Rebuilding Bitubo dampers: Driving me nuts.

[mattdeme]

Have already seen https://wildguzzi.com/forum/index.php?topic=65449.0#lastPost and https://wildguzzi.com/forum/index.php?topic=89277.0

Have used the Stein D rebuild kit.
Replaced all o-rings including bottom chamber. (Not sure if this should have oil or air in it, I left air in it).
Left 6cm space at top of damper tube and used 5W as suggested, this leaves the piston out of fluid at top of stroke. Can't find any definitive specification for how much should be in.  Would love to know what others have found works. (The 70cc in the manual is for the 10W put in the legs outside the dampers I believe).

Both my dampers seem identical but one is smooth and the other locks up when bouncing by hand and body pressure, the difference is obvious.
One returns immediately and the other just crawls back to position.
I have had them apart about 5 times now and cannot work out what I'm doing wrong.
The parts are all in the same and in the same place on both. Same amount of oil in both.

Also had trouble with the nut at the bottom of the tubes un-winding itself and have actually had the adjuster knobs flying off while riding.
Decided best to assemble with bottom nut two turns out from fully screwed on and then attaching knobs. (had been using 3 when dampers dismantled themselves).

Also, does anyone know if any other parts for these are available anywhere (such as the damper adjuster knobs) because mine are getting pretty ropey now.

[Chuck in Indiana]

Hmmm, this is another of those projects that are pretty hard to do through a computer screen. I don't remember any issues except too much oil will lock them up. Sorry. Maybe this bump to the top will find someone that knows..

[guzzisteve]

Maybe try calling these guys

https://www.facebook.com/BituboUSA/

I used to get my Bitubo stuff from Hermy's in PA, didn't need rebuilding. You might call Hermy's also, Port Clinton, PA

[mattdeme]

Thanks for the input guy.
A bit more experimenting with the oil quantity, I tried 8cm below top of damper tube and it now works fine.

[flangeman_70]

G'day Matt

I'll try and help. I have heaps of experience with these things now, after going through all the horrors you are experiencing. Today, I even have multiple sets with different springs and oil weights

Cleanliness is critical. Pull everything apart and clean it and replace the O'rings on the Adjuster rod also

, trust me, it's worth it. I also polished the damper piston/porting seals, I was being anal but I am truly satisfied with the results.



The bottom chamber should have air only in it. When the piston O rings wear out the oil will be pumped in and the air out and you will end up with a very uncomfortable ride. If you check the top of the air piston, you might see the imprint of the damper bottom nut on the air piston as it was hydraulically locked.

If you disassembled the valve body, i.e. removed the E clip and the sleeved bottom nut (this has the rebound adjust port drilled in the side)

,  the damper piston and the damping/porting disc and spring probably fell apart, you might have the port disc stuck offset on the spring retainer and disc guide

. This will cause the rod to sit where you place it in the stroke and it won't damp very well or return, yes I have learnt this the hard way

Filling took some serious experimentation. I ended up using Putoline 7.5W. Make sure the air chamber cylinders are thoroughly cleaned


dice and coin flip
, new O rings on both piston and damper cylinder bottom cap and test pistons are free to move and equally spaced from the tops of the damper body tube. Half fill the damper with the oil. Lube the damper piston seal and then insert the damper rod. gently push this into the oil. Fill the oil to the bottom of the threads. Gently stroke the damper up and down to purge the air out of the oil and leave it sit at the bottom of the stroke. Let them sit like this for about an hour so all the air is out. Add more oil to just under the top of the threads of the damper tube.  Gently withdraw the damper rod so the piston is just under the threads, add oil so it remains overfilled as you do this. Now fit the top cap hold the damper rod up so the top of the top out spring is against the bottom of the damper cap. Oil will overflow until the top cap O ring seals it off. Now test it. The damper should fully extend of it's own accord

I hope this helps.

Best of luck.

Adam

 

[Chuck in Indiana]

Excellent explanation, Adam..thanks for that. 

The bottom chamber should have air only in it. When the piston O rings wear out the oil will be pumped in and the air out and you will end up with a very uncomfortable ride. If you check the top of the air piston, you might see the imprint of the damper bottom nut on the air piston as it was hydraulically locked.

I never fully understood why that happened until now.