YZ250F (1979) Shock Oil Change

[BadgerMan]

I recently acquired a 1979 YZ250F that I have decided to restore. Although the shock is in great shape, I suspect that it has never had a fluid change. Does anyone have some advice for doing an oil change/rebuild of this shock? It looks like a pretty straight forward job but I have the following specific questions:

1. I am assuming that there is no bladder and that the shock has a floating piston in which case, what position (measurement) should the piston be in when you fill the shock with oil?

2. What pressure should the nitrogen charge be set at?

 

[kawamaha]

IMHO it doesn't matter if the piston is 1mm higher or lower when it is anywhere in between the top position and about 10mm below.
to imagine what the piston does you can look at the drawing:

www dot supercross-online dot de/Z/

scroll down to "How does the PDS System work"

for example, if the rod diameter is 14mm, travel is 100mm, piston diameter is 40mm, then the piston will move approx. 12mm, thats not too much.


I'm almost sure even this old shock will need 10bar / 145psi to work properly

 

[Jeremy Wilkey]

Great advice. I would add you can count on the seal head displacing the required volume to position the piston correctly. As long as the rod top out is against the seal head and the piston is bottomed negative right as your o-ring seals.

Another good way to keep the piston positioned is to add nitrogen into the reservoir, this will keep the piston down and make the whole process of getting the seal head at the right point. Once the o-ring seals you will just need to release the pressure to place the seal head.

I'd always check, with a dipstick but it’s a good basic safe rule. Run it through and make sure you have no air and that you get full travel with no pressure rise.

Some applications such as Fox and Elka, have very little margin for error, so be cautious!


BR,

Jer

Jeremy P. Wilkey

MX-TECH Suspension Inc.
4136W. 6940N. Rd.
Bourbonnais, IL 60914

815-936-6277

MX-TECH West
Palo Verde suspension Inc.
5607 N. 53rd Ave
Glendale, AZ 85301

623-463-3488

 

[BadgerMan]

I was hoping for a measurement since the floating piston position during reassembly will have a critical effect on oil volume in the shock body (there is not separate reservoir). The piston needs to reside somewhere between the bottom (top?) of the shock body and the bottomed out top of the shock shaft, during assembly. I assume that it will change position slightly when the shock is charged.

Please have patience with me as I have not dealt with this type of shock before…..only bladder type shocks. One day, I will also work up the nerve to learn to service my KTM PDS shocks.

BTW Jer, are parts available for these shocks? I need a bottom bumper but I also assume I will be needing a seal head, piston ring, and piston o-ring. What other parts typically need to be replaced? The shock seems to have pretty good damping so I suspect that the shock body is serviceable. Also, it is not leaking any fluid.

I will call when I get the unit apart.

Thanks,
Tim

 

[Jeremy Wilkey]

That’s a tough one! You need to end up with 30% more nitrogen volume than rod volume. So I would get a piece of paper and draw some cylinders. Figure out what you have in rod CC's and then add 30% on top of that plus the volume taken up as the seal head is positioned. Figure out the height of the resulting cylinder and that’s your piston position. You may want to fill the gas side with a light fluid that will drain out easily and clean out with no residue using contact cleaner. By positioning the piston and then adding the fluid you can keep the piston in position during bleeding etc. Then after the bleeding remove the fluid, and allow the seal head to seat. Then clean the gas side, dry and gas up. It’s not easy and you will need to check operation if you can feel a pressure rise before the bumper hits you don't have enough nitrogen volume, adjust accordingly.

I don't stock the parts for this shock. RT is your best source.

BR,

Jer

 

[BadgerMan]

It sounds complicated but not impossible. I wonder how they did it back in the day? :coocoo:

Thanks for your help. :nod:

 

[TRexRacing]

Kinda pricey but sweet.Especially the bladder.