This past Saturday I installed Progressive forks on my '91 K75 and thought that I'd write about a few bits of tech and in turn ask the opinion of the presidents about a discovery that I made in the process.
I have a K75RT, and the instructions on the K-tech web page were very helpful. When I pried off the fork caps, I was dismayed to see powdered rust under them around the edge of the top plug. After depressing the top plugs, I found that the snap rings had both rusted substantially but were not frozen in place by corrosion.
After removal of the fork innards, I inspected the inside of the fork tubes. Both tubes had patchy surface rust in spots down to the bottom of where the two plastic spacers stopped. Before installing the Progressive springs I used a rod and a shop rag in a manner similar to swabbing out a gun, and this removed most of the rust but left a few light patches of more advanced corrosion.
By the way, the four month-old BMW fork oil came out a muddy purple-brown color. Clearly there was some water contamination in both forks, but only a few practically microscopic drops of water were left in the bottom of my drain jar.
So, what could possibly cause such water contamination and rusting? I do ride daily in all weather, so perhaps heavy rain penetrated the works? I don't recall being stupid enough to spray a jet stream of water anywhere on the bike so I don't think that washing could be the cause...
When I cleaned everything up and was about to replace the plastic fork caps, I liberally applied synthetic Teflon bicycle grease to the underside. From my years of bicycling I've learned that anything that you don't want corroded should be coated in lube. It certainly can't hurt in this application.
So, a few tidbits that might just make this an easier story for someone else replacing the springs. First, I used the method supplied by Progressive to determine the correct amount of fork oil for use with the new springs. For the K75RT (pre-92) the BMW recommended amount by volume was 550 cc. Using the spring installation method of measuring oil height in the forks (with springs removed and forks collapsed all the way) I used just a shade over 450 cc to bring the level up to 5.5 inches. So, at least with the K75RT I can now reliably recommend 450 cc for those who install Progressive springs.
The second tip will be useful for anyone trying to do fork spring work without a helper. The real difficulty of getting at the springs is reliably depressing the (lightly pre-loaded?) top plug so that the spring clip can be pried out. I overcame this problem with the help of my trusty Craftsman 4-inch gear puller. This handy little tool can be configured for either two or three "grabbing" fingers. I configured it for two hooks and it connected perfectly with the underside of the upper triple clamp. I hand-tightened the central press bolt to depress the top plug, and I had plenty of room to work on the wire retaining clip.
Yet another tip for your consideration is the method that I used to remove the retaining clip. I have not seen anyone describe their particular methods, so perhaps mine will be food for thought. I first used a small flat-bladed screwdriver applied to one end of the clip and was able to work it down out of the groove with a few light taps of a small hammer. I then inserted a VERY thin knife blade (thank you, Mr. Victorinox) between the fork tube wall and the clip end. Now I used the flat bladed screwdriver again to gently pry the clip end up, allowing it to slide up the knife blade, over the groove and out over the end of the fork tube enough so that it may be grabbed with a needle-nosed pliers. Once I figured this method out on the first fork, it worked reliably and most importantly it was easily repeatable on the second fork.