Stripped Thread

[Slim]

As title, I've stripped a thread

My steel water tank is under the forward cockpit floor and has an inspection hatch some 24" x 18". The builder (some 23 years ago) recessed the hatch into the cockpit floor (6 mm) by grinding a 'halving joint' into the floor and the hatch. Thus the hatch and the floor both are ground down to about 3mm for about 40 mm around their edges. The hatch is bedded on closed cell foam tape and secured with 18  8mm stainless steel countersunk socket bolts. The stripped thread is thus 8mm by a maximum of 3mm. The pitch of an 8 mm thread is 1.25. The obvious answer is to re-tap to the next available size, 10mm. This would have a pitch of 1.5mm. My concern is that any thread would be very weak. Long term the answer appears to be either fill with weld and re tap or back with some kind of captive nut. Unfortunatly the boat is not out of the water until September. Can anyone suggest a short term solution along the lines of a cavity wall fixing. Equally views on re tapping to 10 mm

I've already tried a bodge of wrapping to bolt in gas PTFE tape without success.

 

Frank

 

Many rhanks

[Tacet]

Araldite a nut or (better still) and drilled-and-tapped length of strip metal to the underside.  

[matty40s]

Ask the other half to get in the tank and hold a nut underneath whilst you tighten it.

[Alan de Enfield]

4 minutes ago, matty40s said:

Ask the other half to get in the tank and hold a nut underneath whilst you tighten it.

But give her a torch.

[hider]

Put a bolt in from underneath with Araldite or Loctite  and the nut on the top.

[ditchcrawler]

You could drill and  tap a 5mm hole  beside it to seal the lid down and just put the 8mm one in wit sealant to stop the water creeping through

[philjw]

41 minutes ago, Slim said:

Can anyone suggest a short term solution along the lines of a cavity wall fixing. Equally views on re tapping to 10 mm

There are things called flanged threaded inserts that are like pop rivets that have an internal threaded hole. You screw a mandrel the right size onto the tool, like a big pair of pliers then the insert on to this  Close the handles and this squeezes the insert onto the material. remove the tool and you have a threaded hole.  The downside is that you need the tool and the right size insert plus there will be a part of the insert remaining on the top of the material.
https://www.sealey.co.uk/product/5637203453/threaded-insert-rivet-nut-m8-splined-pack-of-50 

 

[Mad Harold]

31 minutes ago, hider said:

Put a bolt in from underneath with Araldite or Loctite  and the nut on the top.

My well deck is fastened down with screws into the channel which has threaded holes.

Two are stripped and Araldited bolts are working fine.

If you hacksaw a slot into the threaded end of the bolt,you can hold the bolt firm with a screwdriver while tightening or slackening.

Edited June 1, 2019 by Mad Harold

[Bod]

25 minutes ago, philjw said:

There are things called flanged threaded inserts that are like pop rivets that have an internal threaded hole. You screw a mandrel the right size onto the tool, like a big pair of pliers then the insert on to this  Close the handles and this squeezes the insert onto the material. remove the tool and you have a threaded hole.  The downside is that you need the tool and the right size insert plus there will be a part of the insert remaining on the top of the material.
https://www.sealey.co.uk/product/5637203453/threaded-insert-rivet-nut-m8-splined-pack-of-50 

 

+1 from me.

These can be set with a nut & bolt, rather than the proper tool, if your just doing a few.  Get the correct ones for the thickness of metal, countersink ones are available.

 

Bod

[Phoenix_V]

if i have understood this right there are 17 other bolts so surely just putting soe sealant in the damaged one would be fine?

[nicknorman]

1 hour ago, Mad Harold said:

My well deck is fastened down with screws into the channel which has threaded holes.

Two are stripped and Araldited bolts are working fine.

If you hacksaw a slot into the threaded end of the bolt,you can hold the bolt firm with a screwdriver while tightening or slackening.

 

1 hour ago, Bod said:

+1 from me.

These can be set with a nut & bolt, rather than the proper tool, if your just doing a few.  Get the correct ones for the thickness of metal, countersink ones are available.

 

Bod

I call them rivnuts. Cheap to get a kit from eBay. Eg https://www.ebay.co.uk/itm/Threaded-Nut-Riveter-Rivet-Gun-Rivnut-Insert-Tool-86-Rivets-M3-M8-Nutsert-Kit/283243963102?epid=27023360074&hash=item41f2a7e2de:g:iMoAAOSwusNbjnGQ

[dmr]

3mm is really not thick enough to take an m8 thread, let alone an m10, especially as they need it quite tight to really compress the foam seal. As said above, if its only one hole thats failed then just seal it up for now. Longer term maybe get 4 lengths of steel bar (flat), maybe 6mm thick, and drill and tap new holes, then get these bars welded on underneath, just a few short welds will do, and drill through the existing holes. Second best is a whole set of captive nuts but get them welded on really well, and you might need to re-tap them afterwards.

 

...............Dave

[MtB]

Tapping M8 into 3mm steel is engineering bodgery at its finest. 

 

As others have said, the simplest solution is M8 Rivnuts. 

 

[Mike Tee]

33 minutes ago, nicknorman said:

 

I call them rivnuts. Cheap to get a kit from eBay. Eg https://www.ebay.co.uk/itm/Threaded-Nut-Riveter-Rivet-Gun-Rivnut-Insert-Tool-86-Rivets-M3-M8-Nutsert-Kit/283243963102?epid=27023360074&hash=item41f2a7e2de:g:iMoAAOSwusNbjnGQ

Very neat - a bit like instant Helicoils

[dor]

1 hour ago, nicknorman said:

 

I call them rivnuts. Cheap to get a kit from eBay. Eg https://www.ebay.co.uk/itm/Threaded-Nut-Riveter-Rivet-Gun-Rivnut-Insert-Tool-86-Rivets-M3-M8-Nutsert-Kit/283243963102?epid=27023360074&hash=item41f2a7e2de:g:iMoAAOSwusNbjnGQ

Now that looks like something I should have had years ago, but didn't know they existed.

 

Used Helicoils decades ago, but they had limitations; these look really useful for putting a thread into sheet steel. Especially as mentioned above, when the sheet thickness is not really enough to take enough thread.

[dmr]

Can't quite put my finger on it but my engineering instinct is that Rivnuts are not the right thing for narrowboat engineering, they are more of a sheet metal thing, and boats are plate rather than sheet, and if one breaks loose and spins it will be a pig to sort out.

 

................Dave

[nicknorman]

9 minutes ago, dmr said:

Can't quite put my finger on it but my engineering instinct is that Rivnuts are not the right thing for narrowboat engineering, they are more of a sheet metal thing, and boats are plate rather than sheet, and if one breaks loose and spins it will be a pig to sort out.

 

................Dave

Yes they are a bit weak, on the other hand they are quick and effective and the bolt is only to hold a hatch down with the assistance of gravity. Good engineering is always a compromise!

[WotEver]

2 hours ago, Mike the Boilerman said:

As others have said, the simplest solution is M8 Rivnuts. 

When I built/modified an aluminium bodied kit car many moons ago I single handedly kept the rivnut industry going

 

All the bits and pieces under the bonnet - coil, windscreen washer pump, windscreen wash bottle, expansion tank - were held on with rivnuts and a very neat job they did. The instructions often called for nuts & bolts where a rivnut did a much neater and more convenient job. 

[MtB]

1 hour ago, WotEver said:

When I built/modified an aluminium bodied kit car many moons ago I single handedly kept the rivnut industry going

 

All the bits and pieces under the bonnet - coil, windscreen washer pump, windscreen wash bottle, expansion tank - were held on with rivnuts and a very neat job they did. The instructions often called for nuts & bolts where a rivnut did a much neater and more convenient job. 

 

Totally agree.

 

Very satisfying to set, & neat and professional in appearance. 

 

[Slim]

A big thanks to everyone for their suggestions. It's presently gone back with a dollop of sealant. What seemed like a bodge looks like a sound engineerig solution after a few pints of Old Hookey? I'll overfill the tank tomorrow to test for leaks. I'll look into the rivnut ldea but suspect the real answer is to put 6 mm backing plates in. I agree with whoever said 8 mm into 3 mm is an engineering bodge and I've held my breath every time I've put them back in over the years

 

Frank

[Chewbacka]

For m10 and really m8 rivnuts this tool is better

https://www.ebay.co.uk/itm/Rivet-Gun-M3-to-M10-Rivnut-Nutsert-Rivetnut-Mandrels-Blind-Riveter-Hand-Tools-UK/392294574735?hash=item5b56942a8f:g:nuQAAOSw2QxcZng4

 

the little hand held tools are ok, but mine collapsed when I set a few m8 nuts.  The long handles on the above tool give a lot more leverage, even then I find m10 hard work, though I put that down to age............

Edited June 1, 2019 by Chewbacka

[Slim]

19 minutes ago, Slim said:

A big thanks to everyone for their suggestions. It's presently gone back with a dollop of sealant. What seemed like a bodge looks like a sound engineerig solution after a few pints of Old Hookey? I'll overfill the tank tomorrow to test for leaks. I'll look into the rivnut ldea but suspect the real answer is to put 6 mm backing plates in. I agree with whoever said 8 mm into 3 mm is an engineering bodge and I've held my breath every time I've put them back in over the years

 

Frank

Just found 5 x 8mm s/s rivnuts on e bay for a fiver. If they prove to be useless for the job I've not exactly lost a fortune.

 

 

Frank

[Chewbacka]

1 minute ago, Slim said:

Just found 5 x 8mm s/s rivnuts on e bay for a fiver. If they prove to be useless for the job I've not exactly lost a fortune.

 

 

Frank

Having set stainless pop rivets they take a huge amount of force to set, I think you will struggle to set those without a long handle tool, interested to see how you get on.  

[Slim]

15 minutes ago, Chewbacka said:

Having set stainless pop rivets they take a huge amount of force to set, I think you will struggle to set those without a long handle tool, interested to see how you get on.  

Because I could get to both sides of the rivit i was thinking of screwing a bolt in then winding a nut on from the other side. It's all an experiment at this stage and will try a dry run at home in my workshop. It may be a few months but I'll feed back how I get on.

[nicknorman]

6 minutes ago, Slim said:

Because I could get to both sides of the rivit i was thinking of screwing a bolt in then winding a nut on from the other side. It's all an experiment at this stage and will try a dry run at home in my workshop. It may be a few months but I'll feed back how I get on.

Doing that will just strip the thread in the rivnut. Winding the bolt in hard will just result in the rivnut rotating. The only way to do it is with a (non-rotating) straight pull.