COBRA Hydraulic Jack

I should add a couple of things on this topic.

- While the bottle jack approach is just as good as the scissors jack's when making fine UPWARD rigging adjustments (and great for fast coarse raising the fuselage), but is very much inferior when trying to make fine DOWNWARD rigging adjustments. IMHO.
- The foot actuator on the original "Cobra" hydaulic jack fits nicely onto the replacement bottle.
- The original jack has a cleverly bent lever attached to the release valve so that you can use your fingers to open and close it. The replacement bottle's valve is basically a short bolt with the head cut off and a slot cut into the end. You then use the pipe (used to pump the jack up/down) to open/close the valve by rotating the bolt. I use a large screwdriver instead.. Not quite as easy to use as the original bent lever. I was unsure about moving the original lever to the replacement bottle. Same threads? Same ability to close the release valve properly? That all comes down to the machining in the factory which, I fear, is pretty "iffy". Anyone re-invent this?

My $0.02,
John (OHM)

For those considering replacing their leaky Cobra hydraulic jack, better to replace the seal than deal with the cheap replacement that does not really fit. At least that is what I found with the Harbor Freight solution. I think it does not have the same stroke as I recall. I replaced mine about 8 years ago on now a 20 year old bottle and still working fine today.

Cost $5.42

http://www.mcmaster.com/#mechanical-seals/=ekh96p

part number 9505K82

Buzz Graves

On Wednesday, November 25, 2020 at 12:32:05 PM UTC-5, BG wrote:
For those considering replacing their leaky Cobra hydraulic jack, better to replace the seal than deal with the cheap replacement that does not really fit. At least that is what I found with the Harbor Freight solution. I think it does not have the same stroke as I recall. I replaced mine about 8 years ago on now a 20 year old bottle and still working fine today.

Cost $5.42

http://www.mcmaster.com/#mechanical-seals/=ekh96p

part number 9505K82

Buzz Graves

That's a good choice, Buzz! The key with all Polyurethane seals is to keep the fluid as dry as possible, meaning to keep the water content as low as possible. Polyurethane suffers from hydrolyses, the process by which water slowly penetrates and then dissolves the material from the inside out. The higher the pressure and the higher the water contents the faster the process progresses. Pressure is low in this application but the amount of water that accumulates in fluid may be high, so draining and replacing the oil in the jack may not be a bad idea to keep the seal happy.

Uli
'AS'