STP in shocks

fiddlybits

fiddlybits

Senior Member
If you are considering putting STP in your shocks I have one word..... Don't. (ok, technically it is two words contracted)

My shocks were damping but were just not right, and I had a little leakage around the outer shell, so I decided to open them to replace the seals and fully change the fluid.

What I found was black goop that smelled like burn tar and Stoddard solvent, MEK and brake cleaner does not dissolve it. I resorted to scraping and soda blasting it off.

Here are the front shocks.
 

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I would think it would not generally be a good idea to add anything to a shock that might change its viscosity of it's fluid.
 
See vol XIII No. 2 MAN P17-18

Ed Francis researched the engineering on the shocks. He converted the viscosity to somewheres between SAE 20 to SAE 70 Mineral oil and Castor oil. Non foaming is important.

This may translate to motorcycle shock oils. I have not got that far. I just know that ATF is too thin for a very tight shock and does not come close the timing as prescribed in the Service Bulletins with my guess of the length and weight of the KRW lever arm.

A shock with a very small amount of clearance has much less resistance.
 
Mike.. I didn't add anything. What you see is the way I got the shocks. After I opened them and could not disolve the goop I contacted the previous owner and asked what he had put in them. He proudly stated "STP. Makes them run smooth".

Yeah, right. It turns into ugly black goop the plugged up every orifice. It took almost 2 hours per shock just to get all the passages clean.

Kevin: vol XIII No. 2 MAN P17-18 hey, that's great. . . . of course I have no idea what you are referencing so it doesn't mean a thing.

I've had great success with Modern Motorcycle fork oil in all of the pre-50's shocks. It's compatible with all the seals, including the leather ones, and it's viscosity is temperature stable.
 
Understood,

So if I understand you, the shocks are still good, just needs cleaning up. My Model A has no shocks, getting new ones made...
 
mshmodela said:
Understood,

So if I understand you, the shocks are still good, just needs cleaning up. My Model A has no shocks, getting new ones made...
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Yup, they "just" needed cleaning up.. :eek:

The o-Rings are shot (dry rot) so it will be a few days before I can close them up and mount them but the are functioning again.

The interesting thing is that the car drives better with all the shocks out . Probably because only one shock was doing anything before. Now it's just bouncy bouncy bouncy.:D
 
The Scientifically Treated Petroleum bandage. I run friction shocks on my Model A; they function very well and don't ever leak.

Chet
 
I have an old can of Fidelity, "Houdaille Shock Absorber Fluid". Probably from the 30s or 40s and it's a very light oil, maybe 10 or 20 weight. No where near the consistency of STP. In fact, I was surprised at how light it was. I would have expected something heavier.
Terry
 
Me too, Chet!!! I also run friction shocks. I've heard of running STP or 600W gear oil in worn out original shocks but never tried it. I have never had much interest in tring to fix old original shocks
 
Isn't hydraulic jack oil the same viscosity( or close ) as the shock oil sold by dealers. Any reason it should not be used?
 
Original shocks didn't use oil . I can't think of the name of the fluid right now but it may be glisarine (probably not spelled correct). anyway the two substances may not mix. hydraulic oil is usually around 10 weight I believe.
 
I have rebuilt many original Model A shocks over the years and have used straight STP or a mixture of STP and 150W gear oil. It depends on how much wear the shocks have. I have nearly 60,000 miles on my daily driver with STP & oil in the shocks and they are still working fine. The shock is a very simple device, the principal operation is moving liquid from one chamber into another. The more clearance or wear between the parts the thicker the oil must be to compensate for the wear. Now I admit living in San Diego, cold weather is not an issue so I suspect STP in the winter up north would probably give a very stiff ride. Again for proper operation (60-40 action) it all depends on the internal wear of the moving parts and the viscosity of the liquid used.
 
Purdy, My can of "Houdaille Shock Absorber Fluid" (Not oil) has a copy right date of 1937 (fine print) but no indication of what the "Fluid" is. It smells very petroleum like. Old petroleum like! Sort of like the tractor shed on our farm, but I can't say what it actually is.
Terry



Purdy Swoft said:
Original shocks didn't use oil . I can't think of the name of the fluid right now but it may be glisarine (probably not spelled correct). anyway the two substances may not mix. hydraulic oil is usually around 10 weight I believe.
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The original shocks were filled with Glycerine and will not mix with oil . You need to clean with alcohol maybe denatured before adding new fluid . The original fluid contained Glycerine with 10 percent Alcohol when temperatures were below 0 they would add more alcohol 1/2 to 1 ounce per service bulletin .
 
Terry said:
Purdy, My can of "Houdaille Shock Absorber Fluid" (Not oil) has a copy right date of 1937 (fine print) but no indication of what the "Fluid" is. It smells very petroleum like. Old petroleum like! Sort of like the tractor shed on our farm, but I can't say what it actually is.
Terry
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Terry, I really don't know, but with a copy right date of 1937 on the lable maybe after model A-B production, Houdaille may have changed the fluid that was used. Dennis nailed it in post 16 about the use of glycerine as the fluid used in the model A shocks . I don't know anything about glycerine but remembered reading about it in my model A books. Glycerine is a word that I don't often see and I couldn't remember how to spell it . Oil wouldn't cause rust in the shocks and that would be a good reason why they would have changed the composition of the shock fluid.
 
It's probably apparent from reading the previous posts, but I'll put it down anyhoo: you can't mix some petroleum products (e.g. Oil, STP, grease, hydraulic fluid) with non-petroleum products (e.g. Alcohol, brake fluid). They are incompatible and will react in undesirable ways. I doubt just plain STP mixed with nothing else would hurt, but it's probably the wrong viscosity for the shocks. Adding STP when there's already something else in the shocks is a different story.

What you have looks like a mix of petro- & non-petro fluids, possibly STP & brake fluid or STP & glycerine.

Disclaimer: I'm not a chemist but I do know what happens when someone mixes red (aviation) hydraulic fluid with automotive brake fluid, and it ain't pretty. FYI most light aircraft brake systems use red (petro based) hydraulic fluid rather than brake fluid. Makes a real mess if the owner puts in his automotive brake fluid instead.
 
Alcohol was no doubt added to the originally used glycerine to prevent freezing. It was a common practice by some shade tree mechanics back in the day to add brake fluid to the old 50s cast iron powerglide transmissions to swell the rear seals and slow down fluid leaks .
 
I don't use STP in anything. I had 5 trucks on the road back in the 70's and 80's and one of my drivers thought STP was "the stuff". Cost me one power divider and two front spindles and wheel bearings and probably an engine.
 
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