BMW R75/5 to 900CC Conversion (on a budget)

If you are looking for specific technical guidance for this conversion, you might as well look else where....this is more of a boring story about the procedure with just enough good information to get you confused. Luckily I started life as an auto mechanic so had some knowledge before taking this project on and wasn't about to spend thousands of dollars doing it.

Craigslist Donor Bike & Sidecar, total cost $1200.00

I know just enough about this subject to be dangerous. I took on the task myself and have ridden the bike 12000 miles so I guess it was successful......

I started out with a 43K mile 1974 R90/6 donor bike, which also had the Jupiter Sidecar attached. I think that some of the later year R90/6 cylinders are incompatible for this conversion, but not sure what years.(?) The R90/6 engine had sat for 7 years , luckily in a dry climate so no rust was present. I had a run of the mill but reputable machine shop ball hone the cylinders and regrind the valves. BMW gurus warn "always use BMW Motorcycle specific machine folks". Well with that fact your typically gonna have to ship things around the country at a significant cost, plus their gonna charge you out the nose. I spent a whopping $90.00 for the machine work. After honing one cylinder showed some sort of "stain" on the surface but was not measurable with a micrometer so didn't worry about it.

I gathered up a set of std. size piston rings, new piston pin clips, pushrod tube seals, new valve cover gaskets, head gaskets and base gaskets.

Parts cost was around $250.00. So at this point I have roughly $350.00 into the project. The

donor BMW R90/6 with sidecar doesn't really count against the cost since I E-Bay'd about $800.00 worth of parts when I pieced it out, plus I kept the good engine bottom end for a spare and I obtained a (rough) vintage 1974 Jupiter Sidecar.

Project sat dormant for awhile until I finally got off my ass and tore into my R75/5 engine. Supplier sent me the wrong piston rings so had a delay there. I followed information found in Shop Manuals and from the Internet for torque specifications. Some after market shop manuals are notoriously wrong for torque values so beware and double check those things. Dissassembly was easy. Just marked every part and orientation upon removal. Clean all mating surfaces completely. My piston pins were hard to remove due to some varnish. DO NOT BEAT THEM OUT ! I used a heat gun and heated the piston a little and the pins slid right out.

I used Duane Asherman's (spelling?) method to reinstall the pistons using the "built in" ring compressor featured on all BMW cylinders. No ring compressor tool required. It is easy to put the head gaskets on wrong so pay attention to push rod holes. My new head gaskets would not fit due to the extruding "guide" collars sticking out of the cylinder head. Had to carefully grind the gasket holes larger with a dremel tool. The supplier said no other head gaskets were available.(?). These "guide" collars must not be present on later engines. (?)

Reassembly was the opposite of dissassembly, The R90/6 rocker arms are a needle bearing type and one i had was missing one needle in one bearing. I bought the new bearing but could not get the old one out - new one in without a vice or press. I just used the R75/5 bushing type rocker assemblies and so far so good. They are a little noisier but there are no needle bearings to fall out or fail. They make some special tool to center the rocker arm assemblies but not having that I just made sure the push rods were as close to center of the bore, after slightly snugging up the head bolts. Again, I had all parts oriented from where they came from and made sure they went back on the same. Don't know how critical this is but that is what I was taught as an auto mechanic. I do know the rockers can be installed wrong which will cut off the oil supply, resulting in no oil to the top end....not a good thing.

I screwed up when installing the cylinders as I did not push the pushrod tube collars out to relieve pressure on the pushrod tube seals. When Things were done it over pressured the pushrod tube seals resulting in deformity. After 12k miles they still don't leak, but they will at some point. I think the collars need to be tapped into the seals after the head torque is complete. They make a little tool for that. Looks like one could be made from a piece of metal conduit easy enough.

My top notch BMW mechanic told me to re-torque the heads after no more than 100 miles, which I didn't. Had some seepage before I re torqued. Re torqued twice after completion and everything is good up til now. Be sure and follow the patterns and progressive torque values recommended, Not following these can result in warped heads or stripped head studs.

I used my R75/5 32mm Bing carbs and rejetted them with the R90/6 needle and main jets. I had serious "pinging" under loads so had to play with the carbs. I found that the stock R75/5 needle jet is a bit larger than the R90/6 needle jet, so I put the R75 needle jet back in. also went to a 160 main jet rather than the 150 specified main jet. This completely healed up the "pinging" issue. The 160 main jets cause it to run a little rich and at higher elevations I install a smaller pair of jets. The carb. main needle position must also be adjusted as per the R90/6 spec.'s.

The old "Hag"

In my opinion these old German machines are a simple engineering marvel. They are much more forgiving to work on than one might think. There are many wrong ways to do things but with common sense, research, proper shop manuals, and patience almost anyone can work on them. I performed all this swap in my 14' cargo trailer without hundreds of dollars worth of special tools.