1988 K75S (Sport Fork) Fork Oil Weight Question

[jay1622]

While I don't want my wrists sore after a spirited ride, I do like a slightly stiffer suspension regarding response, feel, and overall performance. The book recommends a five weight for the original spec.  Would you all recommend a 7.5 weight instead?

 Moderator, would this be better served in the workshop?  Still new here and getting a feel for the land.

[TimTyler]

I'd try using slightly longer spacers in the fork tubes first. It's just inexpensive PVC that sits on top of the springs. A little goes a long way, like maybe 0.5" longer.

[jay1622]

Thank you for the timely response! The fork rebuild will be one of the first things I do to the bike. I have my new seals and progressive spring kit ready to go. The kit came with the PVC pipe you speak of. 1/2 inch additional to spec should do it? That should be easy enough then. If I don't like it, I can just as easily cut out that extra half inch section, huh? You would stick with the 5 weight then?

[TimTyler]

I just switched from 7wt to Honda 10w. Will put some miles on this week and report back.

When you disassemble your forks measure the spring length and spacer length for each side. When you put it all back together just make things a little longer.

I'd also recommend a couple of replacement "Guide Rings" part # 31421450517.

And get fresh crush washers from the dealer. Anything you find in your local auto part store won't fit right. At least two each of 07119963030, 07119963041 and 07119963073.

[jay1622]

Thank you for the heads up. The crush washers are on their way; however, I had not considered the guide rings.  Are these a wear item? Also, the fixed tube has a near mirror finish from the previous seals. Should I scuff that area with anything or leave it polished looking? I have the rancho 1952 boot covers on the way FYI.

[TimTyler]

I had not considered the guide rings.  Are these a wear item?

Mine were brittle and one broke when I removed them.

[TimTyler]

Anybody else have any suggestion re the original question? I'm certainly not a suspension expert.  :dunno

[K75RT Keith]

What weight and brand of fork oil are you using currently?

Fork oil weights are not consistent descriptions like motor oil.  So, what one company calls 10w another may call 7.5w or 15w. 

[jay1622]

Good afternoon, and thank you for following up and asking. The bike is a rebuild that my son and I are doing together. I bought this bike from a gentleman about three weeks ago. Before me picking it up from him, it sat in a storage bin for about five years.  I have yet to crack the forks, so I have no idea of what's currently in there, which includes quantity, set-up and condition.  What I do know though is that just rocking the bike back and forth with the brakes set there is significant dive in the front forks. There are no signs of leaks or damage, so I am guessing this almost 30 year old bike is just due for a rebuild.  I am going to paint the slider tubes and replace the drain seal washers, the fill seal washers, the springs, the seals, spacers and the guide rings.

[K75RT Keith]

Dive is common with k75 & k100 variants. Progressive springs help some. Spacers help some changing out the entire fork assembly for k1100 or gsxr front end pretty much resolves the issue

[analogman]

I'm interested in this k1100 suspension change you are referring to. What does it involve? Which model year would I need?

[K75RT Keith]

K1100RT or RS. Either in the liberry here or on Duck's web page is a write up on how to do it. Other choice is to have a machine sop bore out the fork holes to accept the larger K1100 forks

[TimTyler]

I just switched from 7wt to Honda 10w. Will put some miles on this week and report back.

After 1000 miles I'm reporting that the Honda 10w fork oil works great in my naked K75 loaded for touring.