Helicoil vs. Threaded insert?

[Tim Good]

Some M8 bolts are falling out of my mast where a plate is attached. I need to put in some sort of insert.

In terms of strength do we think Helicoil or some kind of solid threaded insert Would be better?

Never used either really.

 

[Laysula]

Could you drill and tap to the next size up?

 

[LadyInBed]

A big self tapper - straight in on some duralac.
That's what I used when a sail gate bolt stripped.

 

[Norman_E]

Rivnuts are what you need. There are lots of types and most thread sizes are available, certainly all common metric ones. Your mast is probably 3 to 4 mm thick, so a bit thin to tap and screw in any but small diameter fixings, and rather thick for most rivnuts, but specialist suppliers will be able to supply M8 ones suitable for your mast thickness. Rivet Nuts | Nutserts, Avdels & Rivnuts | Rivetwise

EDIT: M8 ones in aluminium here Flat Head, Open End Rivet Nuts - Aluminium

 

[mrangry]

I have used threaded inserts recently after drilling out an injector bolt. These things are very strong whereas helicoils not so much

 

[RichardS]

Some M8 bolts are falling out of my mast where a plate is attached. I need to put in some sort of insert.

In terms of strength do we think Helicoil or some kind of solid threaded insert Would be better?

Never used either really.

Is the mast thick enough to use a Helicoil? I would suspect not, and something like re-tapping, a Rivnut or Tappex will be needed.

Richard

 

[rotrax]

I have used threaded inserts recently after drilling out an injector bolt. These things are very strong whereas helicoils not so much

Really?

Perhaps that is why many motorcycle manufacturers use them as OE cylinder head threaded holes in high performance models.

Correctly done they are very strong indeed. A threaded insert is fitted into an oversize threaded hole just as a helicoil type is. IMHO - and extensive repair experience - there is little performance difference between the two types.

 

[Ian_Edwards]

I've used rivet nuts for this sort of repair.
The mast isn't really thick enough for a helicoil or a threaded insert.
Aluminium rivet nuts are much easier to set than SS ones. I found 8mm SS rivet nuts almost impossible to set using a high strenght steel nut and bolt.
However, a SS rivet nut will be much stronger and last longer than an Aluminium one.

 

[Poignard]

Really?

Perhaps that is why many motorcycle manufacturers use them as OE cylinder head threaded holes in high performance models.

Correctly done they are very strong indeed. A threaded insert is fitted into an oversize threaded hole just as a helicoil type is. IMHO - and extensive repair experience - there is little performance difference between the two types.

Yes but the difference here is that the mast wall is too thin.

 

[mrangry]

Really?

Perhaps that is why many motorcycle manufacturers use them as OE cylinder head threaded holes in high performance models.

Correctly done they are very strong indeed. A threaded insert is fitted into an oversize threaded hole just as a helicoil type is. IMHO - and extensive repair experience - there is little performance difference between the two types.

Yes really...I think it is because helicoils are formed of round wire and threaded inserts are solid bushings

I have used these BaerFix Self Tapping damaged Thread Repair Kits Inserts Metric + Imperial Sizes | eBay

 

[Tim Good]

Rivnuts are what you need. There are lots of types and most thread sizes are available, certainly all common metric ones. Your mast is probably 3 to 4 mm thick, so a bit thin to tap and screw in any but small diameter fixings, and rather thick for most rivnuts, but specialist suppliers will be able to supply M8 ones suitable for your mast thickness. Rivet Nuts | Nutserts, Avdels & Rivnuts | Rivetwise

EDIT: M8 ones in aluminium here Flat Head, Open End Rivet Nuts - Aluminium

Thanks. So if I use aluminium rivnuts then presumably still use stainless bolts but just with a lot of Duralac? Or perhaps a decent quality aluminium bolt?

 

[rotrax]

Yes really...I think it is because helicoils are formed of round wire and threaded inserts are solid bushings

I have used these BaerFix Self Tapping damaged Thread Repair Kits Inserts Metric + Imperial Sizes | eBay

Helicoils are NOT round wire, they carry the thread form on both the inner and outer side of each insert.

Just like a helicoil they are only as strong as the metal they are inserted into. They still have an external thread which mates with the thread cut into the item to be repaired.

I have fitted - or had my mechanics fit - thousands of thread inserts in my extensive time repairing and tuning car and motorcycle engines.

If there is sufficient good metal/space, either will be an effective repair. But a proper Helicoil is far preferable to a self tapping type in the kind of applications I was commisioned to repair.

The sort of kit I used to repair was in many cases very highly stressed - 85 BHP From a 500cc single cylinder alcohol burning Speedway engine, 12,000 RPM.

Helicoils were OE. Unless the con rod broke, allowing the piston togive an almighty blow to the head, never had one fail. Only saw a couple of those.

I still have about 15 Recoil kits in my workshop including Cycle threads, 26 TPI. Correctly fitted stronger than a thread directly into the head.

Perhaps no stronger than what you favour, but certainly not weaker, as you indicated.

Getting back to the OP, neither would be much good in a mast repair, for obvious reasons.....................

 

[mrangry]

Helicoils are NOT round wire, they carry the thread form on both the inner and outer side of each insert.

Just like a helicoil they are only as strong as the metal they are inserted into. They still have an external thread which mates with the thread cut into the item to be repaired.

I have fitted - or had my mechanics fit - thousands of thread inserts in my extensive time repairing and tuning car and motorcycle engines.

If there is sufficient good metal/space, either will be an effective repair. But a proper Helicoil is far preferable to a self tapping type in the kind of applications I was commisioned to repair.

The sort of kit I used to repair was in many cases very highly stressed - 85 BHP From a 500cc single cylinder alcohol burning Speedway engine, 12,000 RPM.

Helicoils were OE. Unless the con rod broke, allowing the piston togive an almighty blow to the head, never had one fail. Only saw a couple of those.

I still have about 15 Recoil kits in my workshop including Cycle threads, 26 TPI. Correctly fitted stronger than a thread directly into the head.

Perhaps no stronger than what you favour, but certainly not weaker, as you indicated.

Getting back to the OP, neither would be much good in a mast repair, for obvious reasons.....................

Ah okay...we disagree

 

[Ian_Edwards]

Thanks. So if I use aluminium rivnuts then presumably still use stainless bolts but just with a lot of Duralac? Or perhaps a decent quality aluminium bolt?

Yes, you'll have the same problem with corrosion if you use an aluminium rivet and a SS bolt, so you'll need lots of Duralac or equivalent.
A SS rivet nut would be more durable, but I wouldn't try that unless you have access to the correct tool to set it. A half set rivet nut in hole is a nightmare, you can guess how I found that out.

 

[Norman_E]

Yes, you'll have the same problem with corrosion if you use an aluminium rivet and a SS bolt, so you'll need lots of Duralac or equivalent.
A SS rivet nut would be more durable, but I wouldn't try that unless you have access to the correct tool to set it. A half set rivet nut in hole is a nightmare, you can guess how I found that out.

Agreed. The Aluminium rivnut will give you a greater depth of thread than you can get by simply tapping a hole in the mast wall. For most applications it will be strong enough, but for some, e.g. mast steps Stainless rivnuts would be preferable, and need a decent tool to set them. You can set rivnuts using a bolt plus washers and a nut, with two spanners, but its not an easy method, particularly if you are some way up the mast.
Ideally you want a good rivnut setter, something like this 13" 330MM Blind Rivet Nut Gun Rivet Nut Tool Hand INSER NUT Tool Manual Mandrels M3 M4 M5 M6 M8 M10 M12: Amazon.co.uk: DIY & Tools

 

[Tim Good]

After this entertaining debate I’m still feeling at a loss which will be best. Perhaps these photos will give a slightly better understanding of the application.
In the photos it shows the large alloy place which has been taken off the mast. You can see the holes in the mast and the holes in the grey plate. Through which the M8 bolts go.
The plate does - few things:

- holds the boom on the mast
- holds the foot of the gas rod kicker
- hides the furlong gear and holds the outer furling drum

needless to say it doesn’t take masses of force like rigging does but it will still be substantial.

In my inexperienced mind I’d tend to feel a nutsert would be better as it would bind on either side of the mast and not rely purely on threads to hold it in place.

I agree I could set an aluminium Nutsert myself but an M8 stainless could well be an issue.

I don’t feel the m8 bolts need to be particularly tight but simple secure to ensure the plate is in place. And loading on the bolts will be mostly shear and not outwardly pulling.

Many thanks for all your help.

 

[Boathook]

I would use monel rivets for that. Align the fixing up with a few bolts and drill new holes for the rivets. If the item needs to be removed for maintenance reasons then ignore my suggestion.
I've never used rivet nuts so I would then drill and tap to 10mm as being the easiest and quickest option.

 

[rotrax]

Ah okay...we disagree

OK, we disagree.

But, I hope, we wont fall out over it. Life is too short...........................

 

[penberth3]

I would use monel rivets for that. Align the fixing up with a few bolts and drill new holes for the rivets. If the item needs to be removed for maintenance reasons then ignore my suggestion.
I've never used rivet nuts so I would then drill and tap to 10mm as being the easiest and quickest option.

This comes back to thickness. How many threads would you get in the mast at 10mm dia, say 1.5mm pitch? Not enough.

 

[Beneteau381]

Really?

Perhaps that is why many motorcycle manufacturers use them as OE cylinder head threaded holes in high performance models.

Correctly done they are very strong indeed. A threaded insert is fitted into an oversize threaded hole just as a helicoil type is. IMHO - and extensive repair experience - there is little performance difference between the two types.

Thats what the RGB Weslake did to stop the barrel studs pulling out