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Rope trick gone bad

scard

Well Known Member
Advertiser
The other day I performed some minor exhaust valve maintenance using the standard rope trick. I used 3/8" rope, which I'll never do again. Somehow I must have hit the lottery because the rope tied itself into the most perfectly tight slip knot inside the cylinder. I was on the verge of pulling the cylinder because there was just no way this knot/rope was coming out.

Tanya said, "Don't pull that cylinder just yet. Not until I get a crack at it." She loves puzzles and this one was a doozy. So I took a deep breath and sat for a few minutes, all the while thinking "I could have this #2 cylinder off and back on in an hour and a half."

Was I really going to let her show me how this is done? I don't think so!

I drug out the borescope and computer for detailed evaluation. Yep that is one tight slip knot. With the borescope in the bottom plug hole, looking at the computer screen, a hemostat and flat head screw driver went in the top hole. To my amazement, in about 45 minutes, I was able to mostly untie the knot in the cylinder and extract the rope. Wow, that was close.

I think a smaller rope, maybe 1/4", would be more appropriate such that if it tied itself in a knot, it could still be pulled out. I'm sure some unsuspecting mechanic has already learned this lesson. Now I have too.
 
For the benefit of those learning from this...

In the automotive world, compressed air is the common method for holding the valve while compressing the spring and removing the keepers. Just hook up your compression tester (leak-down type) then throw 80 or 100 PSI at it. Works well. It is common for the hot rod guys to go through several sets of valve springs when tinkering with cams and valve float (i.e. hitting max rpm) and we always use air.

Be sure that have someone hold the prop for you so you don't hurt yourself or tie it to something.

Larry
 
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For the benefit of those learning from this...

In the automotive world, compressed air is the common method for holding the valve while compressing the spring and removing the keepers. Just hook up your compression tester (leak-down type) then throw 80 or 100 PSI at it. Works well. It is common for the hot rod guys to go through several sets of valve springs when tinkering with cams and valve float (i.e. hitting max rpm) and we always use air.

Be sure that have someone hold the prop for you so you don't hurt yourself or tie it to something.

Larry

+1

The rocker assembly is off, so push the piston to BDC, quite safe there. The retainer will have to be smacked off axis with an inertial tool to loosen the keepers as they are usually pretty tight. Then, you are golden. 100 psi is a lot of force on that valve to keep it there during spring removal.

Scott, Good you ( and Tanya ) were persistent and got it out.
 
One online source said they used 1/2" mountain climbing rope, not the 3/8" nylon rope suggested by the Lycoming SB.

I think the demo I saw at a Grumman convention also used 1/2" rope.

Do you think fatter rope would be harder to get knotted internally?
 
I'm at a loss here...what minor exhaust valve maintenance work was Scott attempting to do with the rope?

Jim
RV9a
160 hours
 
I'm at a loss here...what minor exhaust valve maintenance work was Scott attempting to do with the rope?

Jim
RV9a
160 hours
,
I would speculate he was doing the "wobble test" looking for valve guide wear or carbon build up. The spring needs to come off for that test. The standard "in situ" test for valve guide/stem clearance is measuring the lateral movement of the stem tip.
 
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I'm at a loss here...what minor exhaust valve maintenance work was Scott attempting to do with the rope?

Jim
RV9a
160 hours

Lapping exhaust valves. And sure, a wobble test while we're at it. It's time to make sure you're ready for Oshkosh you know :).
 
Lapping exhaust valves. And sure, a wobble test while we're at it. It's time to make sure you're ready for Oshkosh you know :).

OK, I thought I understood the rope trick for reaming valve guides, but would you explain the lapping process when used with the rope?
 
,
I would speculate he was doing the "wobble test" looking for valve guide wear or carbon build up. The spring needs to come off for that test. The standard "in situ" test for valve guide/stem clearance is measuring the lateral movement of the stem tip.

The spring doesn't have to come off for the wobble test if you use the Lycoming specified fixture.

As you bolt on the fixture it pushes the valve down to the required measuring point.

If you need to fix a tight valve after the test, then the spring has to come off.
 
For the benefit of those learning from this...

In the automotive world, compressed air is the common method for holding the valve while compressing the spring and removing the keepers. Just hook up your compression tester (leak-down type) then throw 80 or 100 PSI at it. Works well. It is common for the hot rod guys to go through several sets of valve springs when tinkering with cams and valve float (i.e. hitting max rpm) and we always use air.

Be sure that have someone hold the prop for you so you don't hurt yourself or tie it to something.

Larry

I started with air at BDC but could only bring myself to apply about 75psi as my helper wasn't there as she usually is to police the prop. 75psi wasn't doing squat. I know that BDC should be fine, I've done that before in a past life with cars without this huge metal whacker attached to the front. I backed off to what I felt was the less "dynamic" approach.

Had I read Mike's story about how he tied a knot in the rope and had to pull the cylinder anyway, I may have manned up and put more pressure on it :). Hey Mike, there is a thread somewhere that you shared that critical nugget, right? :).
 
Question...

Does the knot form because one end of the rope is loose in the cylinder as one shoves the rope in through a plug hole? Could that possibility be eliminated by feeding the end through the opposite plug hole, and then shoving the rope in to fill the cylinder? Seems like it would be hard for a rope to knot without a free end. Just wondering...
 
BOTTOM is as far as you can fall.

I started with air at BDC but could only bring myself to apply about 75psi as my helper wasn't there as she usually is to police the prop. 75psi wasn't doing squat. I know that BDC should be fine, I've done that before in a past life with cars without this huge metal whacker attached to the front. I backed off to what I felt was the less "dynamic" approach.

Had I read Mike's story about how he tied a knot in the rope and had to pull the cylinder anyway, I may have manned up and put more pressure on it :). Hey Mike, there is a thread somewhere that you shared that critical nugget, right? :).

You do know that BDC is Bottom Dead Center, right? Pressure is only on that cylinder. It can not move with pressure on it. The prop won't hit you unless you move your head into it. Let's not confuse people with the safe aspect of that.

The keepers stick in the retainer, so this is why you smack it with a hammer, to loosen the two. Then, no problem.

Lapping insitu, a whole other discussion.
 
You do know that BDC is Bottom Dead Center, right? Pressure is only on that cylinder. It can not move with pressure on it. The prop won't hit you unless you move your head into it. Let's not confuse people with the safe aspect of that.

What he said...
 
You do know that BDC is Bottom Dead Center, right? Pressure is only on that cylinder. It can not move with pressure on it. The prop won't hit you unless you move your head into it. Let's not confuse people with the safe aspect of that.

The keepers stick in the retainer, so this is why you smack it with a hammer, to loosen the two. Then, no problem.

Lapping insitu, a whole other discussion.

Yep, not stupid or uneducated. We all make our own choices when we're solo in the shop.
 
Yep, not stupid or uneducated. We all make our own choices when we're solo in the shop.

I've read enough of your posts and dinner conversation to know neither is the case. No fault offered on the choice.
 
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