GM Helicoils

[Snowshoe]

I have been gone for a while. I see there is some sort of contoversy over GM Barrels with helicoiled breech plugs in them. I have a .40 with one and it works fine. I am in the aviation industry, any thread that needs to be stronger has a helicoil in it in an aircraft, and that's from the factory. Every sparkplug hole has a helicoil in it. I tend to agree with GM. When I saw that my .40 had an insert in it I was pleasently surprized.
Shoe :hmm:

 

[Mad Professor]

I have been gone for a while. I see there is some sort of contoversy over GM Barrels with helicoiled breech plugs in them. I have a .40 with one and it works fine. I am in the aviation industry, any thread that needs to be stronger has a helicoil in it in an aircraft, and that's from the factory. Every sparkplug hole has a helicoil in it. I tend to agree with GM. When I saw that my .40 had an insert in it I was pleasently surprized.
Shoe :hmm:

I'll bet (most of) the helicoils used on airplanes are on soft non-ferrous metals such as aluminum and magnesium. These are not only softer metals but are also subject to galvanic corrosion when in contact with a steel bolt. Ever try to pull a sparkplug from an aluminum head auto with 30-50,000 miles and had not been neversiezed? You usually get all the aluminum threads removed with the plug or snap off the plug in the head. I'll bet all the aircraft heads/engine cases are either aluminum or magnesium.

P.S. I'll bet almost none of the threads in steel aircraft parts are helicoiled.

 

[roundball]

I have been gone for a while. I see there is some sort of contoversy over GM Barrels with helicoiled breech plugs in them. I have a .40 with one and it works fine. I am in the aviation industry, any thread that needs to be stronger has a helicoil in it in an aircraft, and that's from the factory. Every sparkplug hole has a helicoil in it. I tend to agree with GM. When I saw that my .40 had an insert in it I was pleasently surprized.
Shoe :hmm:

I'll bet (most of) the helicoils used on airplanes are on soft non-ferrous metals such as aluminum and magnesium. These are not only softer metals but are also subject to galvanic corrosion when in contact with a steel bolt. Ever try to pull a sparkplug from an aluminum head auto with 30-50,000 miles and had not been neversiezed? You usually get all the aluminum threads removed with the plug or snap off the plug in the head. I'll bet all the aircraft heads/engine cases are either aluminum or magnesium.

P.S. I'll bet almost none of the threads in steel aircraft parts are helicoiled.

I agree...no way a stainless steel nipple screwed directly into a carbon steel breechplug is weaker than being screwed in a helicoil that's been screwed into the same breech plug...instead of only two surfaces to contend with, the addition of a helicoil doubles those surfaces to four...and I worked in jet aircraft maintenance for 4 years and never saw a helicoil.

 

[Blizzard of '93]

you are correct, I've been mechanicing for more years than I like to admit and seen many a seized plug in non-ferrous heads. ferrous too for that matter. A tip for mechanics here - any bolt or spark plug that is black-oxided - use never seize on it. I'm a Ford fan but those dern Autolite plugs are trouble without you use some thread coating.

 

[TennesseeJed]

I recently bought a .58 caliber GM barrel and it came with a helicoil in the breechplug. In the short term, I don't have any real concerns about it coming loose or blowing out. But long term, I do have concerns about how it will hold up. I didn't get a response from GM to my question about if they did any long term testing to see how they held up.

 

[FTW]

I have installed helicoils in sparkplug holes in
cars,boats,motorcycles,aircraft,lawnmowers,generators
and in the holes for the studs to hold the exhaust pipes on for all the above.If a man shoots 50 shots a week out of the same gun 52 weeks in row thats 2600 shots.No where near the abuse the threads in your engine take on a trip to the range, for me about an hour each way.Heli-coils don't bother me.

 

[Zonie]

M.P.: For the most part, you are correct about the use of helical thread inserts being used in non-ferrous materials in aircraft parts however, they have been used in steel and nickel alloy parts when a thread locking feature is desired.** (Heli-Coil inserts come in locking and non-locking styles).

For the most part, the strength of a Heli-Coil is greater than the parent material including the steels used in muzzleloaders.
On the negative side of this use is the fact that a Percussion nipple's diameter is usually not sufficient to seal against the breech plug face. This results in a leak path between the Heli-Coils outer threads and the threads in the rifle.
This leak path will be fouled by the black powder gasses and may corrode and/or erode.

Another problem that has been reported in this Forum with the Heli-Coils being used by GM is that some of them are too long.
This length problem ends up in blocking off part or all of the flame channel leading to the rifles bore.
If your barrel has a Heli-Coil installed, you might want to see if this has happened.

zonie

** My knowledge of this is a result of my former job as a Lead Designer of jet engines (now retired).
The use of self locking Heli-Coil inserts was generally against my old company's rules, not because they were weak but because they often would unscrew from the parent material with the fastener leaving a trapped fastener on the removed part and a really PO'ed mechanic who was interested in getting the aircraft airborne again.

 

Been seeing this Helicoil thing on here quite a bit, I am not taking sides with GM or anyone here,I have a GM-38"-.40cal barrel that has the helicoil and as mentioned above the threads were a bit long but not anymore figured out a way to take care of that,now about the powder gases leaking,mine looks to have some sort of epoxy around the outer threads if it does that should take care of that I guess only time will tell.I do feel that GM should not have used them I will keep the board posted on the outcome here as I have put only about 75 shots throught the barrel so far

 

[barebackjack]

Whats a helicoil?

From what I gathered its something that screws onto the breech end of the barrel and the breech plug then screws into it?

What is the specific reason for it?

 

[faw3]

I'll try and write this so I understand it, I got a 58 and wasnt to worried untill I ckd the area under the nipple, do you know what kind of gas cutting these are doing to the breech? Since GM hasnt come back with a answer Im really starting to worry if I have to replace the breech plug every 100 shots to keep it shooting 6" target at 100yds, I know its beeen a problem with many rifles (gas cutting at bottom of nipple in target rifles) any coment? BAREBACK it seems they messed up and are saveing the barrels not doing the "right" thing Im going back to Long Hammock my self. Fred :hatsoff: :blah: on GM

 

[Snowshoe]

Most Helicoils are stainless steel (even the store bought ones). Most Cylinders are aluminum with a helicoil insert. I would imagine the High heat and corrosive salts from burning 100 octane fuel with continued high pressure pulses would give the insert a fair work out. There are applications of steel/ helicoil applications for example Cessna engine mount lugs which are made nickel steel and are helicoiled from the factory.
Shoe

 

[FTW]

I have two GM barrels with heli-coils with over 300 shots through the 50 and 150 thruogh the 54.I don't think you have to worry about replacing any thing for quite sometime.If you have a 2 cylinder 4 stroke motorcycle engine with heli-coils in the sparkplug threads,at 2000 rpm each plug fires 250 times.15000 times per hour per plug.Why ain't the sparkplugs blown out of the head?

 

[Zonie]

Most spark plugs have a face or surface which is larger than the Heli-Coil insert. The face (or cone) seals directly against the cylinder head. This prevents the leakage of gas past the helicoil.

If the outside diameter of the Heli-Coil threads are smaller than the outside diameter of the nipple, then the face of the nipple will seal the same as a spark plug.

If GM is using an Epoxy during the installation of their Heli-Coils, that should seal off this potential leak path however IMO, the outside diameter of the nipple still needs to be larger than the Heli-Coils outside thread to prevent leakage thru the inside threads.

To answer the question "What is a Heli-Coil?" it is a diamond shaped wire wound tightly into into a cylinder. The inside part of the diamond creates the inside thread while the outside part of the diamond creates the outside thread.

As for a Heli-Coil blocking off part or most of the flame channel, a fix would be to remove the "cleanout plug" and run a drill which is just slightly smaller than the inside diameter of the threads down thru the channel.
This will drill out any part of the insert which is blocking off the flame channel.

zonie

 

[roundball]

For those who may not know, these are helicoils

 

[faw3]

I guess it doesnt count that I pulled 7 head bolts out of a Datsun 510 one race season (scca) since the head isnt steel, but no one is saying anything about gas cutting at bottom of nipple that can screw up a shot after as few as 25 shots! :shake: Fred :hatsoff:

 

[Kentuckywindage]

Just to let you all know how strong heicoil is, the guys building high performance buick engines drill out their 1/2" mainbolt holes out to 9/16" and heicoil it. It is strong and ive seen these buick motors putting out 600 horses and running them for 20+ years with no problems. The 3.8 v6 guys also do this when they start building them for the 800hp class. Its strong stuff and takes a beating.

 

[ldykeman]

But....how many psi develops in the chamber of an internal combusion engine, compared to the psi generated by the combustion of BP in a barrel? Not being contrary, I really don't know.
Larry

 

[Mad Professor]

But....how many psi develops in the chamber of an internal combusion engine, compared to the psi generated by the combustion of BP in a barrel? Not being contrary, I really don't know.
Larry

The highest cranking pressure I've measured was ca. 225 psi on a 12: 1 motor, diesels are probably higher. I'd imagine running ups this quite a bit but not into the range of BP chamber pressures that can be ca. 10,000 psi or more.

 

[roundball]

I don't have a clue as to how safe these things are...but it's a REPAIR on a brand new barrel, in a place close to my eyes/head trying to contain a lot of pressure in an enviroment that we already know is corrosive.

So to me it's a no brainer...I simply won't have one...others mileage may vary.

 

[Kentuckywindage]

225psi on a 12:1 motor? That doesnt sound right at all. we had a 95 isuzu trooper with 10:1 compression and it put out 220psi on 5 out of 6 cylinders. In order to get a true 12:1, things like milling the heads any where from 10 to 30 thousandths is required to get it close. For example the motor im building with have 1970 GS350 10.25:1 pistons. But i would have to mill the heads .030 and the intake .020 to get a true 10.25:1 compression ratio.