phillipm
Post #1
Thought I'd make a little FAQ given a lot of the forum are on rebuildable dampers (GAZ, twin tube Koni, KW, etc), on changing the oil and replacing the seals, save you a few pennies.This guide is NOT for monotube and high pressure gas struts, these need de-gassing before use and special kit to regas them, you'll take your eye out opening them and then have to harass me to refill them
) My old pair of Gaz are a little past their best, and the casings were rusty, so they got bunged in some rust remover to clean them up:
After checking them on the dyno I've decided to go the whole hog and rebuild them, so they're currently in bits, it's a fairly simple job (although I'm altering some bits on mine, changing the piston shims to the same as my current set of GAZ and some other mods), so I'll update this thread and go through it over the next couple of weeks, given there's damper bits all over my kitchen table I may as well do it now
I'll go over what's needed to do the same to standard dampers next month too, although that needs more tools than most people will have available.
Pretty simple to open up, just hold the damper casing in some soft jaws and undo the top nut with a decent set of molegrips or adjustables, it'll be bloody tight, ~80lbft, [pic later]
The insert is held in - and sealed by - a nitrile o-ring just below the thread, carefully lift this out - you'll need to replace this with a new one, don't be tempted to re-use it, they distort under the amount of torque and never seal properly the next time, the top o-ring on the nut is re-usable but for the money it'll cost - just replace it.
Anyway, the inner tube (cartridge) will just lift out at this point, so you can see the oil and the footvalve.
You'll see now why it's not a concern that you're clamping the outer tube in soft jaws to undo the nut - it's simply a reseviour for oil and gas, so a little distortion doesn't affect it.
OK, the standard oil in most of these is a straight hydraulic-grade 5w mineral oil, it'll be fooked after a couple of years and a lot thinner than it started out.
Personally I'm going to replace mine with a 7w RDC synthetic ('cause I get it free from the guy who supplies the dampers for the rally car), but you will be best sticking to original weight oil as otherwise the damping will change.
My personal preferance is for either Silkolene 02, or Rock Oil's SVI 5.
Both are multigrade synthetics so they'll retain the damping better when hot, last longer, and they both run specific moly-enhanced additive packages - these compromise some aspects of the oil very slightly, but the friction and heat reduction they offer are of a lot more benefit for a strut which is taking lateral suspension loads, and will help reduce wear.
For the same reason do not use an oil designed simply for shock/damper use, it needs to be a fork/strut oil.
For GAZ you'll need 260ml per strut standard, KW's I think are 275ml, my own settings use far higher but I'll get onto the whys and wherefores of that later.
Anyhow, you'll have to run the damper insert through it's stroke several times to remove most of the oil, don't worry about getting it all out as we're going to open the cartridge up to check the wear, seals and piston shims anyway, but be very careful not to use a lot of force as you can damage the piston by bottoming/topping it out whilst pumping the oil out.
By now you'll have a spare tub of none-too-healthy looking oil, 2 fooked o-rings that need replacing, and 3 bits of damper.
On the upside, you should have 2 shiny new neoprene o-rings, and a bottle of shiny orange/blue synthetic damper oil.
Now, you can just refill the damper with your new oil, bleed the cartridge (put it in the oil in the strut and pump it a few times to get the air out, you'll feel it in the stroke when it's gone), then, this is important - extend the piston rod to it's full travel before fitting your new o-ring and screwing the top nut back on, voila, fresh oil and better damping again.
The purpose of extending the strut rod first is to trap a cushion of air in the damper, which give a slight air spring effect at the bottom of the dampers travel and helps prevent bottoming.
This is also why you need to stick the exact amount of oil in you removed - fill it too far and the damper will experience a hydraulic lock at full bump and bend something expensive...
For those of a more curious nature, or with a big hammer and punch they're itching to use, we're going to open the cartridge up...
To be continued...
________________________________________
- Bespoke rollcages/additions/adjustments. Half cages right up to complete custom spaceframes - MSA/FIA spec, CDS, ROPT, T45, etc - PM meEmail me!
Custom-made polybushes available - need an odd size or fitment? - anything from batch work to one-off pieces.
Posted 10th Apr 2009 at 04:03
