Fork oil weight

[pasteurized]

I am doing an upgrade to dual discs on my GS750. I will have all the parts in about a week. I will want replace the fork seals and flush and refill the forks. I am a pretty big guy (6'1", 225). I figured on about a 20wt oil. Can I use 20wt motor oil instead of "fork" oil? I'm wondering what the differences are, if their viscosities are measured equally.

[GSstiny]

My roommate just did the forks on his katana and he used 15 wt. He weighs about the same as you and he said it made HUGE difference. He says its almost too stiff when its cold. Going to 20 may be a little extreme. Hopefully somebody with a real answer will chime in.

[8trackmind]

I am doing an upgrade to dual discs on my GS750. I will have all the parts in about a week. I will want replace the fork seals and flush and refill the forks. I am a pretty big guy (6'1", 225). I figured on about a 20wt oil. Can I use 20wt motor oil instead of "fork" oil? I'm wondering what the differences are, if their viscosities are measured equally.

Why not just use fork oil?

[tconroy]

my bible(the clymers manual) says to use sae 10or20 fork oil or automatic transmission fluid mixed 1:1 with sae 10w/30 motor oil.This is in the 77 thru 82 750 manual.

[tfb]

Well you can use motor oil or just about any oil you like, but fork oil is supposed to be formulated to minimise 'frothing' and the resulting loss of damping.

One argument against fork oil, let's face it, is that it's so expensive. And not only that, but the wiley manufacturers ALWAYS set the bottle volumes so that you need just over a bottle full to do your forks, thus forcing you to buy two bottles when you really only need 1.1

Mike.

[rustybronco]

Well here is my thinking, transmission fluid is about a 7wt oil hi detergent, 10w30 is 10wt base stocks with an additive package to make it an effective 30wt when at a hot engine temp so... it will be about 8.5 wt oil when done, maybe a little more @ 100 degrees.
I'm doing my forks this evening and refilling them with 10wt engine oil, i just have to make the pcv spacer for preload.
all i can find around here is 5wt or 7.5wt fork oil.
My thought is they have drains on the forks so why not try it!!!

[KGB]

On both my bikes....wait I sold the 100E.....

Anyway on both the 1100E and 1000S with new Progressive suspension installed, I used 20 weight fork oil.
HUGE difference. Firm but not too firm.
The front end on both bikes feel/felt planted.
No shimmy or shake, no wobble.
Under hard braking yea some dive which is normal.
I never used the anti dive on the 1100E.
The 1000S does not have ant dive on the forks.
I knew a guy that used 20 weight in one leg and 15 in the other.
he said it would balance out, I never noticed a difference.
I think it was in his head..........

I'm about the same size as you 6ft 245lbs.


As someone said, you have drain plugs on the legs try it.

[mhardig]

Don't know nothing about fork oil

but if fork oil is like automatic transmission fluid, I'd be careful. The additives in ATF are not compatible, for instance, with the additives in engine oil, and vice-versa. It's an old used-car-lot trick to throw a quart of ATF into a leaky engine, which causes leaky seals to swell temporarily, so the car seems solid. I'd be concerned about the compatibility of the seals- it could be a lot more complex than just draining the bad oil out, you might end up changing all your seals too.

Just my .02

[rkubik]

substantial differences in viscosity ratings

If you click the "June 1998" option in this link

http://www.maximausa.com/technical/lubenews.html

it has a PDF that explains the differences between viscosity ratings.

A related question: can anyone advise how to post a PDF or link to same? Do you do it with photobucket?

[rustybronco]

discussion time... used in an engine, brake fluid as a seal sweller a.t.f. as a cleaner (lifters ect.) kerosene (or #1 fuel oil) cleaner and oil thinner, correct me if i'm wrong.
I assembled my sons yamaha 650 with type "F" a.t.f. last year still going.
i'm the guinea pig.

Let the testing begin...

[rustybronco]

oops, forgot about a.t.f. as a seal softner, it's been a long time.
the mind doesn't work as fast as it did...

[edbot]

I'm looking at the 1977 GS750 factory service manual page 3. It states front fork oil a mixture of SAE 10W/30 motor oil and ATF, the ratio being 50 to 50 percent. Good luck with whatever you do.

[edbot]

I'm looking at the 1977 GS750 factory service manual page 3. It states, front fork oil a mixture of SAE 10W/30 motor oil and ATF, the ratio being 50 to 50 percent. Good luck with whatever you do.

[Cajun Cycler]

Well you can use motor oil or just about any oil you like, but fork oil is supposed to be formulated to minimise 'frothing' and the resulting loss of damping.

One argument against fork oil, let's face it, is that it's so expensive. And not only that, but the wiley manufacturers ALWAYS set the bottle volumes so that you need just over a bottle full to do your forks, thus forcing you to buy two bottles when you really only need 1.1

Mike.

Are you kidding me? Honda sells fork oil in qt bottles and you need about 30cc or less per leg. How can any bike use an entire quart of fork oil and then some?? BTW a bottle of fork oil is NOT expensive, unless you're buying it a Sachs Fifth Avenue.

[rustybronco]

For me the cost is not the problem @ 8-9 dollars a quart, just trying to find out what will substitute when no one around had 10 or 15wt fork oil in stock (5 dealers)
I have read the shop manual where it said one can use atf or a blend of motor oil and atf and that is about 8.5wt which is TOO light in my opinion, i found 5 and 7.5wt around here not the higher weights.
I used the factory fork seals NOT the aftermarket seals that were in my forks when i got it, they only lasted about a year! so it's not about the economy of it.
this summer i will use a blend of 20wt and a.t.f. (atf has anti foaming properties) to try to get a 12-15 wt oil. so for me it's a test...