Question about why helicoils don't fail...

[Lassen Forge]

We all know about those wonderful devices called helicoils, how they can save your *** (especially in soft metals like Aluminum or Magnesium) when you overtighten that fastener and you end up stripping out the hole. God knows I've used a boatload of them over the years, everything from VW exhaust studs to Model T head bolt blocks to Honda 750 cases...

But one thing has always had me curious, and no one has provided a decent answer. What keeps that helecoil insert from re-pulling out like the original stud/bolt/etc. did? True the new threads are stainless where the originals were in whatever they were, but what keeps that insert from pulling out?

 

[Junkman]

I have had a problem with them coming out of the spark plug holes in my Corvair Spyder Turbo. I don't own this car from new, so I don't know who installed the helicoils or when. The cure was to replace the heads with a pair of NOS ones that I pulled off a dealer's parts room shelf back in the 1980s. A friend rebuilt the engine for me, and he encountered a few pulled threads in the head studs, and he put helicoils in those holes with no problems.

 

[larry_g]

SWAG here. When you pull a virgin thread in the hole a couple of things may be at play. One is that the fastener galls to the thread and shears it off as the fastener is rotated. The other is that the fastener may use the wedging action of the tapered treads to open the hole up just a bit so that instead of having 75% thread engagement it is reduced until failure occurs. With the Helicoil in place these things do not happen. The Helicoil is hard metal reducing the galling and has the strength to handle the load at reduced thread engagement. As the load on the Helicoil is increased it is wedged into the parent material biting harder to prevent pullout.

As said, just a swag but it makes sense in my mind.

lg
no neat sig line

 

[vavet]

Since we’re on the helicoil topic, I’ll pose a question a former coworker asked me. If you were pulling a head and had to helicoil one hole, would you do all the head bolts? The patient is a high mileage 02ish Camry with a 4 cylinder.
would your answer change if I said it needed 2? What about 4?

he was leaning towards all of them. The idea was that they all suffer the same conditions that led to failure, so why not get ahead of it. I lean towards the minimally invasive route and do the least amount.
what’s your approach?

 

[Steve from Socal]

The surface area of the insert is far greater than the original thread. I have seen plenty of failed inserts but, they are good option for many situations.

As far as blocks, I have done aluminum Mercedes blocks with inserts all around, as noted one failed hole is just a start

 

[In The Doghouse]

The helicoil has a larger diameter with more OD surface contact Add in the hardness of the thread contact with the fastener they just work.

 

[Rusted Nut]

Helicoils are kept in place by springloaded tension. The coil springs outward and grip the parent material.

 

[larry_g]

Helicoils are kept in place by springloaded tension. The coil springs outward and grip the parent material.

In addition to what I said above this also makes the idea of 100% thread engagement possible as the Helicoil expands to reach the root of the threaded hole.

lg

 

[RoninB4]

In addition to what I said above this also makes the idea of 100% thread engagement possible as the Helicoil expands to reach the root of the threaded hole.

-Just as an FYI a 77% thread engagement is the target most shops use. The last little triangular portion of a sharp "V" thread does very little for strength. Many thread forms have a truncated crest and a radius at the root for this very reason.

 

[larry_g]

-Just as an FYI a 77% thread engagement is the target most shops use. The last little triangular portion of a sharp "V" thread does very little for strength. Many thread forms have a truncated crest and a radius at the root for this very reason.

I do understand that and that many thread forms are not a sharp V. However does this apply to STI taps or do they form a thread that is ultimately near 100% fit to the OD of the thread insert considering that the insert expands to fit?

lg
no neat sig line

 

[CapriMikeC]

For the truly curious, see attached specification which describes all details of hole preparation to accept helicoils including drill size, countersink angle/diameter, and dimensions of the helicoil once installed. This is the standard used by all aerospace for inch inserts. MA1567 is the spec for metric helicoils.

 

[whateg01]

Since we’re on the helicoil topic, I’ll pose a question a former coworker asked me. If you were pulling a head and had to helicoil one hole, would you do all the head bolts? The patient is a high mileage 02ish Camry with a 4 cylinder.
would your answer change if I said it needed 2? What about 4?

he was leaning towards all of them. The idea was that they all suffer the same conditions that led to failure, so why not get ahead of it. I lean towards the minimally invasive route and do the least amount.
what’s your approach?

I had to repair 2 plug holes in my 00 f150 5.4. I did not do the rest even though the cal van kit has the other 6. I do still have the kit in the truck for the day it's needed, though.

 

[whateg01]

One of the things that helps is that the inserts are larger in diameter than the threads that get cut. When you use the tool to turn the insert by the tang on the bottom, it pulls the insert in, making the od slightly smaller. When you stop pulling on it, it relaxes back to its larger od, kind of binding it in the hole.

 

[RoninB4]

However does this apply to STI taps or do they form a thread that is ultimately near 100% fit to the OD of the thread insert considering that the insert expands to fit?

-That's a good question Larry. I can't answer that with any authority but I'll take a WAG. I think the form of the STI, for a Heli coil anyway, has more to do with the shape of the extruded wire they make the coil with. Putting a tiny radius on the root or truncating the crest would seem, to me, use more expensive tooling to extrude and require clocking/orientation to wind into a coil. IMO it would be simpler to just have a triangular cross section for the wire. QC would be easier, it's a standardized form, and presents less chances for poor fitment when tooling wear allows wire form tolerance to approach the limits. Acceptable speculation or not?

-The coil type inserts may expand but it sure doesn't seem like very much binding occurs when being installed or when the fastener itself is put in. The friction between the coil and the fastener threads allows the coil to back out with the fastener after 1-3 in/out cycles when I've used them. I've noticed other toolmakers having the same results but none of us has had adequate time (production waits) to use Loctite on the coil either so this may be the preferred way to install them. The solid wall inserts, obviously, don't expand. The thread form for those, at least some of them, advertise that standard taps are used so I'd guess that the thread form for those is all about the simplicity of using standard tooling instead of needing the special tap required for the coil type. I've noticed that at least one maker of solid wall inserts has thread locker pre-installed on the threads. That's all just supposition on my part so if you (or anybody) has a different opinion I'd be interested in hearing it.

 

[whateg01]

... a triangular cross section for the wire. QC would be easier,

Not sure I understand that. The wire is a parallelogram with 60 degree points for the threads.

-The coil type inserts may expand but it sure doesn't seem like very much binding occurs when being installed or when the fastener itself is put in. The friction between the coil and the fastener threads allows the coil to back out with the fastener after 1-3 in/out cycles when I've used them.

The coiled insert is wound slightly large. When the tang is pulled (rotated) into the hole it reduces the diameter slightly, in much the same way it's easier to pull a wire through a conduit than it is to push. Are it's not being pulled in, it tries to relax back into it's expanded shape. I would guess after a few screw insertions/removals, there are probably small abrasions in the surface of the insert that the screw thread grabs and wants to pull it back out.

 

[RoninB4]

The coiled insert is wound slightly large. When the tang is pulled (rotated) into the hole it reduces the diameter slightly, in much the same way it's easier to pull a wire through a conduit than it is to push.

-Well thank you but I'm well aware of what happens when the coil winds into the hole. I did err by calling it triangular, it's more of a double triangular shape (or parallelogram) to accommodate both thread flanks.

 

[whateg01]

-Well thank you but I'm well aware of what happens when the coil winds into the hole. I did err by calling it triangular, it's more of a double triangular shape (or parallelogram) to accommodate both thread flanks.

You stated that they inserts expand but that's not really true, unless you are saying that they try to expand to their original diameter, which is slightly larger than the tapped hole. Once inserted, they are actually smaller in diameter than in their relaxed state. I don't doubt that you understand that but the way you described it would mislead somebody who doesn't understand how they work.

 

[RoninB4]

You stated that they inserts expand but that's not really true, unless you are saying that they try to expand to their original diameter, which is slightly larger than the tapped hole. Once inserted, they are actually smaller in diameter than in their relaxed state.

-They ATTEMPT to expand, they are constricted/limited by the major diameter of the tapped hole. I thought that should be quite obvious to anybody but thanks for proofreading my early morning ramblings.

 

[iagsxr]

Ever notice that higher end aftermarket aluminum heads come from the manufacturer with the bolt holes Heli-Coiled?

The only ones I've seen fail have been into soft metal that it seems like the OD was tapped sloppily.

I'm waiting on my nephew to Heli-Coil a few brake rotor bolt holes in a pair of cast steel 9" Ford floater hubs. He'll drill and tap them in a mill so everything should be perfect. I have no doubt they will be stronger than the original bolt holes.

I could do them myself freehand, but see above. If I get the tap a little crooked or waller out the OD hand tapping I'd be going backwards.

 

[rustyzman]

I think that as stated up earlier, the reason for initial thread failure is usually more than just a tension failure pulling out the threads. That face to face sliding action of the fastener thread to the threaded hole also has a galling and shearing action that helps cause the original failure.

Once the helicoil is inserted, the softer newly re-tapped STI hole and the helicoil are essentially stationary (and of a slightly larger OD) and the force of the insertion of the bolt is seen as much more of a tension with less of a shearing action at the original base metal. Now the shearing action is more between the fastener and the helicoil, which is inherently stronger and more gall resistant than the original base metal in most cases.

I think of it a bit in the same terms of why that same hole if assembled with a proper stud, tends to not pull threads as easily. The fastener being tightened is not rotating in the base metal, because the stud is essentially fixed rotation wise. The interface with that shear action is between the nut and the stud, both of which usually have higher strength.

Same kind of thought process of how using a washer under a bolt head or nut stops the shearing and galling of the item being clamped together. That action is happening on the face of the washer at the fastener instead.

Another item to consider is that both a helicoil and most commercially made external thread hardware are both formed threads, not cut. Those are inherently stronger than a cut thread as a lot of the stress raisers and microscopic surface tearing that cut threads create are not there on the interfacing/moving threads.

There are absolutely lots of cases where a tension failure alone occurs and a heilcoil or time-sert repair holds up far better in the long run. A big part of that is the fact that the new thread surface area is larger than the original. Particularly on thread insert styles that are not the helicoil type, as they are usually quite a bit bigger OD than the fastener they are holding.

Anyway, that's how my mind sees it.

 

[Xti04]

Since we’re on the helicoil topic, I’ll pose a question a former coworker asked me. If you were pulling a head and had to helicoil one hole, would you do all the head bolts? The patient is a high mileage 02ish Camry with a 4 cylinder.
would your answer change if I said it needed 2? What about 4?

he was leaning towards all of them. The idea was that they all suffer the same conditions that led to failure, so why not get ahead of it. I lean towards the minimally invasive route and do the least amount.
what’s your approach?

Some of that era camry and rav 4s with the 4 cylinder had issues with pulling threads in the block for thebhead bolts. Toyota had a whole kit to repair them. I would do them all. Yes it *****, but pulling the head in 5k miles ***** worse.

 

[Lassen Forge]

Since we’re on the helicoil topic, I’ll pose a question a former coworker asked me. If you were pulling a head and had to helicoil one hole, would you do all the head bolts? The patient is a high mileage 02ish Camry with a 4 cylinder.
would your answer change if I said it needed 2? What about 4?

I've done all of the ones in the "series" - head bolt holes on a model T, exhaust studs on (several) VW's, lower end holes on certain honda bikes. If one pulls, I expect them all to go, and I know that helicoil won't strip. I was curious why they don't pull.... My old shop teacher (years later), who built land speed run buggies as a hobby, ALWAYS told us if you have to do one, you do them all. But never really explained why, other than you won't have a failure there later.

I think that as stated up earlier, the reason for initial thread failure is usually more than just a tension failure pulling out the threads. That face to face sliding action of the fastener thread to the threaded hole also has a galling and shearing action that helps cause the original failure.

Once the helicoil is inserted, the softer newly re-tapped STI hole and the helicoil are essentially stationary (and of a slightly larger OD) and the force of the insertion of the bolt is seen as much more of a tension with less of a shearing action at the original base metal. Now the shearing action is more between the fastener and the helicoil, which is inherently stronger and more gall resistant than the original base metal in most cases.

I think of it a bit in the same terms of why that same hole if assembled with a proper stud, tends to not pull threads as easily. The fastener being tightened is not rotating in the base metal, because the stud is essentially fixed rotation wise. The interface with that shear action is between the nut and the stud, both of which usually have higher strength.

Same kind of thought process of how using a washer under a bolt head or nut stops the shearing and galling of the item being clamped together. That action is happening on the face of the washer at the fastener instead.

Another item to consider is that both a helicoil and most commercially made external thread hardware are both formed threads, not cut. Those are inherently stronger than a cut thread as a lot of the stress raisers and microscopic surface tearing that cut threads create are not there on the interfacing/moving threads.

There are absolutely lots of cases where a tension failure alone occurs and a heilcoil or time-sert repair holds up far better in the long run. A big part of that is the fact that the new thread surface area is larger than the original. Particularly on thread insert styles that are not the helicoil type, as they are usually quite a bit bigger OD than the fastener they are holding.

Anyway, that's how my mind sees it.

GREAT and logical explanation. Thank you!!!

 

[JSGAuto]

Ever notice that higher end aftermarket aluminum heads come from the manufacturer with the bolt holes Heli-Coiled?

I doubt that that is for strength, if it was, they would just design in a larger thread, which would be cheaper. Likely to reduce galvanic corrosion.

 

[Steve_P]

I doubt that that is for strength, if it was, they would just design in a larger thread, which would be cheaper. Likely to reduce galvanic corrosion.

It is for strength. You can't just add larger diameter threads to something like a cylinder head because the stuff that bolts to those threaded holes have been using the same size fastener diameter for 50+ years in many cases; you'd then need custom headers, intakes, thermostat housings, carburetors....

 

[JSGAuto]

It is for strength. You can't just add larger diameter threads to something like a cylinder head because the stuff that bolts to those threaded holes have been using the same size fastener diameter for 50+ years in many cases; you'd then need custom headers, intakes, thermostat housings, carburetors....

Ah, good point, I was not considering legacy interchangeability. But, I am still doubting it's for strength. If it's been that way for 50 years the industry would have corrected much quicker. But maybe it would also serve a better purpose of reducing wear of parts swapping.....since AL threads wear easily. Adding the insert would alleviate this problem.