Why 'Helicoils' can be better than the original thread!

I am an uber big fan of Armstrong 'Helicoil' inserts. I use the trade name 'Helicoil' not as a generic term, but because I prefer to buy the original. Although I have recently had to buy clone brands as they are easier to find.

I first came across 'Helicoils' in 1967 when working on Laycock Overdrives. These had very thin die-cast LM11 aluminium casings and to ensure the threads would hold 'Helicoil' inserts were used, in production, on the main joint threads.


Some little while ago, in answer to a member asking if thread inserts would be OK in the block, for head bolts, I suggested that they were actually better. I recently came across the diagram which shows why thread inserts spread the load better.



I shall be 'Helicoiling' all of the inportant threads in the head, block, and other places during the VWMSification process.

SAAB EXTRA.

You don't need to read this.

Now. When I destroyed my first Saab 96 gearbox internals, I found that Saab did the same, in fact, all of the threads in the alloy box were 'Helicoiled'. These alloy boxes were designed for a two-stroke engine, with a max. standard power of 50bhp, and max. tune power of 90 bhp in the Rally Cars. So were ok for the standard 65bhp of the V4s, but over the limit for the 115bhp of the Weber DFI set-up. The factory used cast iron boxes in the 180bhp cars. I could not afford an iron box, but, back in the early days, Saab cast the two-stroke motor casings from steel, to avoid porosity, and make them uber strong. They cast the original 3-speed boxes from steel, and, they also cast the first few hundred of the 4-speed boxes from steel as well. I scoured the breaker's yards till I found one and bought the whole car for about 25. It also gave me lots of other useful rally spares: spare doors, rear wings, laminated screen and, 'A SPARE ROOF'! Don't ask!

 

[pedant mode on]
How do you tighten the bolt in the diagram as the head is sunk below the upper surface of the upper block.
[pedant mode off]

Can you tell it's Friday, and I'm on holiday next week? lol

 

A rather appropriate `Friday thread` Dave ...

One of the spark plugs in an old Beetle engine of ours was done `as so` ... never a problem

 

[Super-Pedant Mode On]
You failed to mention the glaring lack of washers in those illustrations. Go and sit in the corner with your pointy hat on, Dave

[Super-Pedant Mode Off]

 

I sell loads of thread insert 'Helicoil' kits and never knew they where more efficient at spreading the loading, though it is logical when thought about. Good stuff Daved, cheers for posting.

 

One thing, the thread pitch in the two diagrams is not the same.

If the helicoil and the larger bolt had the same thread pitch, they would be exactly the same.

 

Helicoils are an excellent piece of kit but are only stronger if fitted correctly so many times ive seen these come out when torqueing down when not fitted correctly by so called tradesman!

 

That reminds me I need one of these for the speedo drive in my Mk1!

 

I have a helicoil insert in one of the spark plug holes on my mk3 16v after the cheese-like aluminium head decided it didn't want to have a spark plug in it anymore! Never had a problem!

 

A couple of other points.

Becasuse the new thread in the parent material is a larger OD, the shear stress area is greater, so the actual stresses are lower.

One problem is that the inserts are stainless so some form of barrier is required between the thread on the carbon-alloy steel bolts and the insert.

If you don't want to use a Loctite retainer, Blue or Red, or Green, then one of these is a good idea.



I think you can ignore 8014 unless you intend to cook on you hot motor!

 

I`m sorry, I must be missing something here.

WHY is the load more equally spread on a helicoil ?

Thread pitch is the same, I just can`t get my head round that. Its not from a helicoil PR blurb is it..... ?

 

Im trying to get my head around it too, the helicoil is a bigger diameter than the bolt, this would mean the thread depth is slightly deeper on the helicoil (the helicoil to parent metal side) than the original bolt. Is it this extra depth and the increase in circumference (from the larger diameter) that adds to the higher strength because of the resultant greater surface area of the outside helicoil thread.

The other thing is, surely it is only stronger if the helicoil is fitted to a weaker material than it's self such as aluminium? Otherwise, if you fit a helicoil to the same grade material as the helicoil is made from the weakest part would be the bolt to helicoil thread, not the helicoil to parent metal thread (because of the greater surface area of the threads). Either the bolt strips or the helicoil strips, it would not be the parent material thread because that has a greater surface area making it the strongest out of the two threads, hence this thread!

 

I found the diagram on post #1 on an Irish website when searching for 'Helicoil' inserts.

I have returned to the Irish site and traced the diagram back to a German insert manufacturer named BAER.

Herewith the page from their catalogue from where the diagram originates.

Hopefully this answers some of the questions?

 

I have a good section on heli coils and from a users point of view they are brilliant, one word of advice would the get the correct diameter of the drill bit to do the job right, many are always X.7mm or something to get full surface area on the coil
Thats why I go a different retailer that stocks V-coil (helicoil a brand name like hoover!) V coils are a Australian version of the product (copy?) which comes in cheaper, get more coils & most importantly for me get the drill bit included!

 

For a rough thread, I can see the precision of the helicoil spreading the load more evenly, but on something of higher quality, I`m still not getting it.

I can understand they are stronger as you are increasing the area of the thread, so in aluminium or something, that makes more sense.

 

But it only makes sense if for some reason you can't just use a larger bolt, as the end result is the same.

A helicoil is only used in a circumstance where drilling and tapping and using a larger bolt isn't possible and the original bolt is required.

 

Hope they do, do the trick... i'm off to heli coil my inlet manifold down soon..

 

in some ways on rusty steel it could the the same welding and re tapping, but far easier!

 

Wasn't too soon.. lol

Just been out to helicoil my inlet.. Everything was going well until i tried to put the helicoils in... The first thread would bite but then pull its self out of shape?? I tried on a couple of holes and the same problem... I tested it on a bit off ally liying around, it went in piece of cake..

So figured the tap never went in enough and ground the tip down to get further in the hole, thought i'd cracked it but then the helicoil just started spinning....

Any ideas whats causing this???

 

Shaun. A few ideas that might shed light on your problems.

I am sure you used the correct size drill, but was it new, or re-ground?

And, when you ground down the tap, can you be sure that the end was square, or the chamfer was even all around, or there were no burrs at the cutting edges?

Just like drills that are ground off centre can make over size holes, a tap not ground spot on will follow the line of least resistance and produce over size tappings.

Did you use a good drill/tap lubricant when both drilling and tapping? Some aluminium alloys have a coarse grain structure which tends to break up, as opposed to cut cleanly, with out the use of lube!

Do you have the type of insertion tool where the insert is located in a threaded outer sleeve, like the Armstrong Heicoil one shown below? I have found the more basic ones, without the outer sleeve, are not as good at locating the insert accurately.