Race to Rebuild: The BMW R90/6 Part 2

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The 1974 BMW R90/6 as purchased.
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Associate editor Landon Hall (left) and tech Q&A man Keith Fellenstein at work pulling apart our R90/6.
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Our R90 in pieces.
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We'll need to repair split pushrod tube seals.
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R90 engine, will get a reseal to correct split pushrod tube seals and oil leaks from failing rear main seal.
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A previous owner clearly loved doing their own wiring.
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The failing rear main seal needs to be replaced.
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Center stand pivot bolts missing spacers and washers; it’s a wonder the center stand stayed in place.

Deconstructive surgery is fun. Compared to the exacting process of carefully assembling bits and pieces into a functioning whole, there’s huge and immediate satisfaction in taking something whole and, scant hours later, ending up with a pile of parts scattered around the shop. Ahh, progress!

It’s a good thing we think that way, because deconstruction definitely defines the current phase in the Motorcycle Classics/Dairyland Cycle Race to Rebuild.

Chalk it up to an insanely busy summer, but we’re running a good month or more behind schedule. If things were going right, by this time we’d be showing you sketches and already acquired pieces to define the projected end product in our build.

That’s not to suggest we haven’t made any headway. In a break from previous builds where we’ve just jumped in whole hog, discovering what surprises awaited us as they presented themselves (the holed piston and trashed forks on our CB500 project come to mind) this time we’re heading into our teardown phase with a little more care. We really do learn from our mistakes. Maybe.

For example, we know we’re going to give the engine on our 1974 BMW R90/6 some mild massaging. Nothing radical, mind you, just a few fun and sensible upgrades to give the bike a bit more personality and a leaner look with the benefit of enhanced performance. When it comes to the engine, we’re looking at items such as upgraded carburetors and filters, a custom exhaust and electronic ignition.

On the bench

Since we want to inject a little extra oomph into our Beemer, we decided it’d be fun to put the BMW R90/6 on a dyno so we can document how our project bike ran when we got it against how it runs when we’re done. Fortunately, local dyno expert Kent Clawson likes us, and agreed to take our bike through a session on the chassis dyno.

For our dyno run we tested the bike exactly as received, and the results showed our old Beemer to be pretty healthy. When new, BMW claimed as much as 67.5 horsepower at the crank at 6,700rpm (some reports claimed 60 horsepower at 6,500rpm). That’s taking the measurement at the crank, but we did our testing on a chassis dyno, which measures power at the rear wheel. Factoring in power losses from the transmission, driveshaft and general variations from one engine to another, dyno man Clawson said to expect up to a 20 percent reduction for rear wheel horsepower, with a perfectly tuned bike. In dire need of a tune-up, running rich and on unknown octane fuel likely months old, our Beemer put out 44.54 horses at 6,500rpm and 41.51ft/lb of torque at 4,200rpm, roughly 34 percent down on horsepower and 20 percent down on torque from factory claims. When we’re done with our planned mods, we’re hoping to get within spitting distance of the factory claim, at the rear wheel. Cross your fingers.

Our deconstruction fun came the next week, when along with tech Q&A man Keith Fellenstein we hoisted the Beemer up on a bench and got to work taking it apart. At 9 a.m. on a Monday morning we rolled the Beemer into place. Five and a half hours later, including a break for lunch, our Race to Rebuild project bike was in pieces.

The tear-down revealed thankfully few problems. Chief among them — and hardly surprising given the bike’s age  — was the discovery of split pushrod tube seals. BMW used large, compressible pillow-type seals to seal the pushrod tubes to the block, and ours have given away, victims of time and the elements.

More surprising was the discovery of completely worn-out steering head and swingarm bearings, casting further doubt on the bike’s true mileage. No worries, however, because they have to come out when Stewart Armstrong at Custom Coatings & Metal does his powder coating magic on all of our frame bits.

All told, we were quite satisfied with what we found on disassembly. The bike is, as we thought from the beginning, a fairly honest survivor. Not exactly beat up, it’s a basic rider that clearly hasn’t received the best of care in the past decade or so, a state that’s going to change rapidly now that we’re finally sinking our teeth into things.

Next up

Keith has the R90’s mill at his shop, pulling apart the top end for new pushrod tube seals. That process means new head gaskets and cylinder base gaskets. And, if we think it needs it, a valve job. The engine is strong, with good compression and quiet running, so we hope to otherwise leave it alone.

We like the idea of a twin-disc front end, something we need to decide soon as the stock hoops will be replaced with custom rims laced up by Kenny and the boys at Buchanan’s Spoke & Rim in Azusa, Calif. You might remember the beautiful set of black anodized Sun aluminum rims on stainless steel spokes Buchanan’s crafted for our CB500. We’re thinking the same sort of treatment will look good on our café Beemer.

With the summer show season over, we’re looking forward to getting back in the shop. A lot will happen between now and our next report, when we’ll finally — cross your fingers — show some real progress and give a full description of our project BMW R90/6. In the meantime, go online to follow our project updates and sign up to win our BMW at the Dairyland Cycle Race to Rebuild Facebook page. MC

Read the first Race to Rebuild article about this bike in Race to Rebuild: The BMW R90/6.
What happens next? Find out in the Race to Rebuild Part 3.

Motorcycle Classics Magazine
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