Stemar
Well-Known Member
Another vote for Rivnuts. I'd use a Helicoil if there's the thickness of metal, like for a stripped bolt in an alloy head, but they're different things for different purposes. It ain't apples oranges, it's apples and bananas
Helicoil vs. Threaded insert?
- Thread starter Tim Good
- Start date
Tim Good
Well-Known Member
does anyone know how much space a helicoil takes in the hole? I mean I heard a helicoil is so thin you almost don’t need to increase the hole size if the original threads have already worn away.
so if I went helicoil I’d have at least the fallback of a rivnut if it doesn’t work. Assuming a rivnut would need a significantly larger hole to work.
so if I went helicoil I’d have at least the fallback of a rivnut if it doesn’t work. Assuming a rivnut would need a significantly larger hole to work.
Alfie168
Well-Known Member
Dont use apples or bananas. I have found them both useless for stripped threads.
Tim Good
Well-Known Member
Care to elaborate what you have found useful?
Poignard
Well-Known Member
[TE="northcave, post: 7400216, member: 40225"]
View attachment 101765
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Is that a horizontal split at the bottom of the line of holes?
View attachment 101765
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Is that a horizontal split at the bottom of the line of holes?
Norman_E
Well-Known Member
Having seen the photo lets clear up a few points. Firstly the existing holes took M8 screws, assuming they were standard 8 X 1.25mm thread the tapping size was 6.8mm, and as some at least are stripped leaving an 8mm hole that completely rules out using rivets unless all the existing holes are ignored and new ones drilled through both parts. I am unsure whether its wise to put so many more holes in the mast but monel rivets would do the job and given the numbers involved 4.8mm would probably be strong enough.
My solution given the number of existing holes is still rivnuts, but countersunk ones will be needed. M6 rivnuts are generally about 8.9 mm diameter so existing holes could be enlarged to suit, and countersunk. M8 rivnuts require a hole of around 11mm, and I would just use a couple of those each side to locate the fitting accurately, with the rest in M6 which will be too small for the hole in the outer part, a problem that can be solved easily by anyone with a lathe turning a little step under the heads of each bolt, or by a method I have used in the past, of turning up little "top hat" washers to fit both hole and bolt.
Finally drilling out and tapping 10mm could cause the same issue of stripped threads at a later date. If you do go that route do not use standard M10 X 1.5mm bolts, as you will have too few theads in the thin mast section. M10 X 1.25mm and M10 X 1mm (better) are obtainable, but hard to find in stainless steel, and you will need a tap to suit.
My solution given the number of existing holes is still rivnuts, but countersunk ones will be needed. M6 rivnuts are generally about 8.9 mm diameter so existing holes could be enlarged to suit, and countersunk. M8 rivnuts require a hole of around 11mm, and I would just use a couple of those each side to locate the fitting accurately, with the rest in M6 which will be too small for the hole in the outer part, a problem that can be solved easily by anyone with a lathe turning a little step under the heads of each bolt, or by a method I have used in the past, of turning up little "top hat" washers to fit both hole and bolt.
Finally drilling out and tapping 10mm could cause the same issue of stripped threads at a later date. If you do go that route do not use standard M10 X 1.5mm bolts, as you will have too few theads in the thin mast section. M10 X 1.25mm and M10 X 1mm (better) are obtainable, but hard to find in stainless steel, and you will need a tap to suit.
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penberth3
Well-Known Member
I wouldn't say "significantly larger", but the concensus is material thickness is the decider. Thin - use Rivnuts, thick - tap or use Helicoils.
Helicoil instructions/data sheet must tell you the minimum thickness required? Will also give you the outside diameters for each size.
Moodysailor
Well-Known Member
Going back to my engineering days, a threaded fastner needs 1.5 X the diameter for 100% hold strength (all other things being equal). So for an M8 bolt, maximum mechanical strength is achieved with a minimum 12 mm thread insertion.
Another vote for rivnuts for me, you can go with alu if you don't get or have a rivnut tool, but SS would be the best. I recently fitted Lewmar DC2 clutches to our mast using SS rivnuts, and reminded myself why I like them for these type of applications.
Another vote for rivnuts for me, you can go with alu if you don't get or have a rivnut tool, but SS would be the best. I recently fitted Lewmar DC2 clutches to our mast using SS rivnuts, and reminded myself why I like them for these type of applications.
Norman_E
Well-Known Member
Does the fitting need to be removeable, for example to service the in mast furling, or to change the sail? If so it explains why screws were used and not rivets, and completely rules out rivet use.
Tim Good
Well-Known Member
Is that a horizontal split at the bottom of the line of holes?
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Well spotted but no thank god!!!
pawl
Well-Known Member
hi, just a thought, it's going to be difficult to do anything with the existing holes and I'm not sure exactly how your cover works. However, if it were my problem I might think about using some flat aluminium bar about 5 or 6 mm thick, equal to the length of the cover, which could be pre-drilled and tapped to suit the hole centres and then attached to the mast with rivets or even countersunk self tappers. The cover would stand off a little further from the mast than before but would that matter?
Tim Good
Well-Known Member
yeah that won’t work. The large plate sits flush against the mast. Mast is 5mm thick.
Here is some photos of it now:
Beneteau381
Well-Known Member
Both my Beneteaus were/are riveted on. That thread looks bigger than 5mm by the way? I woukd drill some more holes in that wide area and rivet it.
Tim Good
Well-Known Member
The threads are M8 and the last is 5mm thick.
Norman_E
Well-Known Member
Standard 8mm rivnuts have a grip range up to 4.5mm, though some suppliers quote 5mm. If the mast really is 5mm thick then you need to countersink them a little below the surface to get a good grip. It is worth getting the serrated ones as these seem to grip better. In any event make sure you do not drill the holes in the mast oversize. If it says hole size 11mm, buy a 10.9mm drill and use that, and only open the hole carefully to 11mm if the rivnuts will not push in.
EDIT: This sort of rivnut: Aluminium Countersunk Head Grooved-Serrated Rivnuts-Rivet Nut-Nutserts | eBay
Any holes that still have a good thred, just use it. Only put rivnuts in where threads were stripped.
EDIT: This sort of rivnut: Aluminium Countersunk Head Grooved-Serrated Rivnuts-Rivet Nut-Nutserts | eBay
Any holes that still have a good thred, just use it. Only put rivnuts in where threads were stripped.
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Tim Good
Well-Known Member
Thanks.... I’ll see if I can find some that officials grip to 5mm. Fair point about only doing the ones that need doing. In fact the top 3 bolts I’ve backed inside the mast with a washer and nut since I can get my hand in there and figure there is nothing more bomb proof than a bolt with the nit on the end. May as well leave them like that.
LadyInBed
Well-Known Member
I still reckon self tappers would be easier (#3) 
Poignard
Well-Known Member
M8 self-tappersI still reckon self tappers would be easier (#3)
Hallberg-Rassy
Well-Known Member
Did anyone ask why are they falling out, is the thread in the alloy actually damaged or worn? It looks pretty good in the photo. May be it just needs Loctite?
If they're not bad, LOCTITE PC 3967 Stripped Thread Repair Kit (aka LOCTITE Form-A-Thread) might do without any drilling.
M8 x 1.25 helicoil would require a 8.3mm drill and M10 STI tap, I think. Specifically it's actually 9.7mm but you're not going to hold your drill 0.3 steady on a boat.
If you go that route, there are other products like LOCTITE 2620 (but check for alloy/marine use) to hold it in permanently. Otherwise, what's to stop it pushing right through?
Tim Good
Well-Known Member
Corrosion between the stainless and aluminium. The threads are basically shot. Pretty standard unless you protect with duralac or similar.
Prior owner must have removed and then replaced without using a barrier liquid.
