A $3675.62 helicoil - really.

[DVA]

Welcome back from KOSH to those who airventured out. It was great meeting a few of the folks here, including Don Kaye who was on the Mooney Owners Forum panel discussion.

For those who may be unfamiliar with a helicoil, it is a device that is used to place threading in a hole so a stud or something that screws can be, well, screwed in and tightened. One of the most familiar places you might find one of these $30 delights on your airplane would be in the hole your spark plug screws into.  The alternative to inserting a helicoil is to actually drill a hole of proper size and then “tap" the hole, cutting threading directly to the metal. The disadvantage to this, in some cases, is that if you accidentally cross-thread and bugger up the threads, you may not have any options left expect replacing the entire part.

If a helicoil gets buggered up, you can usually replace the helicoil and voilà! you’re all fixed up for cheap.

Because we all have excellent engine monitoring, on my way to KOSH I noticed that the #5 CHT started to climb; my JPI 830 is set to alert at 400dF.  As you’ll see from my previous posts my #5 CHT usually ran about 350dF in cruise and it was heading past 400dF. Oddly, nothing else indicated any sort of problem. I was running 2400/29 LOP and the adjacent cylinders hadn’t moved at all in temperature, FF was steady, EGT was unchanged, oil temp the same, etc. Oddly, when I opened the cowl flaps the CHT on the #5 would drop quickly, much faster than the rest.  It looked suspicious to me and due to good instrumentation and a little knowledge of how things work I did not feel as if I had any cylinder or combustion event problem. I pressed on for the last 30 minutes of the first leg of the flight.

On the ground at KIAG we de-cowled the plane and took a look. There was a small but clear exhaust leak at the #5 manifold, you could see the light tan soot spewing directly at the CHT probe - problem found! We re-torqued the nuts (the one in the back was a a little lose), closed things up and I went on my way.

For the next hour of flight, all was normal, then once again the #5 CHT started to climb. I figured the manifold gasket got burned enough to begin leaking, this will be a simple fix at KOSH.

Anyone got a clue yet?

The folks at New View Technologies on the field at KOSH were very nice and capable (although not a Mooney shop), and since I was going to be there for 9 days they had plenty of time to fix the exhaust leak which now had grown in size. (this leak at this area that is not particularly dangerous yet, but what will happen is that the more the hot exhaust gasses blow through the gasket, they can begin to erode the surface of the mounting area, acting like a cutting torch and reshaping the metal. After that, gaskets usually won’t seal well and you have to machine the surface flat again)

You can see the CHT probes above, the one on the spark plug is the one that goes to the factory panel CHT gage, the one behind it is for the JPI engine monitor.

A few hours later I get a call just as I am settling in to watch the first airshow. “We found the problem - you should come down to see it”.

When the mechanic was attempting to loosen the back nut on the stud to remove the flange on the #5 manifold, the stud twisted out very easily. The pictures below show the mangled helicoil, and the stud:

 

 

Upon inspection of the mating surfaces we found them to be flat and in fine shape, just a little cleaning needed so no damage was done there. However, there was not enough surface area in the hole where the helicoil pulled out to kit-in another one.  (side note: I did not know that helicoils were used in cylinder head stud holes)

I was told I needed a new cylinder. (I’m trying to find the proper emoticon to insert here, but none of them are cursing up a proper storm)

I only had about 100 hours on the “old” cylinder, so this was a disappointment and a shock. Since all the experts in the world were in a two mile radius of me, I looked for advice, and it was unanimous - don’t try to tap in another helicoil.

I am now the happy owner of a new #5 jug, piston, valve train, gasket and labor.

Take aways:

1. I thought I saw a little bit of soot near that exhaust manifold when I changed the oil about three weeks ago. I recall looking at it and determining that it was just too small to worry about, and I’ll check it next oil change. (I should not have ignored it - lesson learned)
2. Because I actively get involved in (and preach) owner maintenance and I sought to learn about how my airplane engine works (do you?), I was prepared to do a calm differential safety diagnosis as I saw the CHT rise:
   1. If it really was a true CHT rise, the close proximity of the #3 cylinder would have shown some small rise in its temp being so close. But there was none.
   2. If that cylinder was really getting 50-75dF hotter, I might have seen a corroborating oil temp increase, there was none.
   3. If a combustion event was occurring (detonation, pre-ignition) I would have seen EGT changes, but there was none.
   4. If there was an injector problem causing a change in the mixture for the cylinder, I might have seen a FF change, but there was none.
   5. The airplane’s other engine data points were unchanged: (RPM, MAP, IAT, CDT)
   6. When I opened the cowl flaps, the temp on that #5 CHT dropped precipitously, but the other cylinders only dropped slowly. You could not cool a steel cylinder that fast - it just didn’t make any sense.
3. Had I not invested in proper engine monitoring, I would have relied only on my panel CHT gage which did not change.
4. There are real reasons why you might consider new parts over rebuilt parts when it comes to things like cylinders. (Save for another discussion)
5. Helicoil's suck.

BUT - After all of that...

#OSH16 was a BLAST! and I had a great time, learned a lot from the experts and I just love to fly that Mooney!

Have fun and fly safe!
DVA

 

[carusoam]

DVA,

You win a prize for best post of the day!

- highly technical.

- supported with clear photographs.

- well written.

- demonstrating fabulous (writing) control under extreme (financial) pressure. 

- recognizing the rate of cooling of the instruments with cowl flaps open is strong engineering-fu.

 

1) Usually, finding a helicoil in your machine is a sign that somebody has already buggered up the threaded hole that used to be there.  Apparently helicoils are now an ordinary part of putting hardened threads into an aluminum surface.

2) Our machines are so well developed, any extra meat used to drill a hole oversized has been eliminated for weight savings.

3) Knowing the power of the Bravo's uncontained exhaust stream, it was wise to get it contained quickly again.  Similar to having a leaky o-ring on a solid rocket booster.  The cutting torch may be randomly aimed at an important target...

4) Speaking of cutting torch...  Hopefully they didn't have to loosen any of the Bravo's exhaust clamps.  If they did, they may need to be replaced.  Make sure you are aware of this situation.

Thanks for sharing your experience.  The thought of being in the world's most concentrated aviation technology center and having a problem gave me the willies.  It is a challenge to get ordinary services like fuel there...

Nice job!

Best regards,

-a-

[moxfox]

Kind of strange that the  cylinder only had 100 hours on it and it had a helicoil in it? It must have been  ( Bogered up) to have that in it , must times if the standard stud comes out and the threads are still there but lose with a standard stud ,you would go back with a oversize stud . The threads must have came out when the standard stud came out? Was this a new cylinder, or overhauled one ?? was the gasket the thick steel no-blow type? Can't tell from your picture. 

[DVA]

Just now, moxfox said:

Kind of strange that the  cylinder only had 100 hours on it and it had a helicoil in it? 

Moxfox, hi and thanks for writing back!

Your comment falls into my #4 point above:

50 minutes ago, DVA said:

4. There are real reasons why you might consider new parts over rebuilt parts when it comes to things like cylinders. (Save for another discussion)

All of the cylinders were replaced prior to my purchase. They are all reconditioned, so what does that mean exactly? I think it depends, but I can can clearly see that a reconditioning would include lapping valve seats, installing new valves, springs, etc. making sure the cylinder is honed and meets specs, inspection for defects / cracks, etc., and ... possibly replacing original helicoils with new ones.

[DVA]

54 minutes ago, carusoam said:

4) Speaking of cutting torch...  Hopefully they didn't have to loosen any of the Bravo's exhaust clamps.  If they did, they may need to be replaced.  Make sure you are aware of this situation.

First, thanks for the kind words. I really enjoy writing and sharing experiences that hopefully will help others.

It is absolutely clairvoyant of you to mention the V-clamps in this case. All of my problems were on the right side of the engine, nothing was wrong on the turbo side. There was no need to remove any of the v-clamps around the turbo. All that was needed was to loosen #2 and #4 on that side at the exhaust flange to relieve the pressure on the cross pipe so you could tinker with the other side.

Recall above I parenthetically added “they are not a Mooney shop” Well, I was going to add this other part of the story but I decided it would make another topic. However you, in your infinite and correct wisdom have forced me to share ... this:

Yep, there it is - exactly what you spoke about.  The mechanic, not being up to speed on some of the idiosyncrasies / gotchas / SBs / knowledge thought nothing about un-wiring and taking off the v-clamp. I had even mentioned to the shop owner before I left, that he should make sure that the A&P knows to stay away from the v-clamps and to make sure he checks out all of the SBs.

When I saw this, I told him, in as kind of a way as I could, that he just cost me another $800 because he was unaware of the issues around these fasteners. The V-clamp (use once - torque only twice - then throw away) is about $400 at that position (the big one around the turbo is close to a grand), AND the gasket that can only be used once is also about $400.

I own both of those too, now. (although I did get a nice discount from Aircraft Spruce)

There is a reason why we should take our Mooney’s to a Mooney shop. In this case I had no choice, but I did try to mitigate the risk albeit unsuccessfully.

Mic drop!
DVA

[Edit: BTW, add about $700 to the above cost to really complete this job to account for this]

[Edit: I want to stress that the A&P, and the folks at New View were very nice, accommodating and fair! I would take my airplane to them again. The point is that, most places that fix airplanes can’t be up on all of the models, types, etc. and may miss something, like this, inadvertently.]

[carqwik]

Well look at it this way...now you know how many times that V-Clamp has been torqued...!

[DXB]

Thanks great post  - I learned a lot, including what the soot from an exhaust leak actually looks like.