Cobra bottle jack failure

I had a failed jack that I took apart to see why it was not
maintaining the extended position. In my case the ramp would slowly
lower back to collapsed postition, which besides annoying, can be a
danger to the fuselage. Turns out that it was the synthetic plunger
on the main piston leaking. So even under pressure the oil would seep
past the plunger and the piston slowly looses position till it
eventually collapses. The culprit does not seem to be any of the
various ball-bearings in the base of the jack.

Anybody know if a replacement plunger could be obtained. I am sure
its much cheaper than a new jack.

On Oct 28, 1:53*pm, db_sonic wrote:
I had a failed jack that I took apart to see why it was not
maintaining the extended position. In my case the ramp would slowly
lower back to collapsed postition, which besides annoying, can be a
danger to the fuselage. *Turns out that it was the synthetic plunger
on the main piston leaking. *So even under pressure the oil would seep
past the plunger and the piston slowly looses position till it
eventually collapses. *The culprit does not seem to be any of the
various ball-bearings in the base of the jack.

Anybody know if a replacement plunger could be obtained. *I am sure
its much cheaper than a new jack.

Darren

Most people just replace the jack ~$80/$90 or so and not waste time
messing around. Unfortuantely Cobra uses junk jacks and will likely
fail in a few years again.

Williams Soaring should have them.

Darryl

Yep, actually I already did. But before doing that I had attempted
to fix it and was disappointed when I couldn't figure it out. Till
last night when a cracked it open. Incidentally took a monkey wrench
and a propane torch to get the top off and I really had to lean into
it.

You might be right though that any replacement part would just fail
again... Just seems wrong though to toss a 5 lb chunk of metal into
the garbage can.

Darren

Most people just replace the jack ~$80/$90 or so and not waste time
messing around. Unfortuantely Cobra uses junk jacks and will likely
fail in a few years again.

Williams Soaring should have them.

Darryl

On Oct 28, 5:52*pm, db_sonic wrote:
Yep, actually I already did. * But before doing that I had attempted
to fix it and was disappointed when I couldn't figure it out. *Till
last night when a cracked it open. *Incidentally took a monkey wrench
and a propane torch to get the top off and I really had to lean into
it.

You might be right though that any replacement part would just fail
again... *Just seems wrong though to toss a 5 lb chunk of metal into
the garbage can.

Darren

Most people just replace the jack ~$80/$90 or so and not waste time
messing around. Unfortuantely Cobra uses junk jacks and will likely
fail in a few years again.

Williams Soaring should have them.

Darryl

If you figure it out, you could have a nice overhaul biz...

On Oct 28, 4:18*pm, Dave Nadler wrote:
On Oct 28, 5:52*pm, db_sonic wrote:





Yep, actually I already did. * But before doing that I had attempted
to fix it and was disappointed when I couldn't figure it out. *Till
last night when a cracked it open. *Incidentally took a monkey wrench
and a propane torch to get the top off and I really had to lean into
it.

You might be right though that any replacement part would just fail
again... *Just seems wrong though to toss a 5 lb chunk of metal into
the garbage can.

Darren

Most people just replace the jack ~$80/$90 or so and not waste time
messing around. Unfortuantely Cobra uses junk jacks and will likely
fail in a few years again.

Williams Soaring should have them.

Darryl

If you figure it out, you could have a nice overhaul biz...- Hide quoted text -

- Show quoted text -

Changed the subject to something more apropos ; ).

On Oct 28, 4:54*pm, db_sonic wrote:
On Oct 28, 4:18*pm, Dave Nadler wrote:





On Oct 28, 5:52*pm, db_sonic wrote:

Yep, actually I already did. * But before doing that I had attempted
to fix it and was disappointed when I couldn't figure it out. *Till
last night when a cracked it open. *Incidentally took a monkey wrench
and a propane torch to get the top off and I really had to lean into
it.

You might be right though that any replacement part would just fail
again... *Just seems wrong though to toss a 5 lb chunk of metal into
the garbage can.

Darren

Most people just replace the jack ~$80/$90 or so and not waste time
messing around. Unfortuantely Cobra uses junk jacks and will likely
fail in a few years again.

Williams Soaring should have them.

Darryl

If you figure it out, you could have a nice overhaul biz...- Hide quoted text -

- Show quoted text -

Changed the subject to something more apropos ; ).- Hide quoted text -

- Show quoted text -

Good move. I think there's a club member here with the standard cobra
bleed-down problem. If he replaces his, I'll try to talk him out of
the old one. It's just a hydraulic jack, and I have more than one
that are over 30 years old and never leaked a bit, so I'm pretty sure
it's possible. I have access to a machine shop. Who knows? I might
make a fortune selling 1 or 2 rebuilt jacks a year!!

On Oct 28, 2:53*pm, db_sonic wrote:
I had a failed jack that I took apart to see why it was not
maintaining the extended position. In my case the ramp would slowly
lower back to collapsed postition, which besides annoying, can be a
danger to the fuselage. *Turns out that it was the synthetic plunger
on the main piston leaking. *So even under pressure the oil would seep
past the plunger and the piston slowly looses position till it
eventually collapses. *The culprit does not seem to be any of the
various ball-bearings in the base of the jack.

Anybody know if a replacement plunger could be obtained. *I am sure
its much cheaper than a new jack.

I took a look at a Cobra jack for a friend. It's not actually a
"bottle jack" but more likely a replacement part for a "floor jack" -
those long handled things on caster wheels you see in auto shops. The
giveaway is the trunnions on the hydraulic cylinder which are not seen
on mere bottle jacks.

In the car jack application, "leak down" isn't an issue since it just
has to hold long enough to put jack stands under the car after which
the jack is lowered and removed. That makes me think that you will
never get it to stop leaking.

Scissor jacks are terrible since they have the least mechanical
advantage when you need it most. The best solution I've seen is
scissor jacks with the crank handle replaced with a large nut which
the owner turns with a humongous slow speed cordless electric wrench
bought specifically for the purpose.

Bill D

"bildan" wrote in message
...
On Oct 28, 2:53 pm, db_sonic wrote:
I had a failed jack that I took apart to see why it was not
maintaining the extended position. In my case the ramp would slowly
lower back to collapsed postition, which besides annoying, can be a
danger to the fuselage. Turns out that it was the synthetic plunger
on the main piston leaking. So even under pressure the oil would seep
past the plunger and the piston slowly looses position till it
eventually collapses. The culprit does not seem to be any of the
various ball-bearings in the base of the jack.

Anybody know if a replacement plunger could be obtained. I am sure
its much cheaper than a new jack.

I took a look at a Cobra jack for a friend. It's not actually a
"bottle jack" but more likely a replacement part for a "floor jack" -
those long handled things on caster wheels you see in auto shops. The
giveaway is the trunnions on the hydraulic cylinder which are not seen
on mere bottle jacks.

In the car jack application, "leak down" isn't an issue since it just
has to hold long enough to put jack stands under the car after which
the jack is lowered and removed. That makes me think that you will
never get it to stop leaking.

Scissor jacks are terrible since they have the least mechanical
advantage when you need it most. The best solution I've seen is
scissor jacks with the crank handle replaced with a large nut which
the owner turns with a humongous slow speed cordless electric wrench
bought specifically for the purpose.

Bill D


About a year ago, I started a similar thread. I got three responses to an
offer to do rebuilds. I couldn't do it for a lot less than a "Cobra"
replacement.

The hydraulic in a cheap car jack is similar, but not exacly the same as a
"Cobra". A custom bushing or two would do the conversion. To make it worse,
Cobra has changed the dimensions at least once.

So--if you can do your own machining, you can easily replace the hydraulic.
You also could "clean up" the innards of the "Cobra" jack and it would work
better.
But compared to other stuff in gliding, the "Cobra" replacement is so cheap
that it may not be worth the effort.

Hartley Falbaum

On Oct 28, 1:53*pm, db_sonic wrote:
I had a failed jack that I took apart to see why it was not
maintaining the extended position. In my case the ramp would slowly
lower back to collapsed postition, which besides annoying, can be a
danger to the fuselage. *Turns out that it was the synthetic plunger
on the main piston leaking. *So even under pressure the oil would seep
past the plunger and the piston slowly looses position till it
eventually collapses. *The culprit does not seem to be any of the
various ball-bearings in the base of the jack.

Anybody know if a replacement plunger could be obtained. *I am sure
its much cheaper than a new jack.

Unscrew (remove) the bleed down screw dig out the old oring replace
with a new one. Worked for me.