Helicoil advice please

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CJH
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Helicoil advice please

Post by CJH »

The bottom alternator bracket on my 1.9DG has worked loose, again. It's held to the block with two M8 studs. When I first realised the bracket was loose I found out that it was because one of the studs was missing altogether, as was the nut on the other stud. I replaced the missing stud (but it was clear that the threaded hole was already worn/enlarged) and I put a new nut on the other stud. That repair lasted about 250 miles - it got me to Cornwall - but once again the stud in the loose hole had gone AWOL, as had the nut on the other stud. So I fixed it again on the campsite - I put a bolt in the missing stud hole and a new nut on the other one. That repair lasted nearly all the way home!

So now I want to do this properly. I obviously need to do something about the loose stud, so I believe I need to helicoil the hole. So the specific questions I have relate to how to specify the helicoil size.
- Is an M8 helicoil referring to the internal or external thread? To reline the hole to accept an M8 stud, do I need an M8 helicoil or something bigger?
- And if an M8 helicoil is intended for an M8 bolt/stud, what about the external size, i.e. the size of the hole I need to tap into the block - if the helicoil is described as M8 x 1.25, does that mean I need to add 2 x 1.25mm to the M8, making M10.5?
- Finally, does the length of the helicoil matter? The stud hole is at least 20mm deep, but typical M8 Helicoil kits are only half as long as that.

As for stopping this from happening again, am I best off using loctite? Or spring washers? Or Nyloc nuts? Should I perhaps use bolts instead of studs? This bracket is obviously subject to a lot of vibration - maybe for the missing/loose stud I should just enlarge it to M10 and use an M10 bolt?
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itchyfeet
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Re: Helicoil advice please

Post by itchyfeet »

Yes for an m8 you need an m8 helicoil
tap will be with the kit you buy, often drill is not but they are standard

just gogle helicoil drill sizes,
http://www.thorintl.com/Drill-Sizes-Hel ... tric.shtml" onclick="window.open(this.href);return false;
m8x1.25 is 8.3mm

yes length of coil should be as much as you can for max strength, so measure the hole depth, the lenghts are on the chart on the link

dont go m10 yet save it for later
Last edited by itchyfeet on 27 Jul 2014, 16:46, edited 1 time in total.
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itchyfeet
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Re: Helicoil advice please

Post by itchyfeet »

Actually just looked at my spare in bits engine, it has studs so on second thoughts fit studs with threadlock is probably better
measured it an you must not drill more than 15mm deep, you wont get the tap to thread righ to the bottom so you may not even get a 1.5d 12mm coil in, so id buy 1d 8mm coils
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Re: Helicoil advice please

Post by CJH »

Thanks Itchy

I'll go with 8mm studs with threadlock then. I also have a spare-in-bits engine (that's where I got the stud from for my first repair), and the hole is definitely over 20mm deep, but I take your point about not going too deep, since the water jacket is just beyond the end of the hole.

I've made another quick repair this afternoon, pending a proper helicoiled job later. I've used a couple of bolts with spring washers for now, since I don't have any threadlock yet. Access is quite good once the rigid water pipe is removed, but I'm not sure I'll get a drill in there to open the hole out before tapping it for the helicoil. Since the existing hole is a worn/loose M8 thread, and you're suggesting an 8.3mm drill bit is needed, in your opinion do think I'd get away without drilling it? Could I just go straight for the tap?
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itchyfeet
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Re: Helicoil advice please

Post by itchyfeet »

Ok if the hole is 20mm then go for a 1.5d helicoil

as for drilling the hole even a small difference can mean the tap rips rather than cuts, better to get the drill in there if you can
assuming you have access to a tap wrench then file flats on the drill bit ( they are not hardened on the shank) fit it into the wrench handle and hone it out by hand, as you say its not much so should be possible
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Re: Helicoil advice please

Post by CJH »

Thanks Itchy - good advice regarding the drill bit.

Any reason not to go for an even longer helicoil? M8 1.5D is only 12mm - what about a 2D (16mm) or even a 2.5D (20mm)? I guess 20mm might be cutting things a bit fine, but 16mm would seem to have enough slack.
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Re: Helicoil advice please

Post by itchyfeet »

Go for the longest you can but bear in mind tap wont cut a full thread to the bottom of the hole
Personally id go for a 1.5d = 12mm one now you tell me hole is 20mm deep, thats close to the water jacket dont want to be removing any more metal than necessary and 1.5d is generally accepted as enough thread for any size :ok
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Re: Helicoil advice please

Post by CJH »

I bought a kit that came with 10.8mm coils, and I've just finished the job (for the fourth time in recent weeks! - once before I went on holiday, a temporary fix while on the campsite, another temporary fix when I got back, and this, hopefully, final fix).

Although access was good, there was no obvious way to get a drill bit in there - there wasn't even room to twirl a tap handle without removing the oil filler tube. But in fact the worn M8 hole was so close to the drill size that I figured I'd get away without drilling it. I used an improvised collection of sockets and adapters to get a ratchet handle on the tap, and cut the thread with no problem.

I bought a pair of stainless studs with allen slots and aerotight lock nuts. I've used stud lock to hold the new studs in place, and the aerotight nuts pulled up nice and tight.

Image

I hope I don't have to do it a fifth time - I'm fed up with having to remove that metal coolant pipe each time, meaning I have to bleed the coolant.
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Re: Helicoil advice please

Post by itchyfeet »

Fingers crossed for you
20Nm I hope with a torque wrench
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Re: Helicoil advice please

Post by CJH »

itchyfeet wrote: 20Nm I hope with a torque wrench

No idea I'm afraid! I didn't think to get my torque wrench out as I wouldn't know where to find the torque setting - is it in Bentley? I did them 'quite' tight with a small (1/4") ratchet handle. The nice thing about those aerotight nuts is that they grip the stud even when they're not tight, so I'm hopeful that they won't shake loose. I was more worried about the studs shaking loose in fact, or perhaps pulling the studs out with the nuts in future, hence the stud lock.
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Re: Helicoil advice please

Post by itchyfeet »

CJH wrote:
itchyfeet wrote: 20Nm I hope with a torque wrench

No idea I'm afraid! I didn't think to get my torque wrench out as I wouldn't know where to find the torque setting - is it in Bentley? I did them 'quite' tight with a small (1/4") ratchet handle. The nice thing about those aerotight nuts is that they grip the stud even when they're not tight, so I'm hopeful that they won't shake loose. I was more worried about the studs shaking loose in fact, or perhaps pulling the studs out with the nuts in future, hence the stud lock.


Torque wrench settings are pretty standard, 20Nm for m8 is normal unless you are high tensile
http://www.wtools.com.tw/STANDARD-BOLT- ... RQUE.shtml" onclick="window.open(this.href);return false;

having said that Bentley says 25Nm :oops:

stainless studs are good for that even though probably out of spec, I have torqued them to destruction and its way past up at 40 to 50Nm

there is a risk though with stainless is that they could snap in time with vibration, stainless is brittle compare with an 8.8 stud or bolt but my guess is it will be ok
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Re: Helicoil advice please

Post by CJH »

Thanks Itchy. On a related point I felt happier winding the new stud into the helicoil than I did winding the other one into the undamaged aluminium thread, as it felt like the static helicoil would take the strain better and didn't have so much potential to damage the aluminium around it. Similarly, using studs rather than bolts seems like the right decision after all, since the thread in the aluminium won't wear each time the bracket is removed.
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Re: Helicoil advice please

Post by itchyfeet »

CJH wrote:Thanks Itchy. using studs rather than bolts seems like the right decision after all, since the thread in the aluminium won't wear each time the bracket is removed.

i agree but the nuts you used imo are not great for studs as the stud will probably turn out even with thread lock, a stud with a locking nut is really not much different to a bolt
The correct torque is enough to stop them rattling out, you had problems becuse of a a damaged thread or incorrect torque

im not even sure you can torque locking nuts accurately as they add resistance ? ( goes off to think about that one)

cheers
Paul
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Re: Helicoil advice please

Post by CJH »

Paul

I think you're right in theory, about stud+locknut=bolt, and about the additional torque needed to overcome the grip of the aerotight nuts. But in practice it's not a lot of additional torque. It felt quite similar to a Nyloc nut, the difference being that it's achieved without a nylon insert so it's suitable for high temperature applications. Those were exhaust studs that I used - overkill in my case but they handily came with the studs. I'd hope that the stud lock will resist this small extra torque.

Hopefully this is all hypothetical now anyway, as I'm happy that both nuts were done up tight and shouldn't shake loose. Time will tell - if they're still there in 300 miles' time they'll be doing better than my last few attempts. If not I'm a dab hand and getting to them now.
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Re: Helicoil advice please

Post by ghost123uk »

How long does stud lock take to cure ?
Got a new van, but it's a 165bhp T4 [shock horror] Accurate LPG Station map here

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