Tuesday, September 9, 2008, 08:30 AM ( 7354 views ) - BMW Cafe Racer - Posted by Administrator
At long last the r60/5 cafe racer was coming back together. High on the list of priorities was a swap to dual disk brakes. I'm very familiar with the dual ATE brakes on my R100RS, and I think they're ideal for an airhead.

Surprisingly, even the earliest tiny /5 gas tanks have a cutaway area underneath in front to leave room for mounting the undertank ATE master cylinder. That makes plumbing the brakes a pretty stock operation. The dual disk swap has been well described, it means replacing the sliders with a pair of sliders that hold the ATE calipers.

I opted to paint these black, like the RS did but mainly to continue with the vintage feel. The end result- it feels vintage even though there are disk brakes. I know, to the purist vintage racer this is blasphemy, and it will keep me out of vintage racing. But I don't care, I like to stop well on the highways where I ride. Getting this together was very good news.

Now, it may seem like at this point you could bolt it all together and ride off into the sunset. I thought so too. But I knew too well that there was no guarantee that these parts would work together. I had already proven that it was not adequate to merely rotate the engine through a couple revolutions and conclude it was safe to run.

The documentation with the 336 cam warns that it may be necessary to notch pistons for clearance, and I was pretty sure that with the high-lift cam this would be the case. I planned to use clay to find out how much clearance there was between the valves and piston. But as it turned out, I didn't even get that far.

The problems began when I realized that I couldn't rotate the crank through a full two revolutions (2 crank = 1 cam revolution) At first I thought I was colliding a valve with the piston, but nope. Turns out that the valves were running into each other, and it was serious.

After some thought, it was staring me in the face: the 336 cam wasn't compatable with the large-valve heads. It lifted the valves too high for the large diameter valves to clear each other. So then I was faced with a choice- which one to use?

The regulars on the MC-chassis list were helpful there, pointing out that the larger exhaust valve probably didn't make that big a difference, the size difference was most important with the intake, which wasn't that much larger than stock. And it turns out the stock valve diamters are considered close to optimal for a 600. Both would increase flow, but the cam would also raise the power band, effectively increasing the usable power even more. Oh okay, twist my arm. I was biased- I'd installed the 336 cam already, I didn't want to take it back out. So I was back to stock heads.

All along I was lucky to have the good advice of several friends including Tom Cutter, who was most generous with his knowledge of BMW airhead racing development.

One thing Tom impressed on me right away, when he saw that I couldn't be dissuaded by the fact that racing an airhead is controlled folly in the modern age... that out of the way, he made it very clear that the only way to success was to be prepared to take everything apart and then put it back together again, over and over and over. Every time something changed- valves, cam, valvetrain geometry- it all had to be checked for clearance. Once you were out of stock territory, it was no-man's land, and you had to take precautions.

I had asked him what clearance he recommended- the BMW docs said 1.5mm. "2mm everywhere" was his answer. Everywhere? He meant not only valve-to piston, on the face but also along the sides, and between the valves themselves at overlap- exactly the problem I'd run into.

But how do you measure the clearance between the valves inside the cylinder when you have to assemble it to get all the valvetrain installed and adjusted correctly?

In order to do that, I followed Tom's advice, with some modifications. I cut a set of tubes that were exactly the same length as the cylinders, so they could be bolted up with heads and pushrods and the crank could be turned and the valves could be observed in action. Tom had used aluminum, and replaced the valve springs with lighter springs to prevent the aluminum from being warped out of alignment. I opted for steel tubes with steel washers and aluminum washers on the ends to prevent nicking the heads.

And here we can see the valves in action. As we approach TDC, the exhaust valve is open:

Now we are at TDC, both valves are open briefly (overlap):

And after TDC, the exhaust valve is closed, Elvis has left the building. The intake is open and the next charge is being drawn in:

Here we have a closer look at overlap. This is what I was waiting in suspense for: would there be a 2mm clearance as Tom had advised? Indeed there was, 5mm of clearance, more than enough. I was very glad to have seen it and to know for sure.

It made me think too of the reputation BMW racing engines had all the way back to prewar days: they were bulletproof, indestructable. Now I was beginning to see reasons why- if engineers observed extra-large margin of clearance, it would allow parts to be run way beyond normal tolerances of heat and expansion without catastrophic results.

OK, one hurdle accomplished. Now it was time to notch the pistons.

Now, I don't know how to exactly describe how hair-raising this was, except to say that at times I found it easiest to wait until I was in such a black mood that I didn't care if I did ruin it and grind a hole clear through. I'm nto sure I was really confident until I had a set of backup pistons. But once the grinding began, it was quick to actually do the shaping, but hard to go very deep without wanting to go back and check again with clay. I did this many times.

Install piston on connecting rod, using blowtorch to heat piston so wristpin slides in smoothly. Apply clay to piston, and spray it with WD-40 to keep it from sticking to the valves.

Rotate the crank through two full revolutions, a couple of times, and then disassemble it again (don't put the blowtorch away yet). What you get is a piston with lumps of mashed clay on it.

And if you take a gentle blade (I used a brass feeler gauge instead of a razor blade) you can carve away cross-sections of the clay and measure how much there is, how much gap was left between the valves and the piston.

I must have done this a million times.

When it was finally, finally right, I breathed a BIG sigh of relief and turned to the other piston. With the first one as a model, I got the second one done in maybe a half hour. It figures.

Here's a comparison of a notched piston with the stock piston, which has much smaller notches:

When both pistons were shaped and I'd tested them with clay and smoothed and polished them, it was time for balancing. I weighed them... and was surprised to find that they were both 424.0 grams. Amazing! And a very good sign- time to bolt it together!

It was all picking up momentum now. I got the motor back in the frame, and bolted up the 5-speed I'd been waiting to swap in. It would be a big improvement over the 4-speed, and it was one of the rare early 5-speeds with a kickstart lever, so I could continue to run without a starter.

Now I mostly worked to tie up loose ends. I got the body panels all repainted and pinstriped (more on that some other time) and did some needed repairs on the seat and the subframe fender mounts. There were a lot of little details I ran into- the neutral light didn't work, for instance, because the /6 neutral switch had a seperate ground wire, like the taillights, insted of grounding to the chassis. Ran a new wire, problem solved, next.

Finally when I got it together enough I tried starting it- and on the 3rd try it popped into life! I was delighted, and took it out for several test runs. Things were not right obviously, the carb wasn't jetted right, but it ran.

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