Bonneville America Forums Forums Technical Forums Performance Mods Fork oil

I'm going with the progressive front springs & thought while I was doing that I'd change the fork oil.
Anyone know what viscosity is stock?
How much oil per side?
If you've changed the oil did you go synthetic? ( I plan to)
what brand of oil?

I just recently changed my fork oil when putting in the Progressive springs. the shop manual calls for Kayaba G10 fork oil. I believe it's 10 weight. I used an equal mix of Belray 10 and 15 to give me 12.5 after reading about it in here. The shop manual says that each fork holds 548cc(19.5oz) and that the fork oil level should be 166mm(6 1/2") below the inner tube upper surface when fully compressed.

I poured out the oil into a large Big Gulp type of cup from a Carolina Mudcats game, marked it's level with a Sharpie, poured out the old oil, and then filled it up to that mark on the cup with the new oil. Poured that into the fork, compressed it like the book says, and then measured. It came out close to 6" on both forks. I just left well enough alone and put everything back together. The Progressive fork springs I put in had the tight windings down so I guess they displaced a bit more oil than the stock springs which is probably why I ended up 1/2" high. Now I know that adding too much fork oil might possibly lead to a blown seal, but I haven't had a leak so far.

FWIW, the regular Bonneville/T100 calls for a fork oil level of 120mm when fully compressed instead of 160mm for our bikes. I wonder why that is?

Hi Gurdy,

I did put 15 weight oil in my forks but i am going back to 10 weight.

15 is to much mabye 12,5 is the best like Salmaglie´s done?

Do you need to remove the forks to change the oil how is the change done?

Hi Salmaglie,
I am replacing my fork springs with progressive springs as part of my front end rebuild. I looked at the documentation and it says that you should use less oil (it doesnt say how much less) because of the greater displacement of the progressive springs. 'OK then' I think , lets check technical notes on the ol' board; bound to be someone who knows there: but....

I am confused (nothing new there then )

From your earlier posting:

Quote:

---

the fork oil level should be 166mm(6 1/2") below the inner tube upper surface when fully compressed.

---

Surely thats with the springs OUT i.e. - no displacement at all.

When you refilled with same amount as you took out you measured the oil to 6" - half an inch higher than recomendation ....

Quote:

---

The Progressive fork springs I put in had the tight windings down so I guess they displaced a bit more oil than the stock springs which is probably why I ended up 1/2" high

---

How does that work then With the fork springs out the level should be the same!

In the manual it definately says measure oil first THEN put springs back in - or is the manual wrong?

Can anyone help with the correct oil level when using progressive springs?

Popeye

Yes, the measurement is taken with the springs out. I'm not sure I did it absolutely the correct way, and if I had to do it over again, I'd put in less oil NOT the same amount like I did. Overfilling isn't a good idea as it could lead to blown fork seals. How much less oil? Good question. I suppose if you had all the numbers of how much more the Progressive springs displace the fluid than the stock springs, but we don't.

OK - I have a plan.
I will put the correct amount of oil in and put in the original springs. Then measure the level with the spring IN.
I will then take out the spring - check the level again and work out the loss of the oil on the spring. Re-set to the correct level and put in the progressive spring and measure again. I will then 'adjust' the level to be the same as it was with the standard spring in, take out the progressive spring, measure the level and make the same allowance for oil loss - that should give us the correct oil level for the progressive spring - I'll post when its done. It wont be for a week or two though as my fork bottoms are at the chromers.

Popeye

That'd be great Popeye and a keeper for the archives. I didn't post my initial Progressive fork spring and oil change as a guide, but was just posting what I did. I'm by no means a stud mechanic and don't pretend to be one. I would hope that if I'm doing something wrong, somebody would point out the errors of my ways.

I might be taking mine back apart soon to get my lower legs powder-coated black since I won a $100 gift certificate from a local p-c shop. That's IF I get up the courage to do it. I've never had to mess with fork seals before, and Mike(bella corse) backed off from tackling the job and farmed it out instead. I'd certainly be interested in a description on how to do it with any tips. How about it Popeye?

I must admit its the first time I have attempted front forks. I wouldnt have done it now except they got damaged when the police car reversed into them! I was lucky though, most of the damage seems to have been confined to the lower nearside fork shroud, but I had to take them apart to be sure. Since I have them apart anyway I decided to go with the progressive springs and refinnish the lower legs where they had been scraped by the patrol car. The only tricky part is undoing the screw which holds in the damper tube, the problem is that it screws directly into the tube and as you undo the screw the tube turns with it. I got around this by pushing a broom handle down the forks and up against the damping tube then leaning on the fork end (upside down and pressed into the corner between the garage floor and the wall) while I turned the screw - tricky, but it worked, and its definately not in the manual! The next problem I foresee is drifting the bushes and new fork seal back into place. There is a special tool to do this, and rather than buy one (I dont anticipate doing this again) I approached my Triumph dealer who has kindly offered that If I take the forks along there when I get to that point, then they will do that for me - or lend me the tool so I can do it. After that I start playing with the oil. Other than that its all in the manual, thye relevant pages for which was kindly scanned and emailed for me by Dafydd.