For sale: R100RS

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Gift horse - part 6

Alright, it's time to get in there, off with their heads!

First things first, as the valve cover is removed, the center nut comes out taking the stud and a chewed up thread with it; we're gonna need a helicoil:

Once under the cover though it looks ok:

And when you flip it over, it's really not that bad:

The cylinder bores look ok on both sides, while the piston tops understandably show considerable carbon deposits:

The left-hand head is in better shape:

And here's the right-hand one back from a good scrub. Good to go for now, at least to see if and how this engine will run:

The exhaust lockrings came off without having to resort to a cutting disc, which surprised me as I thought for sure these would have been seized solid. They'll go back on with some copper grease as a precaution.

The balance pipe and retaining clips are corroded, luckily we have some second-hand parts from an R100RS we can reuse:

This other balance pipe is looking good:

And the replacement pipe polished up really well.

Gift horse - part 5

A quick update to show you the front fork going back together.

There are new felt strips in the rubber boot:

You may have noticed the right-hand crashbar is off, this is because the bike was in a crash at some point, and that crashbar did its job fairly well, but got all bent up in the process.

We tried to have it straightened but it was too far gone and it cracked as they were attempting it. So, we're now looking for a replacement. You might also have noticed the steering damper and its replacement:

This and its operating rack and pinion is a serious bit of equipment, definitely more effective than other accessories I've tried on other bikes (the Morini and some Commandos for example).

Oh, and before I forget, here's that rubber cap that wasn't included in prior posts, just to put your mind at ease that it is there (on both legs of course):

Gift horse - part 4

Another problem that became apparent right away was the condition of the engine internals, as you will see in the next horrifying photo: this poor bike had been left to rot out in the open, and, in addition to old fuel, water also obviously got in, causing damage to the carburetters and, downstream, to the heads and cylinders. Beyond? who knows!

Above: a peek inside the right-hand cylinder head reveals a serious situation, and as (bad) luck would have it, the valve remained open this whole time. The prognosis for the rest of the engine internals is looking bleak at best.

This motorcycle is equipped with the so called "flat-top" Bing carburetters, a short-lived variant that I understand was in use only a couple of years. Our right-hand one was severely clogged with the same sediment or limescale you can see above, which had stratified in the float bowl. Once that was cleaned, it revealed that the float bowl itself was punctured, so needed to be replaced anyway! The jets assembly was completely seized and required a drill to remove it, destroying it in the process, and allow us to clean the rest of the fuel passages.

If this is what the float bowl looks like, I dread to think about what happened further down the line.

Above: with hole, below: no hole.

Some other parts after cleaning, fine to reuse:

The butterfly valve and actuating mechanism was removed to clean its housing and also replace the o-ring. This requires grinding down the two screws, which then obviously have to be replaced:

New jets, needles, gaskets and diaphragms were procured, and thus was the right-hand carburetter reassembled, ready for use again:

Gift horse - part 3

The fork's internals were way beyond useable and it required an extensive list of spare parts to bring back to life. The only modification here is a set of progressive springs, everything else is stock.

Above: fork sliders, damper rods and brake calipers with associated parts. Bonus content: original toolkit's feeler gauges, a starter relay, and the seat cowl lurking in the darkness.

We rebuilt the damper rods, replacing as many of the internals as were needed, including the wiper rings at the top. 

Above: damper rod rebuilt and ready to be installed. The spacer towards the center-left of the photo is part of a check valve that sits at the bottom of the stanchion.

The original stanchions were pitted, scored and showing rust throughout (see below) so we thought about having them re-chromed vs. buying replacements. 

In the end we went for these "JA-HON" pattern parts that I believe are made in Taiwan. They were by far the cheapest option, and... you get what you pay for.

As with all things BMW, the front fork is a masterpiece in complication, an engineer's show-off and an unwavering middle finger to the concept that "less is more". One of the many, many parts that make up the forks is a pair of threaded rings that screw into the bottom of the stanchions. The Asian stanchions came with new threaded rings and retaining circlip already fitted. "Neat!" I thought. Oh, no, it was not neat. It was a real pain in the ass to take out (and I know you probably don't have to, but it is better to do it this way). In the end it took lots of penetrating oil and a Sanley FATMAX heat gun to get those buggers to budge.

You should be able to see two identical threaded rings with a plain spacer sandwiched in between. Once screwed into the bottom of the stanchion, this valve is retained with a circlip. This is what holds the damper rod captive in the stanchion, and together with the small nut you'll see further down, they are what keeps the forks together. Think about that next time you ride...

Once the stanchion had cooled, it was time to insert the damper rod, and this is where you really ought to have a special tool to guide the aforementioned wiper rings into the stanchion (the idea being to avoid catching the rings on the stanchion's threads or inner edge). Of course there are a number of MacGyverisms you can use to get the job done, but the right tool makes this way easier. Often (especially if you're lucky enough to own multiple bikes), it may seem like a bit of a waste buying a special tool that you're going to use just once or twice, but if you want to do the job yourself (which you should) then you have to get what's needed. In this particular case there is a cheaper option than the original BMW tool (available from Motorworks in the UK) and it's a 3d printed nut with an internal taper that matches the inner edge of the stanchion reasonably well. I was skeptical about a 3d printed thread being able to work but damn, this really works. It turned out to be an awesome little tool (even though on my first attempt I ended up snagging one of the brand new wiper rings and I had to bin it. On closer inspection I should have used some grease and driven the tool fully home); this is brand new technology applied to classic motorcycle repair. What a time to be alive.

After the damper rod is installed, a polyurethane buffer needs to go in:

You can then reassemble the check valve starting with the first threaded ring, aluminum spacer, second threaded ring and securing snap ring:

At this point it's over to the slider, to fit a new main seal as well as a tiny seal for the caliper pivot: I suspect the reason here is that the top of the pivot bolt (which is an eccentric adjuster) has a small cup machined into it, so this is probably to keep some grease in there. Just more engineering overcomplication...

There's a burr on the other slider where the brass bush is pressed in. It's awkward to get at but we need to machine it down before the new seal can be fitted.

The slider accepts the stanchion/damper rod assembly, and the damper rod is secured to the slider's bottom cap. That itself is a complicated part, that includes a new rubber buffer (the old one had disintegrated into a gooey pulp, as you can see below) and a large copper sealing washer, the type used on exhaust pipes.

Above: this is the damper rod protruding through the bottom of the slider. It is secured to it via a large cap that screws into the slider itself:

Good Lord, I am aware that there is a rubber cap missing in the photo above! We have two brand new ones ready to be fitted, don't worry! Geez.... you BMW anoraks are the biggest nerds I've ever encountered. Relax.

"My fork is not as smooth as it used to be... I wonder why!" Yeah, mm-hm.

Looking at the completed assembly and, crucially, understanding how it all goes together and how it works, I can't help but make a comparison with the Norton's Roadholder and the BSA's fork; they're both just fine, and I really can't fault them in any way, on the contrary I've always liked them. And it's perhaps a bit of an unfair comparison since this comes from a later period in history, but what I'm getting at here is that BMW really did put a lot of thought into what they were making. Was it needed? Necessary? Maybe not. Does it work as advertised in the press of the time? Only riding it will tell.

Of course this applies to every area of this motorcycle, and it is apparent that what stands before you is the result of a complete and thorough engineering philosophy, from start to finish, that has gone very much its own way. That's why BMWs are so fundamentally different, why a comparison with British or Italian machinery will reveal them to be in a class of their own.

Better? Worse? Probably just different; and in the end it all depends on what you need the motorcycle to do for you: if it works, then it's a winner, no matter how odd.