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krw5927

Well Known Member
I'm looking for advice or information on threaded insert (heli-coil) repair for exhaust studs on a Lycoming O-320. Hopefully Lycoming publishes some sort of service instruction or maintenance publication.

My #4 cylinder has one stud that has apparently already been repaired once with a heli-coil, and the stud was quite loose. Digging further, the stud threaded out of the cylinder by hand and brought the heli-coil with it, and the female threads in the stud hole aren't pretty. I'm not sure if there is such a thing as an oversize heli-coil to repair this with. The stud hole is getting pretty big.

I have heard that cylinder repair shops sometimes weld the stud hole closed and then re-drill and tap for the stud. If it's best to go that route, wondering what the cost might be. I doubt a repair shop would want to send out a cylinder that hadn't been fully reconditioned...

Thanks in advance!
 
If you pull the cylinder off any good welding shop should be able to weld the hole shut and you could re-tap for a standard-sized stud. No need to send it off to a cylinder shop.
 
If the threads for the heli coil aren't totally toast, you could try this: Lyc makes oversized studs. Screw the heli back in by hand and then screw a plus size stud into that. It should tighten up and might work. Cost? About $2. Otherwise the weld job will for sure work. A jug shop won't charge much. Less than 2 way shipping for sure.
 
There's not an oversize stud available that would be larger than a helicoil thread. However, one could make one with a suitably strong steel on a lathe.

That would be the route I'd go first since I have the capability of doing this.
 
No, not larger than the heli coil. Just oversize from standard. This will expand the heli coil tighter into the aluminum threads. I've seen where someone made "step-studs" by taking a 3/8 stud and turning-threading one end to 5/16. It's ugly. the 3/8 end usually breaks out into the exhaust port exposing the threads to exhaust gas.
 
Thanks for the input, guys. If a jug shop would weld it up, drill it out, and tap it good as new for small change, then that sounds like a good option to me. Would they have a problem doing that and sending it back out without inspecting everything? I guess I'm not asking them to yellow-tag it....

The tapped threads in the jug are pretty much toast from what I can see with a flashlight. Not much for a heli-coil to grab on to.
 
krw5927 said:
Thanks for the input, guys. If a jug shop would weld it up, drill it out, and tap it good as new for small change, then that sounds like a good option to me. Would they have a problem doing that and sending it back out without inspecting everything? I guess I'm not asking them to yellow-tag it....

The tapped threads in the jug are pretty much toast from what I can see with a flashlight. Not much for a heli-coil to grab on to.
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They would do just what you ask them to do, but since you have it off why not rebuilt the whole thing? :confused:
 
Actually Larry makes sense. Your going to spend like $40 bucks each way to ship. New guides, ground seats, and a hone should run only $150 or less. You should have a hone to reseat your rings anyway. They will have a fixture also for locating and aligning the new stud hole. That should only be a few bucks.
 
A cylinder shop will do just the repair that you want them to do. I would not suggest overhauling or reconditioning the cylinder if everything was running good. Good compressions, no oil leaks and such. I have had customers overhaul a cylinder just because it was off and then had issues start afterwords, either high oil consumption or temps... If it ain't broke don't fix it! Well it is broke but just fix the stud :)
 
I suspect the threads have failed due to over torquing, so whatever you do be sure not to overdo it when you put it all back together.
 
rocketbob said:
I suspect the threads have failed due to over torquing, so whatever you do be sure not to overdo it when you put it all back together.
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Bob, are you suggesting the stud may have been over-torqued when it was inserted or that the nut was over-torqued when the exhaust was mounted?

The Lycoming overhaul manual I have for the O-320 says exhaust studs (not the nuts) should be torqued to 40 in-lb minimum, and then a couple pages down says the driving torque upper limit is 100 in-lb. Will following the manual strip the threads?

This is a higher time engine (but below TBO) that was running just fine and had good compressions on all cylinders when it was removed from a skyhawk for upgrade to an O-360. Oil analyses were good, and inspection of cam lobes and followers revealed no evidence of spalling. Because it's higher time, my goal is to fly with it for a while and then overhaul. So pouring money into a full cylinder recondition is not something I'd prefer to do, since I will be buying 4 new jugs at overhaul time.

If I pull the jug to send it out, should I pull the piston pin and send the piston with the jug, or just secure the piston and rod so they don't touch the case and send just the jug? Will I need a new set of rings if I don't have them hone the cylinder?

Thanks again for all your help!
 
Following the manual will not cause the threads to strip on install. As stated the 40 inlbs is the MINIMUM. 100 inlbs seems about right but driving torque is different then regular torque. Driving torque takes into account the friction of the fastener as its being installed. I have to deal with this all the time on helicopters. First I have to figure out what the torque of the fastener is to move it and then incorporate that into the final torque value. Someone probably over torqued it when they installed it. I have also seen techs take the exhaust off only to watch the stud come out with the nut when it was removed. The tech then screwed it back in without torquing it, so it can happen.

As for the piston and pin it depends on the shop. The ones I deal with would not have me send the piston or the pin just for an exhaust stud weld and redrill. I have heard of some others wanting everything to check to make sure the head didn't warp when it was welded. You will reuse the same set of rings, just make sure you stagger them correctly when you reinstall the cylinder. Do have the shop through in a new cylinder base o ring and pushrods tube gaskets when they send the cylinder back to you. This will save you from having to order them in separately. You should then get a repaired tag from the shop stating what work was done and that it is airworthy, but it will not be an overhaul. They will also not hone the cylinder unless you tell them to.

Other than shipping, a good shop will probably charge you in the neighborhood of 40 bucks to weld and retap the exhaust stud. At least that is what I have been charged in the past.
 
krw5927 said:
Bob, are you suggesting the stud may have been over-torqued when it was inserted or that the nut was over-torqued when the exhaust was mounted?

The Lycoming overhaul manual I have for the O-320 says exhaust studs (not the nuts) should be torqued to 40 in-lb minimum, and then a couple pages down says the driving torque upper limit is 100 in-lb. Will following the manual strip the threads?

This is a higher time engine (but below TBO) that was running just fine and had good compressions on all cylinders when it was removed from a skyhawk for upgrade to an O-360. Oil analyses were good, and inspection of cam lobes and followers revealed no evidence of spalling. Because it's higher time, my goal is to fly with it for a while and then overhaul. So pouring money into a full cylinder recondition is not something I'd prefer to do, since I will be buying 4 new jugs at overhaul time.

If I pull the jug to send it out, should I pull the piston pin and send the piston with the jug, or just secure the piston and rod so they don't touch the case and send just the jug? Will I need a new set of rings if I don't have them hone the cylinder?

Thanks again for all your help!
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It was either bottomed out or overtorqued, or both. Thats the only way the threads could have been damaged.

Theres another option you could explore, and its called a Twinsert. Basically its a helicoil within another helicoil. http://www.helicoil.com.sg/HeliCoil-Oversize-Twinserts.shtml

If it ain't broke, don't fix it. Leave the piston in it and once the cylinder is out push the pin back in as well as the piston. No need for a new set of rings even if you pull the piston out.
 
I'm not a gearhead, so I throw this out as a question. Would Timesert's or Keensert's work in a hole that the helicoil would not hold in? I used a Timesert in an aluminum Corvair head one time and it worked great....
 
I thought about using a Keensert, not sure if the diameter is large enough to fix the problem. A friend of mine manufactures aluminum gearbox covers for the F-35 and they use Keenserts on those parts.
 
Wow, thanks for the suggestions on Twinserts and Keenserts. It looks like Keenserts come in thin and and heavy wall options. The heavy wall Keensert for a 5/16-18 stud has a 1/2-13 outer thread. I'm sure the cylinder hasn't been drilled out that far....yet!

The Keensert looks like a real winner. A little expensive for what it is, but still cheaper than $hipping a jug two ways and a weld repair. I'll write back and let you know how it goes. Thanks again - I love VAF!
 
rocketbob said:
The Keensert tool to drive the tangs in is fairly pricey, so thats the only negative.
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Kurt, you may be able to borrow a Keensert tool if you post on VAF and someone has one to loan. When I had to install the Timesert I was on a Corvair list and asked to borrow a Timesert tool set and a guy in AL sent me the whole kit, including Timesert's. I used the tools and one Timesert and sent everything back to him with $20 stuffed in the kit. We were both happy!
 
It looks like one can get away fairly cheaply ($20.82 + S&H) with a kit from Mcmaster: http://www.mcmaster.com/#90245A013

It says "Inserts with tool". Not sure how many inserts come with it but extras aren't too bad either.

It says "similar to keensert" which is probably good enough for me.

What do you guys think?
 
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krw5927 said:
It looks like one can get away fairly cheaply ($20.82 + S&H) with a kit from Mcmaster: http://www.mcmaster.com/#90245A013

It says "Inserts with tool". Not sure how many inserts come with it but extras aren't too bad either.

It says "similar to keensert" which is probably good enough for me.

What do you guys think?
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I'm not qualified to give you an opinion on that question, hopefully someone else can...
 
Last time I checked they were a bit more. You'll have to have two taps, one to start the threads and one with the nose ground off to be a bottoming tap. The neat thing about keenserts is they use regular taps unlike helicoils which require helicoil taps. The important thing is to carefully chamfer the hole to prevent cracks from occuring.
 
Soo... did it work?

krw5927,
Just wondering if your McMaster locking helicoils worked for your cylinder? I'm new to the forum and new to the community. Bought a nice O320 RV-4 in November and found a crack in the exhaust. Had it rewelded but noticed that one of the studs just pulled out with less than 5 ft/lb of torque. The stud unscrewed by hand and there was not much left of the inner threads in the cylinder head... All help is appreciated. Thanks!
 
I do a lot of broken bolt and stripped thread repairs.

An oversized big-sert repair, made by time-sert, would be an easy way to fix the cylinder. They are easy to use, affordable and work well.

George Meketa
 
navalaviator07 said:
krw5927,
Just wondering if your McMaster locking helicoils worked for your cylinder? I'm new to the forum and new to the community. Bought a nice O320 RV-4 in November and found a crack in the exhaust. Had it rewelded but noticed that one of the studs just pulled out with less than 5 ft/lb of torque. The stud unscrewed by hand and there was not much left of the inner threads in the cylinder head... All help is appreciated. Thanks!
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Hey Ty,
Welcome to VAF!

Yes, I bought the McMaster locking insert set and a matching three-tap set to tap the stud holes (definitely need the regular tapered tap to start the hole and the bottoming tap to get threads cut all the way down to the bottom of the hole). The taps are good quality and cut well, and the inserts worked like a charm. I haven't flown yet, but the studs and nuts torqued up to Lycoming specs and all seems well. I see no reason they won't be just fine in service.
 
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