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Bike Profile - Posted 28th April 2011

BMW K75RT Part 7 - Loose Brakes and Leaky Forks
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Many classic bike enthusiasts could be described as having a screw loose, but Steve MacGregor has more than most. A cleaning session turned into all Gaps and Gaiters*...

While cleaning my BMW K75 after its winter sojourn, I noticed that a socket head button screw from one of the front discs was missing. Each disc is retained to the disc carrier by nine M5 button screws with nyloc nuts. I wasn't desperately concerned about the loss of just one screw, though I made a mental note to order some replacements. A day or so later I went to measure the offending item and discovered that another screw was missing. And another. I was down to six screws retaining the left-hand disk. I had only done around 50 gentle miles since noticing that the first screw was missing, so this represented an alarming attrition rate. I decided that I had better leave the bike off the road until new screws were obtained and fitted.

Ulp... Notice anything missing? The screw at the eight o'clock position looks OK, but is actually just a head - the shaft and nut are absent.

When I went to fit the new screws and nuts, I discovered to my horror that one of the old screws was a head only - the shaft and nut had broken off. So, I had actually been riding with only five out of nine disc retaining screws in place. Eek! I haven't come across this before, and I don't know what caused it. I can only assume that the loss of one screw placed additional stress on the others, and then another failed, and so on in a chain reaction round the disc. I did examine the remaining screw head where the shaft had broken off and this showed evidence of a crystalline fracture, usually evidence of a brittle failure. As a precaution I replaced all the screws and I'll be keeping a close eye on them from now on.

Now that's more like it... Shiny new screws.

While doing this I also undertook some other work on the front end of the bike. I had noticed a tell-tale oily tidemark on the left hand fork stanchion. A sure sign that the fork seal is on the way out and an MOT fail. And if one seal is worn the chances are that the other isn't far behind, so it's always prudent to replace both. Fitting new fork seals isn't difficult, and I'm surprised by the number of people who won't tackle this job themselves. Then again, I suppose it does depend on your competence and confidence. This job involves removing some fairly essential bits of the bike including the front wheel, brakes and front suspension. So if you're the sort of person who finishes any job with a baffling handful of left-over nuts and washers, perhaps it's best to leave this to the experts.

Oh dear, Steve said 'naked'. That'll set the search engines twitching... Naked forks. Boring.
K75 bits on Right Now......

As a first step, I ordered a set of fork seals. Actually, I ordered a set of the wrong fork seals but the nice folk at nevertheless contrived to send me the right ones. The forks on the K75 changed in July 1991. My bike was registered in November 1991, and so I assumed it required the later type of seal. Fortunately I quoted my VIN to Motorworks when placing the order and they spotted that my bike was actually manufactured in April 1991, and therefore required the earlier type seals. This is one of the benefits of using a knowledgeable specialist supplier for parts.

I also ordered a rather spiffy set of fork gaiters. The chrome on the stanchions had chipped in a couple of places, fortunately outside the area swept by the seals. Fitting fork gaiters will prevent any further chipping and I think they look rather nice.

No snow? What's going on?... Fork. Off.

Fitting the new seals was straightforward. After draining the fork oil and taking off the front wheel, I removed the forks by loosening the pinch bolts in the top and bottom yokes. On the K75 it isn't essential to remove the forks from the bike to replace the seals, but it does make the job easier. The stanchion is secured to the fork leg by a hex bolt which can be seen when the wheel spindle is removed. With this out of the way the stanchion can be separated from the fork leg. The seal sits on a lip inside the fork leg, retained by a circlip. Once the circlip is removed (a dentist's pick is the ideal tool for this), the old seal can be levered out.

An easier job than you'd think... The fork seal lives here.

Then it's just a case of carefully inserting the new seal without damaging it and putting everything back together. The forks can then be re-filled with the correct quantity of oil. This is something that's worth being precise about. Even a small amount too much or too little can affect handling. I used a synthetic 10W oil, though the recommended is 5W. I find that the forks dive rather too much under braking if I use the lighter oil, and the 10W seems a good compromise.

The forks (with new gaiters) were then put back on the bike and the front wheel was reattached. I also replaced the corroded brake calliper retaining bolts with stainless steel items, and that's it, job done.

Mmmm... Soft *and* shiny... Soft, shiny, black rubber gaiters. Mmm…

After a pleasant afternoon's work the front end of the K75 is once again safe and it even looks better too. I can now approach next month's MOT with a degree of confidence.


* A weak pun which will be recognised only by those readers who remember the BBC ecclesiastical sitcom All Gas and Gaiters from the late sixties.


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