Helicoil Inserts - What Did I Do Wrong?

[revilla]

EU3 K Series cam cover. One of the bolts that secure the coil packs to the cam cover had stripped the thread from the alloy. No problem thought I, I'll do a helicoil-style repair, so I bought a kit - not actually "Helicoil" branded but supposedly compatible - like this.

I checked I had enough depth in the blind hole - I reckoned I needed 11mm to allow for the depth of the insert plus the length of the tap that wasn't fully threaded at the end. I measured 19mm so no worries.

I used a bit of tape around the drill provided in the kit to mark the correct depth and drilled to a depth of 17mm, then tapped it as deep as I could using the tap provided (plenty of depth of fully formed thread for the insert). All looking good up to this point.

So the original bolt thread was standard pitch M6 x 1mm. The tap provided was approximately M8 x 1mm, so fine pitch for an M8 but as expected to allow the coil insert to screw in to give an M6 x 1mm internal thread again. All made sense.

I would therefore have expected the inserts themselves to have a thread pitch of 1mm - but they don't, it's more like 0.75mm. If you screw the bolt down the centre of an insert it stretches out as the turns of the coil are expanded from 0.75mm pitch to 1mm pitch. Not having done this before, I assumed this was normal and the insert was designed to stretch out like this as it was screwed in.

So I applied a this smear of threadlock to the outside of the insert and using the special tool provided in the kit (which fits tightly down the centre of the insert, preventing it from collapsing inwards whilst gripping the tang at the bottom of the insert) I screwed the insert into the newly tapped thread. It felt a bit odd, the insert was clearly stretching out so the "tail end" of the insert was lagging behind the head a bit, but it wound smoothly in until the tail end was just below the surface. At this point I withdraw the tool ...

The insert immediately snapped back to its original length, jumping out of the thread I had tapped, leaving it well below the surface and absolutely impossible to get out. Any attempt to screw a bolt into it is quite tight and just cuts the thread off the bolt.

Luckily I happen to have a spare cam cover, but the old one is now a bit of a bodged mess.

I'd like to know what I did wrong - or whether the kit is just wrong - as I'd like to know that this is something I'm competent to do in the future without wrecking the component I'm atempting to repair.

Any advice greatfully accepted, thank you.

[elie boone]

Strange I never had a problem using a recoil set. The sets i have don't even have a drill. The only thing you could try is to try and take a normal tap and try gently to clean up the tread, if it will work is a big questios as the recoil is steel, hope it's a little softer than a tap

[CharlesElliott]

I have never done it - but I would be nervous of an eBay kit for £6. I think I would have at least splashed out on the Machine Mart version: https://www.machinemart.co.uk/shop/product/details/040216677

Charles

[Richard Price]

'Sounds like it's the wrong size to me.

typically, when helicoiling an M6 hole, no drill is required. The supplied tap is simply used to re tap the stripped hole. 

Likwise, I would never use thread lock with a helicoil. They normally look a little too big for the hole, and are compressed as they are fitted.

maybe you have the correct tap, but the inserts are M5 rather than M6?

[Richard Price]

What markings are there on the tap that was provided in the kit?

[John Vine]

It's a very long time since I last used Helicoils (on motorbikes, not 7s). From your description, I think you've been supplied the wrong coils. IIRC, the tapped hole will be slightly narrower than the coil so that, when the coil is wound in fully, it grips securely.  The tang will gradually advance wrt the "tail" as the coil compresses and the diameter reduces.  But, like you, I'd expect the pitch of the coil to match the tap. The coil shouldn't have to stretch at all. 

FWIW, there are removal tools available (basically a V-shaped blade with sharp edges that you drive into the top of the coil and then turn anticlockwise to unscrew).

JV

[Mechanical Moz]

I have managed to get badly fitted helicoils out before by levering the top end of the coil out a little, grabbing it with some needle nose pliers and then uncoiling it from the thread.  This is with the first coil just below the surface though so I don't know if it's possible in your case.  

The kits we use now (Helicoil Plus) have a threaded mandrel that the coil screws on to with a step at the bottom for driving the coil tang, these are much better for not stretching the coil out than the plain type with the slot.  You could try to make one by filing a step on to the bottom of an M6 bolt but it may be a little tight.   Hopefully this picture of the box insert will display large enough to read, first go at an embedded image.

Like Richard I would never use thread lock on either the helicoil or any bolt I was screwing in to it that might have to come out one day. 

Edit: Picture is rubbish, have a link instead. http://www.boellhoff.com/en/de/fasteners/special_fasteners/thread_technology/thread_repair_helicoil.php

 

[revilla]

Thanks for all the helpful input.

@CharlesElliott - Point taken, lesson learned! I think that answers my question as to what I did wrong, I "saved money" by buying some cheap tat!

Message received and understood about the use of threadlock in the future too - my logic was that putting a thin smear on the outside of the insert might make it less likely that the insert would come out with the bolt in future but if it's just not needed I can see that it could seep through and work the other way around and lock the bolt to the insert. Won't do it again!

@Richard Price - The drill bit is marked "6.3 HSS" and a vernier caliper across it makes it 6.28mm. The inserts themselves measure 7.66mm across the outsides of the external threads and 5.46mm across the insides of the internal threads. M6 internal diameter specs are 4.917mm to 5.153mm which suggests the inserts are slightly loose for M6 but this fits with the idea that they are compressed a bit when screwed in. They don't appear to be M5. The bit I don't really understand is the tap is marked "HSS STI M6x1.0" but it's clearly NOT an M6 thread. Due to the shape of the tap (with 3 "lobes" rather than being a complete circular thread) its hard to get an accurate measurement but measuring across the widest profile of the tap its 7.3mm - the actual circular dimater can only be larger, so no way is it M6. I guess the markings must be simply identifying it as being the correct tap for the M6 thread repair kit to which it belongs.

@John Vine - Yes on balance I think you're right, I think the basic problem is the inserts are M6 FINE (M6 x 0.75) thread whereas the rest of the kit is for the standard M6 COARSE (M6 x 1.0) thread.

@Mechanical Moz - Thanks for the link, interesting reading and that certainly looks like better quality kit than I was using!

To be honest I'm not going to bother to try to get the insert out and recover it. I had a spare cam cover which I've now fitted and all is well again. Looking carefully at the old cam cover, al least one of the other bolt holes has already been helicoiled and the threads on most of the others look pretty ropey. I think somebody who has worked on the engine in the past had big biceps and no torque wrench. The one I've fitted looks in much better shape.

The main reason I wanted to ask for advice on this was not to rescue the cam cover I'd made a mess of but to learn from the whole experience so that if I need to repair a thread in future I can make a better job of it. I've learned not to use threadlock, to buy a decent kit and to expect the thread pitch of the insert to be an approximate match for that of the thread I'm repairing! I might even buy a few proper M6 x 1.0 inserts and compare them with what I have and use the old cam cover as a practice piece.

Thanks all :)

[Birkin S3 ZA.]

Yes it does sound like the coils are a cheap copy, I only use helicoils but on the odd occasion i have used the cheap ones and I end up having the same kind of problems you have experienced.

If you can get some genuine helicoils I am sure it would work, to get the old one out use a sharp pointed scriber to bend it then hook it out like a spring if the threads get damaged just put the tap through again.

I always try and use the solid Wurth Timeserts for the smaller sizes they never fail and last a lot longer than helicoil inserts

[Nigel Riches]

If you're not sure of the thread form, place a bolt with the desired thread form, against the tap of the re-thread tool, hold up to a strong light source, and take a good look at the relationship of one to the other, both should mesh without any variation of centres.

This doesn't help if the diameter is different, but the pitch the same. 

Generally speaking the shank of the tap should have the diameter (and form, BSW, UNF, etc) and pitch of the thread identified on it. 

Regards, Nigel.