 2018 by winning best sidecar, despite not having £20ks worth of chrome and paint. It's done a bit of green laning, handles like a dream in snow, carries around a quarter ton of bricks, can sit at around 65-70mph for longer than I can not need a wee (not while carrying the bricks though), and I flog the arse off it everywhere. I'm dead pleased I made it.

In more detail:

Dnepr frames fit the BMW engine perfectly, I'm told Urals need the rails jacking apart. Frame rail does get in the way of the oil filter cover, but I've found I can pull the engine bolts out and drop the motor in the frames to get to it. Everything else stays connected, bar the rear exhaust mounts. Some people fit external filter kits of various types, others have a detachable frame rail. I can't be arsed to do either. Every bearing has been replaced, and every spacer and widget remade to closer fits, better quality and in stainless steel. Expect the fork bushes, which are brass. They should probably be some kind of bronze though. Wheels have new UK sourced rims (they're a standard size) and new spokes, again OE quality is dire.

Any BMW type 247 engine will fit. These are the R60, 75, 80, 90 and 100, from the early 70's /5 to whatever the last airhead twin was. (Note, not the R65, completely different engine) The bigger, the better, and if you even think of breaking an R90S or early /5, you want your teeth knocking in. Engines up to 1979 had a bigger clutch and flywheel, which can mate to the Dnepr box by fitting an R60/2 plate. After 79, they had smaller clutches, a chap in Germany did me a custom plate. You can mix and match BMW engine bits, but be aware that if you change clutches you'll need to reshim the engine. Go to Snowbums brilliant (yet long winded and mildly confusing) website for more info:
Airbox is the Dnepr one, with air hoses to the stock BMW carbs made from garden pond hose. Exhaust is, now, stainless steel. Stock BMW headers with the ends chopped about to fit the stock BMW silencers, which in my case are the much later, shorter ones.

 Dnepr drive trains are apparently stronger than Urals. My gearbox internals look utterly horrible, the box sounds like two skeletons shagging in a dustbin, but it has given me no trouble. Again, new good quality bearings (ask for C3 grade, they have more clearance). The gearbox is adapted to fit the engine by welding up the bell mouth and remachining. Crude, but my DTI says it's not made the box any worse than it ever was. The casting is big and chunky, it didn't appear to distort. Bevel drive runs standard Dnepr solo gearing. Do not mix BMW and Dnepr gearboxes and bevel drives, or you'll have 4 reverse gears and one forward. Also, pay attention to the length of the gearbox shaft. The Dnepr clutch is two plate, if you leave it over length it will interfere with clutch operation, which is how I burnt mine out. Also, the gearbox output is through a rubber doughnut with a steel band round it. These are welded, badly, mine fell out with a tremendous clatter on a dual carriageway. Make a new ring from billet steel before yours falls out!

Stock Dnepr brakes are utterly dire. I found a disc conversion for a Chang Jiang, which mates into the drive spline in the wheel hub. I only ordered the disc, disc carrier and caliper carrier, then modified the latter to fit some calipers off a suzuki SV650 I had lying around. This is probably the most important upgrade. I'm also running a hydraulic drum on the sidecar wheel, on a separate pedal. This is great for corner assisting, and keeping the whole rig straight when braking. Brake itself is a bit weak though, I want to go disc on that too.

Electrics are completely rewired, based on the BMW wiring diagram for the whole bike. Remember you have double the lights on an outfit, so either upgrade the alternator or fit LEDs everywhere. Left handlebar switches are Dnepr, right are BMW for the electric start switch. I cut off the rh bar and had a smaller piece welded on to suit the BMW switches, think they go on a 22mm bar. Battery lives in the sidecar, you want as much weight in the chair as possible.

Fuel tank has two new bosses, one each side, threaded to take the excellent quality BMW fuel taps. You need this, the engine drinks like Boris Yeltsin. Before this, I really noticed fuel starvation problems at any decent speed. This was particularly bad at below 1/3 tank, which was hilarious because the original tank leaked if it was more than 3/4 full. This is now fixed.

There are changes I either will make, or would make if I did another. I would be interested to hear from anyone who has done any of these:

  • 1000cc, heavy flywheel engine. I'd be curious to fit the softer R80 cam, and lower the compression ratio from stock R100 levels (you'll certainly need to do this on an RS engine, they don't like modern fuel). I think it would give a fat, lazy engine, lovely for sidecar work, but don't know for sure.
  • Lower first gear ratio. I think there were some variations among dneprs, but I've no idea what they were other than some military bikes had lower first gears. That said, a bigger engine and chunkier clutch as above might negate the need for this.
  • Fit disc to the sidecar. I've got some bits, but there's not much room, and I gave up and did other things instead
  • If money were no object, have completely new gearbox internals made to better quality.
  • Leading link forks sound interesting.
I might, one day, if I can be arsed, do even more detailed write ups on each bit of the bike, but don't hold your breath.

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