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Mixture Cable Failure

Ironflight

VAF Moderator / Line Boy
Mentor
Last month, I completed the first Condition Inspection on the Valkyrie, so naturally, this month, I had a failure....The past couple of weeks, I have noticed a stiffness to the mixture control, and in the last couple of flights, it got progressively worse - but still functional. It seemed to get better as things warmed up, and I went so far as to remove the cowl last week, remove the cable from the carburetor, and make sure the carb wasn't stiff. It wasn't, and by that time, it was working smoothly.

Today, I went to start things up, and it was very stiff to get into full rich for start. I figured it would act like last week, and get better as it warmed up. Instead, when I went to lean, it was essentially locked in the full rich position (if it's going to fail locked, that's a good place to be!). AS it was not a safety issue at this point, and I was only flying across town to an RV Builder's lunch, I continued, leaving it full rich and shutting down with the key. The flight home was uneventful in full rich.

My passenger today was RV-8 builder William Slaughter, and I have to thank him for being interested enough to hang around and help me with the troubleshooting and diagnosis. (Of course, he mentioned something about "honey-doos" needing to be done if he returned home...) We de-cowled, and disconnected the cable at the carb - the carb was free, the cable was not. We cut some cable ties and clamps, and stretch the cable out straight - still extremely stiff from the cockpit control. I have dreaded the thought of taking apart the throttle quadrant, because frankly, once you get it all in place, getting it out can be a real chore - but that was the next step. I got the cable end off of the lever, and the cable was still virtually unmovable - so there was nothing to do but remove it from the aircraft. To make that easier, we took the forward baggage floor out. (Anyone keeping track of how much re-assembly I have to do....?)

Taking the top off of the throttle quadrant, I was able to get a stubby wrench on to the nuts that held the cable housing to the quadrant bracket, and with some not-do-gently pulling from the front, the cable was out - about an hour from the start of disassembly. The cable is essentially frozen in the sleeve, and it appears to be from a spot where the cable had been riding on the muffler hanger (the piece of radiator hose). It had worn through the plastic jacket of the mixture cable, and started to wear away the metal housing. I have not dissected it yet, but I suspect that the cable was crushed in some way, and has damaged the core.

The lesson here is that just because the metal housing was rubbing on the radiator hose muffler hanger, I should have expected that the softer hose would wear away first! I don't think it had been rubbing before, and probably came in contact as things moved around with flying hours, and settled into their final positions.

I am now in desperate need of a Van's 60" Quadrant Cable (A CT Q-60):

http://www.vansaircraft.com/cgi-bin...337-52-285&browse=controls&product=ctq-cables

Does anyone know if these are identical to the 60" A-920 cable from ACS?

http://www.aircraftspruce.com/catalog/appages/a920.php

Of course this happens on a Saturday before a week when I was planning to do some traveling! I will have to call Van's first thing Monday, and am willing to pay overnight shipping (if they will actually be shipping next week), but am wondering if ACS has the same thing, I might have more luck getting them to ship immediately.

Of course, if there is someone in the Houston area who has their FWF kit and has a cable sitting on the shelf that they might not be using for the next week....maybe I offer to have a brand new one shipped directly to them, and I could come get the dusty one off their shelf....Anyone? It's not like I can fly it the way it is, so what the heck am I going to do all day Sunday??!! :confused:

Paul
 
Paul, be happy it busted at home. :eek:

I can't help you with a cable, but this does illustrate how chafing is a real problem. I went nuts with RTV under my cowl. Every single place where something touches something else, I put a blob of RTV.

Regarding push/pull cables, I have the vernier sort for TP&M, and I plan to replace all three at about 1000 hours which will be about the end of next year. I've noticed that each year the telescoping ends have developed a bit more play.

I'll take this chance to plug the manufacturer of mine (who also makes some of those available at Van's and ACS), an outfit called, yes no joke, ACS. This ACS is not the Spruce variety, and is in Lake Havasu City, AZ. They will make a custom (or at least did 6 years ago) cable (you need to define overall length, covered/not, vernier/not, travel, end lengths and end types. They make these custom things and ship them in about two days. Really unbelievable.

Paul, I hope someone comes through, I'll bet someone will.
 
Pictures

Alex - you're absolutely right about the blob of RTV between contact points - I try to do that everywhere! I either missed this point of contact, or it developed after the exhaust had found it's settling point. Either way - here are a couple of pictures of the cable.

This shows the actual wear point - the cable housing seems to be made of wire strands virtually parallel to the cable, and a spiral band holding these together in a tube. The whole thing is then coated in plastic. This wear point was through the plastic, and also through the spiral band. I'm betting I will find a broken strand on the interior cable:

img0426nl0.jpg


This shot shows that it was pretty close to the carb end of the cable:

img0425sf1.jpg


Paul
 
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The Plot Thickens....

I love failure analysis. One thing it teaches you is that first answers are almost always wrong! So here is more of the story....

I dissected the bad cable using a cut-off wheel on my dremel tool. The first thing I did was to cut the cable housing next to the abraded region (very carefully, so as not to damage the core). I cut it on the side of the abraded region AWAY from the closest end. Once it was free, I pulled the housing off the end - and it moved with complete freedom - the abraded region was not hindering the core's movement at all. OK, that theory's out the door.....

I looked farther along the cable, and saw where the plastic housing had been scuffed a bit. I cut of the plastic, and I saw an area where the metal housing had some blue/green staining - it looked like chlorine corrosion you see on metal in a pool. I made some more cuts in this area, and essentially discovered a region about three inches long where the inner core was moving with great friction, and much difficulty. In reconstructing how the cable was run in the engine compartment, this corresponds to the "lowest" portion of the cable.

This picture shows a cross-section of the cable, and the core lying next to it:

img0432yb7.jpg


You can see that the housing has what appears to be a Teflon tube, surrounded by the wire strands, and then held together by the spiral wrap, and all covered by plastic. the core is all parallel strands, very hard to cut, and wrapped in Teflon (or something). In the region where the core is hanging up, it appeared that the housing's inner Teflon tube had uniformly compressed around the moving core. There were no kinks, bends, or damaged areas. Those are all facts.

My SPECULATION is that "something liquid" got into the cable, and caused either the wire strands of the housing to corrode, squeezing down on the Teflon tube, or the Teflon tube itself was chemically changed and "swelled" to create the same effect. Because the bad region is at the lowest point in the cable run, this liquid attack makes sense. The evidence does not suggest common water-induced rust, because the stains look blue/green (and do not appear to come from the greenish plastic, because they are too localized). It's still a mystery, but from all I can tell, physical abrasion does not appear to be a cause. It does make me want to pay extra attention to keeping the plastic jacket undamaged however - that should help prevent chemical attack. Could something have gotten in there during the pre-paint cleaning? Or did something react to a lubricant? I doubt I'll ever know more for sure.

If nothing else, I've learned more about how these cables are built!

Paul
 
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A-920 is not the same. I learned that the expensive way. It has a different throw, and thus the distance from the bulkhead nuts to the cable end is different.
 
Davepar said:
A-920 is not the same. I learned that the expensive way. It has a different throw, and thus the distance from the bulkhead nuts to the cable end is different.

Thanks Dave - I guess I'll have to hope Van's can ship first thing Monday!

Paul
 
Hi Paul:

I'm a few weeks away from having to install my cables.

Tell you what. I'll bring my cable to work on Monday. If Van's can not express one out to you, call me at work.

I can have our shipping deparatment Fed Ex my cable to you on Monday. You should have it on Tuesday.

You can then have Vans ship me a replacement.

I'll email you contact info.

-Mike Draper
 
Paul,

Did you know those green cables that Van's sells are apparently made for BOATS?

I use 'em, too. :rolleyes:
 
I sort of suspected that might be the case - and I actually stopped at my local Boat US store on the way home fromt the airport last night to see if they had one. Alas....nothing that short in stock....

Mike - thanks very much for the offer! I emailed you, and we'll see what I hear from Van's in the morning.

Paul
 
Cable

Paul:

Dan is correct. Check with your larger boat dealer's parts dept. first thing Mon. You can do that before Van's opens Pacific Time.

Mannan Thomason
RV-8 N161RL
 
Paul,
Great job of failure analysis. Since the "stain" is blue/green and at the lowest point, do you think it might be fuel stain? How about taking a "good" section of the cable, soak with a little 100LL and see what happens?
 
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how about the cables themselves?

How about the cables themselves? On these control cables, there is a plastic (of some sort) sheath molded/wrapped around the cable itself. If that cable sheath gets disturbed in any way, it can cause cables to bind up pretty good. Here's what happened to my throttle & prop cables:

20060208_trashed_cables3.jpg


FWIW, these were in LOW HEAT areas, up by the quadrant.

Paul, I suspect you didn't see something like this on your cables, otherwise you would have mentioned it...but I figured I'd raise the point just in case.
 
stainless corrosion

Paul,

The blue/green color is a tell tale sign of stainless steel corrosion (I assume that the metal strands are stainless). I'm not sure what it was that caused it, but I don't think that primers or solvents would do that.
 
Nice work, Paul. I've heard in passing that some solvents can cause Teflon to swell. How much? NO clue....it's not something I ever really work with. Anyhow, I remember something about Alcohol or Acetone causing the swelling. I'm SURE there's a material's guy kicking around at NASA somewhere that answer this off the top of his head.

I could also have totally misheard the conversation and am sending you on a wild goose chase. Ya' never know!
 
Dan -

Now THOSE are disturbing pictures Dan!! No, I idn't see anything close to that on the internal cable, although there was a slight difference in the pattern of the plastic coating on the internal cable in the "region of interest". I slid that portion of the internal cable through a "good" portion of the sheath to see how it moved, and it was smooth. I am thinking that if I had been able to continue operating the cable, the internal coating could have eventualy ended up looking like your examples.

I have thought about heat being the issue, and certianly can't rule that out, although the bad part of the cable was not particularly close to the exhaust pipes themselves. When I reinstall a new one, I am going to see if I can give it even more separation.

And yup John, I was going to check with some of my friends in Engineering about materials incompatability. Always curious...


BTW, I searched for "Cablecraft" (the name on the jacket), and quickly found the spec sheet, if anyone is interested:

http://controls.tuthill.com/Support... Industrial/Push-Pull_Standard_Industrial.pdf

Paul
 
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Thanks so much for posting this...

Paul,
Yesterday I planned to take a potential builder flying in my 4. We were scheduled to meet another friend in an 8A and spend our last good wx day airport hopping. I loaded him up and as I started taxiing out, I also noticed that my throttle was very stiff. I taxiied around a bit but could not convince myself that it was my imagination. So, I cancelled.

I went through most of the same diagnostics that you did right up to the point that I was about to take the throttle quadrant apart. I called my 8A friend to tell him I was toast :) and he suggested lubing the throttle lever right where it goes into the block. I did this for both throttle, mix and also the rear seat. After it was MUCH better. I took it out for 1.5 hrs late yesterday and it felt ok in flight.

Except ! It's clear that the resistance is not linear from full close to full open.
I move about 1/3 of the way forward and I can then feel resistance for about the middle 1/3 of travel. Then it smooths out again. It's not real bad but now I'm hyper alert to any change in the feel. I think I'll call Van's tomorrow too. Might as well replace the cable with 900 hrs on it just as PM if nothing else.

John
 
Paul, you said in the initial post that the cable stuck in full rich. Is is correct that the cable was forward at that time? In other words, attempting to lean the mixture would put the cable under tension? (I don't have a quadrant, so I don't know the geometry). Depending on the answer, I may have some thoughts.

Several people have posted their failures, are they all Tuthill brand?

Also, ptfe (Teflon) is really inert to most solvents, and I would not expect those found near airplanes to swell it either.
 
Cable

Hey Paul,
I left you a message on your home phone.
I have a cable, here at Conroe.
Give me a call.
Ed O'Quinn
936-443-8859
 
AlexPeterson said:
Paul, you said in the initial post that the cable stuck in full rich. Is is correct that the cable was forward at that time? In other words, attempting to lean the mixture would put the cable under tension? (I don't have a quadrant, so I don't know the geometry). Depending on the answer, I may have some thoughts.

.

Alex,

Yes, when it finally stuck "solid", it was full rich. Leaning was puttign the cable in tension. However, it was also very stiff in compression - it was basically getting harder to move with each operation, so when I got it Rich, I left it there until I got home.


And I am headed out to the airport right now to meet Ed, who has graciously offered me a cable, and to fly it down from Conrore. It's a gorgeous day for flying - very glad I could give him an excuse to get airborne! :D

Paul
 
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Hey Paul
Thanks for the information,mine has just begun with the same symptoms
so I will call vans for a replacement,of both mix and trothle cable,my friend in norway lost his RV4 with a broken trohtle cable,so since I have to rake things apart i Might as well change that too

Sincerly
Jorn Moller
OY-RVS RV8
300 Hours
 
Ironflight said:
....You can see that the housing has what appears to be a Teflon tube, ...
Is it possibly NOT teflon and this is perhaps thermal failure of the liner?

Just a guess.

-mike
 
What part number

Paul .... which part number are you using?

I hope it's the 313/314/315 series with the higher temperature rating of 310F... the other cables at 230F max. sound like they may get overstressed near the exhaust...

gil in Tucson
 
I work in the marine industry and we use alot of cables of that design (Telflex Morris 33c Supreme to be exact) for throttle, transmission shifting, position indicators, etc. As such, I have seen just about every form of failure. The most common failure is simply wear and tear from use, especialy if gunk, grit, solvents, dirt, oil, etc get down in the cables. Eventualy it imbeds with grit and the inner sheeth wears and builds up alot of friction, or starts shedding like dan c's. I have seen the outer cables so rusted up that they double the diameter of the outer sheath, yet still accuate smoothly. My best guess, your cable is simply worn out, possably accelerated by contamination, or wear due to bend in the cable (was there a bend where it was resting on the exhaust hanger?)
 
az_gila said:
Paul .... which part number are you using?

I hope it's the 313/314/315 series with the higher temperature rating of 310F... the other cables at 230F max. sound like they may get overstressed near the exhaust...

gil in Tucson


Good question Gil - just reading the spec sheet, I could guess that it is the higher temp version based on the construction...but then again, this is a cable that Van's sells, and we all know that he isn't known for extravagently picking the most expensive parts.... :rolleyes:

Bottom line is that I really don't know.

Paul
 
Guess I better replace mine too...

Paul,

I've got 106 hrs on my RV-8A this year. A few months ago (probably around 70-80 hrs TTSN) I noticed my mixture cable starting to get stiffer. Thought it might be my imagination at first but it has grown progressively worse. One symptom of my cable is that in addition to being stiff it also has a bit of a "ratcheting" feel to it...the movement is not smooth, it sorta bumps along. I have carefully inspected it along the entire length, and there is absolutely no indication of external abrasion, damage, overheating, etc. The mixture arm in the carb is fine. I thought I'd keep flying it for now but after seeing your post I think I'll replace it before getting stuck somewhere. Like you said, it's a major pain taking it out of the throttle quadrant so I don't relish the thought of doing this task.

One thing about the mixture cable in the carbureted O-360 is that it has some pretty sharp bends as well as proximity to the exhaust. I wonder if the combination causes extra stress there. I really don't know how else to route it though, and if that was the problem you'd think a whole lot of flying RV's would have the same symptoms...

--Mark Navratil
Cedar Rapids, Iowa
RV-8A N2D
 
I thought that I would put my two cents worth. On Grezdlitn I had several cable failures always related to heat. Caused by proximity to exhaust and insufficient shielding or insulation. The sympton was always a stiffining resulting in a loss of quadrant motion. The inner sleeve would liquify in climb then as it cooled in cruise it would sieze (shrink) the spiral wound cable. I disected several before I decided this was the problem.
The cables are identical to PTO cables for big trucks and can be custom built most anywhere at a cost comparable to Vans. The exterior sleeves come in different materials with various heat ratings. I recall the max being about 350 deg. F. If length is a issue you can specify anything you want with the only limitation being the throw (travel). Which is limited by the harware itself.

I have some extra Van's cables if anyone is interested. I think they are 60 inches.
Mark
 
Fire sleeve

Concerning the cables, has anyone experimented with fire sleeve over those areas close to heat?
 
Fire Sleeve

I Fire Sleeved and heat shielded my Throttle Cable in avoid potential problems. Engine is Horizontal Fuel Injected so throttle runs under the engine near the exhaust system.
So far-100hrs- no problems.
Pete.
firesleevedhrottlerv4.jpg
 
Pete,

About your sniffle valve...you don't want that puppy dumping onto the exhaust. You might consider contacting Airflow Performance and purchasing their more compact sniffle valve. It's a modified -4 AN fitting that is more compact, and would enable you to run 1/4" aluminum tubing to it, routing the dumped fuel away from the exhaust.

Anyway, just figured I'd mention this. Seems to be a common issue with some of the 180hp horizontal induction IO-360s. The Vetterman exhaust comes way too close to that sniffle valve imho...
 
Fire down below

Thanks Dan.
I tried to find on with an elbow, but being like a enuch- all cut off- down in Aus. I couldn't find one.
So I went to a hydaulic shop- where they know theie threads- and they fitted me out with a 45 elbow.
The joke was that, when I had a LAME torque the prop and check my FWF brfore the first flight, we couldn't get my brand new IO-360 to run under 1500RPM or even smoothly at that speed.
I commented that it sounded like an inlet manifold air leak.......... and then I remembered the sniffle valve on the bench.
After fitting it it ran smooth as silk and has ever since.
Pete.
 
Gonna Do a Test....Sometime....

I agree those are the ones Dan!

I still have sections of the old cable , and intend to take a "good" section and expose it to some heat for awhile, then see if I can recreate the problem that I had. I have the "bad" section - about three inches long, where the inner cable just doesn't want to slide through.

When I get around to doing the experiment, I'll report back.....

Paul
 
On the lighter side....

My RV-10 pitch trim cables came from the same company with a sticker and big red letters "NOT FOR AIRCRAFT USE" :eek:
 
Better late than never?

It came to me in the middle of the night!
It is possible that a poor engine-to-airframe ground would lead to the starter load to be carried thru the engine control cables or whatever it can find.
I have seen this happen before on spam cans such as a lot of Piper products that were built with light weight aluminium battery and engine ground cables that tend to corrode internally over time upping the resistance to the point where the starter load is carried thru carb heat cables and mixture cables.
During starting load, this can can heat up those control cable housings and the internal wire in them red hot!
I have seen a lot of airplanes including RVs that look to have small or poorly grounded engines.
If you have just one ground cable, is it it big enough? Are the cable ends properly crimped and tight? Is it bolted to a painted surface? Is it attatched only to a small oil sump bolt which can become oily and loose?
The best way to ground a starter would be directly from a starter mounting bolt to the battery post. It would eliminate a lot of potential problems and probably give better starter performance over time.
 
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Interesting Idea Monkey!

Now that's an original thought - I hadn't consider the cables acting like an engine ground. In my case, I've got two big honkin' ground cables from engine to firewall, nd last I checked, they looked pretty good...but it is certainly something to consider. I remember hearing people say that if you forget to connect the ground cable and hit the starter, the mag P-Leads can easily smoke....

I've still got sections of the failed cable sitting on my workbench, and one of these days, I'll try the heat experiment over a hot-plate.

Paul
 
Supposedly, stainless steel has a tendency to eventually develop a non-conductive oxidized coating. I say "supposedly", because although this is plausible, and I've found oblique references to it on the web, I've never seen an explicit statement from a reputable source. Supposedly, this non-conductive coating is very thin, and the point of a multi-meter probe will break right through it, showing an apparently good ground. But the coating will reform, and the increased resistance may affect the quality of the ground.

Assuming this is true, it would seem that grounding straps need to be bolted to a hole that also goes through one of the aluminum angles on the firewall. Same thing with voltage regulators, remotely mounted engine sensors, etc.

I'm not saying that this is necessarily in play on Paul's aircraft, but it is something to keep in mind for other builders.
 
Smart

You are one smart monkey! I have been following this thread with interest but do not feel qualified to comment. I will say that your idea seems to have merit and it is one that we would not usually think about.

Having disassembled the diesel engine on my sailboat 3 times before finding that the failure of the engine was due to electrolysis, I am a believer in the weird and mystical damage that errant electrons can produce.

Good call Monkey. I think you may be on to something.



Monkey said:
It came to me in the middle of the night!
It is possible that a poor engine-to-airframe ground would lead to the starter load to be carried thru the engine control cables or whatever it can find.
I have seen this happen before on spam cans such as a lot of Piper products that were built with light weight aluminium battery and engine ground cables that tend to corrode internally over time upping the resistance to the point where the starter load is carried thru carb heat cables and mixture cables.
During starting load, this can can heat up those control cable housings and the internal wire in them red hot!
I have seen a lot of airplanes including RVs that look to have small or poorly grounded engines.
If you have just one ground cable, is it it big enough? Are the cable ends properly crimped and tight? Is it bolted to a painted surface? Is it attatched only to a small oil sump bolt which can become oily and loose?
The best way to ground a starter would be directly from a starter mounting bolt to the battery post. It would eliminate a lot of potential problems and probably give better starter performance over time.
 
custom cables quickly

Paul,
If you do a search for Tuthill cable vendors, you may find that local shops (industrial and farm equipment) can make a custom length cable on the spot. I tried Vans premade cables and my throttle would just would not fit right. When I called a dealer, they made the cable to my exact specs in about 2 hours. I could have had it overnight if I was willing to pay for FedEx. The price was the same as Vans within a few dollars and the cable was exactly the same except gray outer shield.
The specs are all on the Tuthill web site as mentioned, you should have a local vendor in your area.
Good Luck

FM
 
Thanks FM - but at the time this all happened, it was a Sunday (this thread started last November), so the first option was to use this incredible "builder's network".....If that had failed, I would have looked locally if I'd had the time.

Paul
 
Cable

Paul,

Just scanned your cable problem thread, and made a random postulation. Looking at your photo of the cable, it occured to me that the metal spiral wrap appeared to be extended. Should that spiral wrap be stretched tightly enough, it could cause compression of the core material at the point of greatest resistance to further elongation of the spiral wrap, possibly collapsing the teflon core around the cable at that point. There are a number of ways the cable might be stressed during installation and the movement of the mixture control may have gradually exacerbated the strain, eventually leading to failure. Those cables are not very long anyway. My mixture cable barely makes it from full rich to idle cut-off.

Just my two cents worth.

Regards,
JB
 
My RV-8 mixture control is beginning to act as described in the lead to this thread. Does anyone have advice on how to gain access easily to the throttle quadrant? And, does anyone know where to find a diagram of the quadrant with instructions? I've check the construction manual that came with my aircraft and found nothing useful. Thanks!
 
My RV-8 mixture control is beginning to act as described in the lead to this thread. Does anyone have advice on how to gain access easily to the throttle quadrant? And, does anyone know where to find a diagram of the quadrant with instructions? I've check the construction manual that came with my aircraft and found nothing useful. Thanks!

Hi Jim,

The problem with directing you to drawings is that since all of these airplanes are homebuilt, there are many different throttle quadrants that could have ben used, and many builders do their own custom quadrant mounts and panels - so without knowing what you have, no one can give you exact guidance. The good news, however, is that everything is just bolted and screwed together, so you can just start taking things apart. If you only have the build manual, and not the drawings, you can buy the drawings electronically from Van’s for like $10 - and there are drawings that will show the “stock” throttle mounting - which will give you a starting point.

Welcome to the world of experimental aircraft “second (or letter…) ownership” - you’ll ned to work through to figure out what the original builder did.

Paul
 
Hi Jim,
... If you only have the build manual, and not the drawings, you can buy the drawings electronically from Van’s for like $10 - and there are drawings that will show the “stock” throttle mounting - which will give you a starting point...

Paul

Even better, with the new Van's web store, if you register for an account, and set up your airplane in your profile with your serial number, you have access to all current plans for free, at any time, without waiting for a USB stick to arrive in the mail, and that becomes outdated over time.
 
Thanks to Untainted123 and Paul Dye for thoughtful replies. Gary Keyser at Van's was also very helpful. I was thinking it would involve removing the clevis, but apparently the best approach is to leave the clevis intact and unscrew the cable from the clevis. I will probably post again when I've finished, but for now I'll say NEVER, EVER disassemble the quadrant mechanism if you can avoid it, because it will take a lot of struggle to re-assemble it. Don't ask me how I know.
 
Pre-Emptive

Your story make me happy my new 60" cable is in the mail from VANS. Started getting stiff at forward/full throttle position and in air failing is not what ANYONE wants. Now to find a 10" Hulk Hogan to climb around an put in the new one.
 
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