Topic: Inner fork tube - scratch or meant to be?

[Toby84]

Hi,

I'm rebuilding the forks for my CB750 K7 -77.

When I look into one of the inner forks (just one, not the other) I see this long depressed line. It goes from top to bottom in a straight line. To perfect to be a scratch I believe, but since it's not present in the other tube it makes me wonder. The tubes could be from different bikes / years, that would probably explain the difference.

Main question is, does it affect the suspension having this difference in the inner tubes? Tubes looks great otherwise.

Thanks,
Tobias

[pekingduck]

They are probably a mis-matched pair.  They can be made out of seamless tubing with a piercing mill, or of thick sheet that has been rolled and formed into a tube, with a continuous resistance weld inside.  When the weld is scraped clean, it can leave a line or depression.

This should not be a problem on forks that have an enclosed damper activated by a rod that attaches to the fork nut, as on CB500 four, but most other bikes use the inside of the tube for their dampening, with plastic sealing rings that sweep the inside surface.

[RAFster122s]

Peking duck, he has a cb750, is it an issue for him?

[pekingduck]

The CB750s should ideally have a smooth internal surface ideally.  If the damper ring isn't making full contact, it isn't working properly.  In reality, probably won't feel any difference - could always use a little heavier oil.

[scottly]

My K7 fork tubes also have the seam on the inside.

[Toby84]

Thanks for your input.

I had another similar set of forks in the garage, and checked one of them. It also have the small groove. Unfortunately it's a lot more pitted and rusty, so I can't really use it to get a set of two similar tubes. I'll try it as it is, and see if I notice any problems riding it.

Tobias