Is it possible to rebuild struts? I mean, is the part that wears out the o-ring/grommit atop the cylinder? I have no clue how the struts are assembled in the factory, but to my mind anything that is put together can be disassembled, and in that case, coulnd't the worn parts just be replaced?
I will but putting on NOS struts on my 411 sometime in the next decade (ha ha ha - 12 cars is about 11 more than I have time to maintain!) and am tempted to play surgeon on the strut inserts that I remove.
Justin
Is it possible to re-build struts?
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vwbill
- Posts: 970
- Joined: Sat Feb 15, 2003 12:01 am
Strut rebuild?
Hey check some of Ray's post to me about the struts! They are a work of art! I have somepics of the insides I can email. I think you are better going with a Ray option to start with and then maybe play down the road!
I still have the fluid from one of my tubes but I dont think I would trust rebuilding the originals maybe if you could get someone in that field to do it? bill
I still have the fluid from one of my tubes but I dont think I would trust rebuilding the originals maybe if you could get someone in that field to do it? bill
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raygreenwood
- Posts: 11912
- Joined: Wed Jan 22, 2003 12:01 am
I spent almost 6 months working on 3 sets. They cannot be rebuilt...at least not in any simple manner. Heres why. It helps to have one open in front of you. These struts.....are a work of art. The original oil struts I mean.
They have a very complex fluid path. The strut tube (outer)...other than being a support...is the resoivoir for the oil. The oil, must be of the exact correct vicosity, temperature range....and volume. No kidding. This is all critical. If the oil level is just a bit too high...it will blow the top seal out all over the place, and also be of such high resistance that it can destroy both your strut bearings and ball joints....in the short amount of time it lives. If its too low, the strut starves on multiple short pumps...like going over railroad tracks. Again...it then bottoms out...hammering the ball joints. Wrong temp range....a cold day causes hard as hell struts....broken ball joints.
OK. Weve described the outer tube...that bolts onto the body.
When you remove the top cap...and pull out the strut unit, you see a mushroom shaped head with an O-ring that seals the strut rod core/tube to the inside of the outer strut tube. Thats a fitted O-ring. Its squarish...and must be very well fit...or it either leaks or is a bear to get in without damage.
Take a close look at that mushroom shaped head. It has a sintered bronze rod bearing in it. That "sleeve"...mush be pressed in the exact amount. Too deep....and it blocks a set of return passages. See that ring of holes around the bottom of the mushroom? Like 3 or so? Well...here is how it all works.
At rest....lets say 2/3 of the fluid is outside of the strut core...the silover tube you pulled out. 1/3 of the fluid is inside. 2/3 of that, is on the underside of the piston that is on the bottom of the rod. The piston has a single finely honed and fitted piston ring, more on that later.
OK...so your tire goes into a hole.....the strut extends.. rod/piston pulls upward toward the strut bearing. See that castellated nut on the bottom of the tube? Look in the center...it is holding...a stainless steel plate with a single hole about .020" in diameter or less. That is the metering orifice. It governs how fast that rod pulls up...by how fast the fluid can be pulled through it. Now....other than the spring....what cushions the rod at the top and slows it down? The fluid on top of the piston. It...pumps upward.....to the mushroon seal plug...and flows outward through the 3 holes.... and is metered by them....wherin...it then flows back into the resoivoir of the main strut tube. OK....now the wheel is back on level ground and hits a rock.....the rod compresses. On the bottom of the piston head...there is a nut.....and a spring. In between them...is a stack of flat shims....they are valves. They have cut outs around the inner hole. The number and size of the cut-outs, governs compression. This is your shock valving. They vent to a set of holes on top of the piston.....so they vent their oil to the top side of the piston. They quit venting, when the rod quits moving downward. So if the rod goes way down...like maximum compression of the strut spring......there is a lot more oil on the top side of the piston than there was before...in the at est state. You need that....because with the spring about to unload from its compressed state.....its going to need a lot more oil to flow through the orifices...faster inside of the mushroom at the top...generating harder braking action for the unloading of the spring.
As you can see....its complex....and finely balanced. In fact...most aftermarket struts are really built exactly the same, but with a twin tube construction and sealed in a unit. A tube inside a tube. There are some exceptions like the Monroe/Bilsteins...that have a graduated set of grooves around the wall of the tube that act as the balance and metering valve...and just a basic set of plates in the piston head. Very smooth.
The flaw in the original strut system was this. There is a top rubber shaft seal. It would wear out and allow dust to tear up the sintered rod bushing. Thats your first minor leak point. The next...is the O-ring around the mushroom head. Thats usually major.
What happens is that a good portion of the oil leaks out. The stuts gets hot from motion, poor cooling from low oil and the occasional bottoming out dry in the bottom chamber of the whole piston/rod assembly. Along with this....with moisture seeping past the seal... the oil starts to break down. What you get...is wear to the inside of the tube where the piston ring runs.
To repair these....you need a replacment for the sintered bushing, the top seal....which is quite unique, a new piston ring...unobtanium, and a way to bring back the tolerances of the tube....like chrome plating and honing. Then little things like what viscosity of oil...its similar to brake fluid but is not, and how far to actually insert the bronze bushing to not impede the flow of oil through the upper replenishment orifices. Its touchy.
If you do happen to find a pair that have been sitting, maybe leaking, but have no wear or grooving or rust.....and you can find an oil that works well.....they should run for years. But...they were weakly valved in the first place. This is the same stock part that was in the superbeetle strut in all respects...except different valving. So you can scavenge valve plates, and piston assemblies.Ray
They have a very complex fluid path. The strut tube (outer)...other than being a support...is the resoivoir for the oil. The oil, must be of the exact correct vicosity, temperature range....and volume. No kidding. This is all critical. If the oil level is just a bit too high...it will blow the top seal out all over the place, and also be of such high resistance that it can destroy both your strut bearings and ball joints....in the short amount of time it lives. If its too low, the strut starves on multiple short pumps...like going over railroad tracks. Again...it then bottoms out...hammering the ball joints. Wrong temp range....a cold day causes hard as hell struts....broken ball joints.
OK. Weve described the outer tube...that bolts onto the body.
When you remove the top cap...and pull out the strut unit, you see a mushroom shaped head with an O-ring that seals the strut rod core/tube to the inside of the outer strut tube. Thats a fitted O-ring. Its squarish...and must be very well fit...or it either leaks or is a bear to get in without damage.
Take a close look at that mushroom shaped head. It has a sintered bronze rod bearing in it. That "sleeve"...mush be pressed in the exact amount. Too deep....and it blocks a set of return passages. See that ring of holes around the bottom of the mushroom? Like 3 or so? Well...here is how it all works.
At rest....lets say 2/3 of the fluid is outside of the strut core...the silover tube you pulled out. 1/3 of the fluid is inside. 2/3 of that, is on the underside of the piston that is on the bottom of the rod. The piston has a single finely honed and fitted piston ring, more on that later.
OK...so your tire goes into a hole.....the strut extends.. rod/piston pulls upward toward the strut bearing. See that castellated nut on the bottom of the tube? Look in the center...it is holding...a stainless steel plate with a single hole about .020" in diameter or less. That is the metering orifice. It governs how fast that rod pulls up...by how fast the fluid can be pulled through it. Now....other than the spring....what cushions the rod at the top and slows it down? The fluid on top of the piston. It...pumps upward.....to the mushroon seal plug...and flows outward through the 3 holes.... and is metered by them....wherin...it then flows back into the resoivoir of the main strut tube. OK....now the wheel is back on level ground and hits a rock.....the rod compresses. On the bottom of the piston head...there is a nut.....and a spring. In between them...is a stack of flat shims....they are valves. They have cut outs around the inner hole. The number and size of the cut-outs, governs compression. This is your shock valving. They vent to a set of holes on top of the piston.....so they vent their oil to the top side of the piston. They quit venting, when the rod quits moving downward. So if the rod goes way down...like maximum compression of the strut spring......there is a lot more oil on the top side of the piston than there was before...in the at est state. You need that....because with the spring about to unload from its compressed state.....its going to need a lot more oil to flow through the orifices...faster inside of the mushroom at the top...generating harder braking action for the unloading of the spring.
As you can see....its complex....and finely balanced. In fact...most aftermarket struts are really built exactly the same, but with a twin tube construction and sealed in a unit. A tube inside a tube. There are some exceptions like the Monroe/Bilsteins...that have a graduated set of grooves around the wall of the tube that act as the balance and metering valve...and just a basic set of plates in the piston head. Very smooth.
The flaw in the original strut system was this. There is a top rubber shaft seal. It would wear out and allow dust to tear up the sintered rod bushing. Thats your first minor leak point. The next...is the O-ring around the mushroom head. Thats usually major.
What happens is that a good portion of the oil leaks out. The stuts gets hot from motion, poor cooling from low oil and the occasional bottoming out dry in the bottom chamber of the whole piston/rod assembly. Along with this....with moisture seeping past the seal... the oil starts to break down. What you get...is wear to the inside of the tube where the piston ring runs.
To repair these....you need a replacment for the sintered bushing, the top seal....which is quite unique, a new piston ring...unobtanium, and a way to bring back the tolerances of the tube....like chrome plating and honing. Then little things like what viscosity of oil...its similar to brake fluid but is not, and how far to actually insert the bronze bushing to not impede the flow of oil through the upper replenishment orifices. Its touchy.
If you do happen to find a pair that have been sitting, maybe leaking, but have no wear or grooving or rust.....and you can find an oil that works well.....they should run for years. But...they were weakly valved in the first place. This is the same stock part that was in the superbeetle strut in all respects...except different valving. So you can scavenge valve plates, and piston assemblies.Ray