Thursday, July 3, 2008, 12:32 PM ( 2605 views ) - BMW Cafe Racer - Posted by Administrator
Since the original connecting rod bearings (and most likely the crank) had been well thrashed by their brushes with glory, a bottom end change was in order. Preserving the original r60/5 engine block and crank for possible restoration later, I stripped the bike down.





Note that I'm using an automotive engine hoist, and I recommend it when pulling motors. Not only does it provide 100% positive support that can't fall over even if you lean on it, but since you can't use the centerstand (it comes off with the engine mounting studs) you also don't want to use a motorcycle lift, because that would normally go right under the oil pan- and that's going to get lifted out of the frame.


So an engine hoist works wonders, and a five gallon pail like this one to set the frame on and keep it held steady is just the ticket. You probably can't see in the photo that I pulled the swingarm pivots and unbolted the u-joint and just pulled the whole swingarm and rear end free and left it hanging on the shocks. That's the easiest way.


I began building the new race motor using this good '76 R75/6 bottom end. The changeover from /5 to /6 meant a change in a number of things, including both flywheel (to bigger bolts) and cylinder base size. I could have used an earlier /5 block, but I opted for /6, even though it meant sourcing another pair of cylinders. (r60/6 and r60/7 are not exactly rare, but uncommon, so finding these was not so easy as it would have been for R80, R90, or R100 stuff).






But the tradeoff was worth it, because it allowed the mounting of this lovely flyweight aluminum flywheel (I'm not sure why the light makes one of the flywheel bolts look darker, believe me all five are new bolts):





But that's getting ahead of the story- of course there's a whole lot more three steps forward, two steps back stuff that I'm skipping over but you know it's there. Put it together and take it apart again. Because of course it's the old story: "As long as I'm in there..."


So, yeah. As long as I'm in there, what about another cam? Certainly the stock R60/5 cam could be improved on. But why not a racing cam? As it happens, the famous 336 cam that helped BMW dominate sidecar racing in the 1950's was still available. Whee!


Here we are pressing the timing chain sprocket off the old camshaft.





I'm skipping some ugly steps to get here. What does it take to swap a cam in an airhead motor? Well, besides the front cover you take the alternator and rotor off, and the timing cover, and there's a lot of disconnecting of stuff you want to be sure to reconnect right later. Under that is the timing chain, which on this vintage is a double-row chain running around sprockets on the crank and cam shaft.


And I'm gonna take a stand for being a heretic here. The first time I changed the timing chain on the RS, I cut the old one off, and that was fine, it was worn. This one wasn't worn. So rather than cutting it off and using a new one with a master link, I would rather use the endless timing chain, cause it's cool.


So what I did was used a puller to pull the crank sprocket off the crank -most- of the way but not all the way. If you do this, you can pull the camshaft forward far enough to get the chain loose and off. This is good because when you go to reassemble, the sprocket is correctly aligned on the crank and it's easy to press on- otherwise you have to heat and freeze parts and even then it isn't easy. So to me this is a good trick. I know, I know, the sprocket should be replaced, but if the chain isn't worn and neither is the sprocket, I say run it.


Anyway, here's the 336 cam, compared with a pair of R75 cams. Notice that in addition to the different size lobes, the top R75 cam has a larger lump on the end. That's where the timing cover seal is, and there are two sizes, small (early) and large (late). I chose to use an early small seal, because I wanted to use the original early timing cover that included the speedometer cable for the /5 speedo.





Now a closer look. You can see how much larger the lobes are on the 336 cam, almost 50% more lift and a lot more duration too (wider).






Here we are pressing the cam sprocket onto the 336 cam. Whee!





Every little bit helps when you're riding your cafe racer hard around corners. Years ago I'd come across a BMW police oil pan that included these baffles to prevent windage around corners by keeping the oil from sloshing around. Boy it's fun to pull a part like this out of your special parts stash!





Meanwhile, other changes were in store besides the engine. Having been studying details of 50's as well as prewar BMW's, I decided for fun to detail the cafe racer as though it were from an older era. One of the most striking details of some of the older models was painted rims. I considered all-black but instead opted for the black painted stripe rim of an R51/3. This and a black painted hub would do wonders to give the bike a vintage feel.





And even more important than a faster engine and better paint: dual front disk brakes! Here we see the first mockup with fork brace.





After lying in wait for so long (it seemed) the r60 cafe racer was finally coming back together!




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