1985 K100RS scraped fork tube.

Do I need to do anything about this?

I've always known since I've owned the bike that there was 2 different lengths of fork oil drain screws, and when I looked into it I found out why, but never worried about it until now. The length of the thread in the drain hole is 6mm and the screw that was in there was 8mm (proper screw is 6x6). This is because the first half of the thread (about 3mm of it) is completely stripped, so "a" previous owner found out that putting in a longer screw that would bite more threads worked. Problem solved, right?

What I found last weekend when I started my cartridge emulator upgrade is that the longer screw was sticking out into the slider and impinging on the fork tube as I went bumpity-bump down the road. You can see the scrapes which I can only just barely, slightly feel with my finger nail.

I now have to drill and tap a new hole-8mm probably. That's no big deal. I would like your opinion's on whether I can reuse this fork tube or if I should look for a good used one. I honestly don't see why I can't use it but lots of you guys have more experience than I do. Personally, I've never had a bike where this has happened so I thought I would ask.

Thanks,
Tom

Is the bike a "rider" or a "keeper" If it is a rider then I would monitor the fork for leakage but ride it. If it is a keeper then I would at an opportune moment probably contact "Forking By Frank" (the old name) now called Frank's Forks. https://franksforks.com/

PGlaves said:

Is the bike a "rider" or a "keeper" If it is a rider then I would monitor the fork for leakage but ride it. If it is a keeper then I would at an opportune moment probably contact "Forking By Frank" (the old name) now called Frank's Forks. https://franksforks.com/

Click to expand...

Thanks, Paul. It's a keeper. I put 3,700 miles on the bike this season and it didn't leak a drop. I sent off a quote request to Franks Forks. New and used ones are available from my parts supplier of choice, Motorworks, in the UK, so if I do need to replace it I won't be stuck.

I'll just add this as I got an email back from Franks. They are moving and aren't taking any requests or orders, so I don't know when I'll hear back from them. Not an issue, though, given the fact that as I've mentioned above, availability is there.

Working more with hydraulic cylinders that forks, we always felt for a “snag” when running a fingernail across the chrome. I have used crocus cloth- https://www.mcmaster.com/crocus-cloth/ to extend (possibility) the service life of scratched/nicked chrome tubes.
New seals will put up with quite a bit of scratchiness....squirt a little Teflon lube to keep things moving and not drying out- you may be surprised.
OM

I could be mistaken, but I believe the fork tops out well before that surface ever reaches the seal.

True. If it were me, I would rotate that tube in it's mount so that the scratched area is 180 degrees opposed to it's original orientation and ride on - at least until you can find a better condition tube or talk to Frank.

I think that I am going to get some crocus cloth as per Omega Mans post, polish it even though I can't even hook a finger nail on it, and then when I reassemble rotate it 180 degrees as Greg has suggested and ride on. Thanks, guys.

Tom

The drain screw is at the bottom of the lower end of the fork and for the life of me I can't see how the upper fork portion that fits in the trees and moves up and down would ever come in contact with the screw. The lower rod ( that is held in place by the bolt on the bottom of the lower fork leg ) would be the only thing that could come in contact with a longer screw. It never moves so wouldn't involve any potential for leaks or marks like you show.

The only area for concerns are the ares of the fork tube that come in contact with the fork seal, and that area is about half way up the fork tube. I haven't had a K bike fork apart but have had R bike forks apart and there are usually some bushings etc that can wear and cause your marks.

If you are looking at replacing them, make sure what front forks you have on the bike. The name is on the rear of the lower fork. The early ones where Brembo or Fichtel & Sachs and they were 41.3 mm. Sometime in 1986 model year they went to a different fork, and it was 41 mm. Many of the K bike forks have been changed over the years so you need to know what you are dealing with. My early 86 K100 by way of example has one Brembo & one Fichtel & Sacks. Bike has low miles, but bought used with no real history. If your bike has the one piece fender ( no fork brace built into it ) then it likely has one of the above forks. If it has a two piece front fender with built in fork brace then it likely has the newer version.

The different forks take different amounts of fork oil and the same year of bike be it a factory stock K100 ( no fairing etc ) and a factory stock K100RT/LT used the same internals, they just varied the quantity of fork oil in the fork to compensate for the extra weight of the fairing. I know this is true for the older bikes, not sure if the 90's models had different springs etc. The only reason I know all this stuff is that I've changed the fork oil on my 86 K and also installed a fork brace ( not a plug & play setup - no matter what they say ) so I did a bit of research and reading.

Cycleman2, you're absolutely right. I should have looked at it more closely when I disassembled the fork and prior to posting. The area where the stanchion is scraped comes nowhere near the drain screw. As a matter of fact with the stanchion internals out, it will only go down in the slider to a point where the bottom of the stanchion is visible in the drain hole. And the stanchion spends no time down there with everything assembled. I suppose if I whacked the front end really hard and completely bottomed it out, it might get there. But under normal fork travel - no. So was there contamination of some kind at some point that caused this? I don't know. I polished it out easily with a 1500 grit foam sanding pad. You wouldn't find any evidence of it now unless you really looked hard. It is virtually invisible now.

My lower forks are Brembo (one-piece fiberglass fender) and the stanchion diameter measured out at 41.5mm with my calipers, so 41.3 it is.