/5 Fork tube out of round

Rebuilding the forks on my '71 /5...and installing cartridge emulators and new springs while I'm at it. In removing the fork sliders the right came off smoothly...the left was really challenging to pull off. There was a goodly bunch of sticky mess on the fork tube which I attributed to the seal going bad. Anyway, got everything cleaned & ready to reassemble. Both fork tubes were clean and smooth as glass, no scratches, blemishes or scars on either. They looked great post-cleanup.

Right fork reassembly went flawlessly - left fork, not so much. The slider (with new seal, of course) went smoothly up the fork tube 4" or so and stopped. Took it back off and with a micrometer checked the tube for roundness. Turns out it was way out..almost a millimeter out...at about 4" up. So, a new left fork tube will be in order.

Just curious if anyone else has seen this. I've had the bike two years; the rest of the front end appears perfect and there's no evident reason why the tube would be so out of round starting at that point (the bottom few inches are dead perfectly round). Just very odd.

Another easy way to confirm that a tube is not straight is to put the tube on a completely flat table...glass would be best. Then you can roll it around and see where the light shines through. Compare one fork to the other for straightness. Just to help validate what you already know.

I don't think this is a common thing...something must have hit it hard enough to cause the bend...or maybe the axle nut wasn't tight...or...

Thanks Kurt. It is indeed odd, since there's not a scratch or mark on any other component. Heck, even the tube itself is (to the naked eye) glassy smooth. I had to mike it three times because I didn't believe it at first.

I guess I misread the issue...the tube's not bent but rather it's misshapen at that one area. Does the out-of-round location impact the fork seal? Sounds like the area stays within the enclosed part of the slider.

I take it you're not the original owner?? Could this be something that came from the factory? Forks are pretty important, but I'm wondering what is the potential risk for reuse. I can see that having that in the back of your mind going down the road wouldn't be a fun thing.

Replace it!

If you are confident the fork tube is out of round / bent then I would strongly suggest replacing it, its not something you want to take a chance on. I have no affiliation but have been satisfied with a company out of the mid West, forking by Frank or Franks forks. If you Google forking by frank it should pop up. Another alternative is a used fork tube that may end up being worse than what you have.

Depending on how much you plan to ride the bike and the type of riding you do you may even be good to go with it but I wouldn't trust an almost 50 year old, out of round, maybe bent fork tube just on the chance it has had enough just before I hit that little bump in the road.

Thanks - and it's absolutely getting replaced, that's not in question...mainly wondering if anyone else had seen this sort of issue.

If forced to guess I'd say either someone once put the tube in a vise for some reason or had some kind of clamp on it, causing the tube to deform. The out of round bit was/is right about where the slider/fork seal would be moving up & down. That'd perhaps explain why the fork had gummy residue on it - it was likely eating up the seal due to the issue. I always figured if the tubes had a fault it would have been with straightness vice this issue. Nothing a new tube won't solve. I'll be checking alignment when the new one goes in.

And the moral of the story is....

...the first report is often wrong. In short, there was nothing wrong with the tube - I'd simply done a poor job with my measurements. Don't ask me how I'd gotten it so wrong, but I had.

I was at least smart enough to have the tube accurately checked by a proper machinist with a dial indicator and a runout stand before replacing it - and it was fine. So the issue was NOT an out of round tube.

Upon further examination, one of the two studs that hold the fender bracket in place on each slider was threaded in too far...such that it was making contact with the fork tube. D'oh....it's apparently been like this all along, which explains why the left slider was so easy to slide in for about 4", and then stopped. Fixed that issue, and now the forks are all back together and smooth as can be.

Any day I learn something (and don't have to spend $300 in the process) is a good day. Now I've gotta brush up my skills with a micrometer - a little egg on my face is a cheap price.

Dave -

Good persistence to the get to the real root of the problem! I suspect that there was a little voice in your head that said "this doesn't make sense". Measure twice and cut once!

All's well that ends well. Thanks for the report!