Repairing threaded hole in saildrive bed.

[prof pat pending]

You need a special tool to fit a helicoil. It's not a solid insert, it's more like a spring made of wire that has the form of the thread.

When you wind it into the hole that has been tapped with a special oversize thread, it's a bit like winding in a spring that's a little too big for the hole so locks itself in. If you have it right, the tang breaks of as described above, if you tapped the hole too deep, you can break the tang off, if the hole isn't deep enough it can be problematic.

 

[PaulRainbow]

Will it sheer if the hole is not a blind hole?
Or does the helicoil have a tapered thread that locks once its in?

I won't bother loctiting the actual engine mount bolt in, its not really needed, the only reason I would use it on the helicoil would be to stop it winding through when I put the bolt in, but I won't even put it on that if its not necessary.

The helicoil tang is not there for that reason. It is purely there to fit the insert. It's turned with the special helicoil inserting tool, at it is turned clockwise (right hand thread) the tang tends to coil the insert slightly tighter as it goes in, which makes insertion easier. Once it's in the tang is sheared with a punch. With your hole not being blind you could just wind the insert all the way through the hole and out the bottom. You should also use Loctite to secure the helicoil, not the bolt, this is the method that is recommended by Helicoil.

 

[PaulRainbow]

If you have it right, the tang breaks of as described above.

It does not. You have to break it off with a punch.

 

[prof pat pending]

You should also use Loctite to secure the helicoil, not the bolt, this is the method that is recommended by Loctite.

More trouble than it's worth IMHO, I've installed hundreds of them.

 

[PaulRainbow]

More trouble than it's worth IMHO, I've installed hundreds of them.

As have i and i don't always use Loctite. However, it is nonetheless what Helicoil themselves recommend and in this case (the insert likely to be mostly, if not exclusively, threaded into GRP) i would use it. At the very worst, it cannot cause any harm.

 

[Ceirwan]

With your hole not being blind you could just wind the insert all the way through the hole and out the bottom.

Wouldn't that just leave a loose helicoil rolling around inside my engine bed feet?

Anyway, cheers for the advice everyone, I'll helicoil it following the manufacturers bumf.

 

[PaulRainbow]

Wouldn't that just leave a loose helicoil rolling around inside my engine bed feet?

Yes, i wasn't suggestion you do it, i was pointing out that if you wait for the tang to sheer as suggested, it would just wind all the way through the hole.

 

[prof pat pending]

As have i and i don't always use Loctite. However, it is nonetheless what Helicoil themselves recommend and in this case (the insert likely to be mostly, if not exclusively, threaded into GRP) i would use it. At the very worst, it cannot cause any harm.

If you're going to quote it, quote all of it

In summary, make sure you don't use too much loctite (common mistake hence my comment above), wipe off any excess from the internal threads so you don't bond the bolt into the insert.

 

[PaulRainbow]

If you're going to quote it, quote all of it

In summary, make sure you don't use too much loctite (common mistake hence my comment above), wipe off any excess from the internal threads so you don't bond the bolt into the insert.

I did quote it all.

 

[prof pat pending]

It does not. You have to break it off with a punch.

Try fitting one to a flat bottomed hole and let me know how you get on with your punch

I did quote it all.

Nope, you said Loctite recommend it and it can't do any harm

 

[RichardS]

As have i and i don't always use Loctite. However, it is nonetheless what Helicoil themselves recommend and in this case (the insert likely to be mostly, if not exclusively, threaded into GRP) i would use it. At the very worst, it cannot cause any harm.

Paul ..... I have 12 helicoil inserts into GRP on my boat and when I tried to remove the Allen bolts used into them in the first 2 holes, both of them made a springy graunching sound and the insert dropped out of the bottom on the hole in the GRP. Both of the inserts still have the tang one them and one is stretched open towards the tang whilst the other looks as if it could be re-used but I don't have the correct tool.

I deliberately chose 2 to remove where I knew I could get access to the bottom of the moulding so I was able to just put nuts on the end of those 2 bolts and I have left the other 10 until I have a plan.

The fact that I was able to put nuts on the end of the bolts means that the bolt is proud and is pushing against the tang and presumably forcing the helicoils out of the GRP.

I have never seen helicoils in GRP before and had not realised that Loctite works on GRP so was thinking of using epoxy on the outside of the helicoil but have held off in case the expoxy the bolts in permanently or make it impossible to get the bolt in if I leave the epoxy to go off.

I'm also not sure whether the current holes in the GRP might have been so ruined that the same size helicoil (there're only M5 bolts) will no longer fit. When you fit a helicoil into GRP, do you drill and tap the GRP like metal, or do you just drill the GRP and allow the helicoil to self-tap when you wind it in?

Richard

 

[sailorman]

The helicoil tang is not there for that reason. It is purely there to fit the insert. It's turned with the special helicoil inserting tool, at it is turned clockwise (right hand thread) the tang tends to coil the insert slightly tighter as it goes in, which makes insertion easier. Once it's in the tang is sheared with a punch. With your hole not being blind you could just wind the insert all the way through the hole and out the bottom. You should also use Loctite to secure the helicoil, not the bolt, this is the method that is recommended by Helicoil.

turning the inserting tool anticlockwise, once fully inserted breaks the tang off

 

[PaulRainbow]

Try fitting one to a flat bottomed hole and let me know how you get on with your punch

If it's fitted in a blind hole, with an appropriately sized bolt the tang doesn't need to be snapped off. Thought someone who'd fitted hundreds would know that.

Nope, you said Loctite recommend it and it can't do any harm

I did, i have no idea what you're complaining about.

 

[PaulRainbow]

Paul ..... I have 12 helicoil inserts into GRP on my boat and when I tried to remove the Allen bolts used into them in the first 2 holes, both of them made a springy graunching sound and the insert dropped out of the bottom on the hole in the GRP. Both of the inserts still have the tang one them and one is stretched open towards the tang whilst the other looks as if it could be re-used but I don't have the correct tool.

I deliberately chose 2 to remove where I knew I could get access to the bottom of the moulding so I was able to just put nuts on the end of those 2 bolts and I have left the other 10 until I have a plan.

The fact that I was able to put nuts on the end of the bolts means that the bolt is proud and is pushing against the tang and presumably forcing the helicoils out of the GRP.

I have never seen helicoils in GRP before and had not realised that Loctite works on GRP so was thinking of using epoxy on the outside of the helicoil but have held off in case the expoxy the bolts in permanently or make it impossible to get the bolt in if I leave the epoxy to go off.

I'm also not sure whether the current holes in the GRP might have been so ruined that the same size helicoil (there're only M5 bolts) will no longer fit. When you fit a helicoil into GRP, do you drill and tap the GRP like metal, or do you just drill the GRP and allow the helicoil to self-tap when you wind it in?

Richard

You'd need to tap the holes in the GRP Richard. I think if you epoxied them in, allowed the epoxy to set, then ran a M5 tap through the Helicoils they would work well. Remember to snap the tangs off, the bolts will force the helicoils out, as you describe.

 

[PaulRainbow]

turning the inserting tool anticlockwise, once fully inserted breaks the tang off

Yup, that works on the smaller ones.

 

[lw395]

1)
I would not trust loctite or whatever to work ever so well on GRP that's been drilled and then left under a diesel engine for some years. The grp is likely to have many cracks and broken fibres from being drilled and tapped, and is likely sodden with water and oil and so forth from being in an engine bay.
2)
You really want the metal plate to be taking the load from the thread and spreading the load into the grp.
3)
This is an engine we are talking about. Big heavy vibratey thing with lots of force on it. It needs to be properly bolted down, not screwed to some little spring insert that's glued to a small area of abused fibreglass.
The mteal plates weren't there for a laugh, seems like maybe they should actually have been thicker?

 

[Poignard]

,

1)
I would not trust loctite or whatever to work ever so well on GRP that's been drilled and then left under a diesel engine for some years. The grp is likely to have many cracks and broken fibres from being drilled and tapped, and is likely sodden with water and oil and so forth from being in an engine bay.
2)
You really want the metal plate to be taking the load from the thread and spreading the load into the grp.
3)
This is an engine we are talking about. Big heavy vibratey thing with lots of force on it. It needs to be properly bolted down, not screwed to some little spring insert that's glued to a small area of abused fibreglass.
The mteal plates weren't there for a laugh, seems like maybe they should actually have been thicker?

+1

 

[prof pat pending]

If it's fitted in a blind hole, with an appropriately sized bolt the tang doesn't need to be snapped off. Thought someone who'd fitted hundreds would know that.

I know that, you know that, try telling that to the designer who hasn't specified to correct the correct depth hole and size of bolt as "there just isn't room for that". One of the downsides of working in a weight critical industry.

 

[PaulRainbow]

1)
I would not trust loctite or whatever to work ever so well on GRP that's been drilled and then left under a diesel engine for some years. The grp is likely to have many cracks and broken fibres from being drilled and tapped, and is likely sodden with water and oil and so forth from being in an engine bay.

If he's helicoiling it he'll need to drill the hole out and re-thread it. Clean GRP !

2) You really want the metal plate to be taking the load from the thread and spreading the load into the grp.

3) This is an engine we are talking about. Big heavy vibratey thing with lots of force on it. It needs to be properly bolted down, not screwed to some little spring insert that's glued to a small area of abused fibreglass.
The mteal plates weren't there for a laugh, seems like maybe they should actually have been thicker?

Guess you haven't used many inserts. The big vibrately engine, with all it's force sits on a couple of rubber blocks, glued to metal plates

 

[lw395]

The helicoil only takes a hole out by a small amount. The drill size for an M10 x 1.5 Recoil is only 10.4mm, then the tap will have an OD of what? another 1.5mm? So about one mm penetration into the fibreglass that's been drilled, tapped, tthreadstripped and exposed to various things. Hardly virgin material.
Hopefully the the insert would be working in the steel plate though.
But this means cutting a thread from scratch using the bottoming tap that you get with a thread repair kit? OK in ali, where I've used a few helicoils and recoils, not ideal in steel.
You can get taper taps for Helicoil sizes, but I suspect I am not the only engineer who's not got any?

Personally, I'd look at tapping the steel plate a bigger size, or using something like a rivnut if it can be put right through the plate, which would depend on the space behind it.
Or if a thicker bit of tapped steel can be inserted behind the existing plate, that would be proper.