Fix/replace M12 threads in cast iron

[Danni BurnsParticipant @danniburns84841]

Hi all.

I have a couple of m12 threads that are stripped in cast iron. Its not a blind hole so I can access the rear if needed. Possibly 20-25mm depth.

my initial thought was to drill larger and tap m14, but ideally I’d like to stay with m12 and not sure if bigger will work.

then I’m thinking to make an M15 external, m12 internal Mild Steel bush and loctite it. But will it stay put?

it will be under some pressure because it’s for a vice/clamp on a power hacksaw.

anyone got experience of different solutions or tools?

I’ve seen helicoil used in Ali frame but I’m not confident that is an option (not robust enough), and I don’t have a tool anyway.

cheers

 

[bernard towersParticipant @bernardtowers37738]

Blimey must have been a big spanner!. If you make the bushes top hat shaped and fit with loctite from the rear they should be ok. I have done lots like that on brake calipers and Neve had any trouble.

[Nigel Graham 2Participant @nigelgraham2]

Yes – you could use a bush as you suggest – if possible flanged with that on the thrust side.

M15 is in three pitches in the ISO-M Fine series though, and won’t leave much iron between its roots and the M12 crests.

If possible, M16 X 2mnm (ISO-M Coarse) may be better but still gives only 1mm wall between the threads.

You need also consider how much metal there is between the thread and the nearest surface.

[duncan webster 1Participant @duncanwebster1]

Can you tap it m14, then make a stud m14 one end, m12 the other. Done that on lots of motorbikes in the past

 

[Danni Burns]

On 20 November 2023 at 22:55 Danni Burns Said:

I’m thinking to make an M15 external, m12 internal Mild Steel bush and loctite it

 

They exist. They’re called Timeserts.

A Helicoil will do the job, although if you’re going to go to the effort of drilling and tapping holes, and machining bushes, why not just make them M14 and use M14 hardware.

[Ady1Participant @ady1]

If you’re stripping 12mm holes then you’re using a lot of welly

An insert with a bottom flange will sort you out

[JasonBModerator @jasonb]

What goes into the M12 stud or bolt? You could simply thread to a common M16 and make a stepped stud, M16 to go into the new hole and M12 protruding.

[Nigel Graham 2]

On Nigel Graham 2 Said:

…won’t leave much iron between its roots and the M12 crests.

The observation is good and more so because M12 is a bad one as it is 1.75mm pitch, which no other thread shares.

The Timesert product has its external threads the same pitch as its internal threads and synchronised so wall thickness is maximised (or its function is provided in a minimal-sized package).

I am trying to think through how it might be possible to produce something similar in an amateur workshop. It is easy enough to leave the geartrain engaged to ensure the threads coincide rotationally, and I think you would need to pre-set the cutting tools so the point of the internal and external tools are longitudinally offset by exactly half a thread pitch.

 

[John HaineParticipant @johnhaine32865]

On my mill when I fitted a 3 phase motor I re-used a cast iron pulley which had a large hole.  I made a steel bush a good fit in the hole and bored for the 5/8 motor shaft and loctited it in the hole using the strong stuff.  This on the advice of John Stevenson.  It has held up under repeated starts over several years.  So bore out the hole as large as you can, and make a steel bush with threaded hole, top hat as suggested, and use high strength loctite.  Don’t aim for a great finish on the external diameter, it’ll help it stick.

[noel shelleyParticipant @noelshelley55608]

If you want to stick to M12 then use a heli coil type repair. The kit is not dear . If your near me I can lend you one. Noel.

[Martin ConnellyParticipant @martinconnelly55370]

 

DC31k, you could make the Timesert product with a lathe. Make the insert with the last feature being the outer thread. Cut a male thread on a mandrel that matches the internal thread. Thread the new insert onto this mandrel and cut the outside thread. Only use one point on the threading dial and the same point for the two male threads. Apart from the outside diameter change on the cross slide don’t change anything else before cutting the second thread. If you have an inch based lathe and a metric pitch thread then do not release the half nuts at any point in the process to avoid mis-aligning the two threads. At first look I thought you may need the half pitch adjustment but then realised it was not necessary.

If both threads have the same form then thread milling on a CNC mill would be possible but would require enough material on the flange to grip securely for the process.

Martin C

[Danni BurnsParticipant @danniburns84841]

Guys – I have a Clarke 500M – it’s almost with willpower alone that I get anything done with good accuracy – so screwcutting a Timesert is a step too far Im afraid.

BUT I will happily test out your wares – if anyone wants to take on the challenge though.

 

Have a great day all

 

[Martin Connelly]

On Martin Connelly Said:

 

Cut a male thread on a mandrel that matches the internal thread

Thankyou.

That is a good idea. As you say, you are cutting two identical pitch and phase threads, just at different diameters.

[DC31k]

On DC31k Said:

That is a good idea. As you say, you are cutting two identical pitch and phase threads, just at different diameters.

Except the last couple of internal threads on a Timesert insert taper inwards. The last step is using a form tool that pushes it outwards and deforms the external thread so it has a better chance of staying in the hole.

There is some serious wheel reinvention going here.

[Robin Dufton]

On Robin Dufton Said:

There is some serious wheel reinvention going here.

 

One could, of course, use Loctite to retain a home-made insert (a genuine M12 Timesert would cost me £16.68 delivered from here: https://www.mo-kit.co.uk/threaded-fasteners-hardware/p/wurth-time-sert-inserts-various-sizes-m6-m14).

It is not about reinventing the wheel.

It was a question about how one might produce an item where an internal and external thread are clocked in relation to each other.

[Pete RimmerParticipant @peterimmer30576]

I agree with Noel, stick a helicoil in it and be done. I have the kit and I’m in North Kent if thaat’s your locality.

[HOWARDTParticipant @howardt]

When I was working we would make a headed threaded bush and loctite in place. Making the bush in steel makes for a stronger thread although if you need a matching surface finish make from cast iron.

[DC31k]

On DC31k Said:

 

It is not about reinventing the wheel.

Then the best option is a Helicoil, as I said in the 5th post in this thread, and a number of others have now said.

[Danni BurnsParticipant @danniburns84841]

Helicoil is not suitable for a thread under constant use. If being nipped up and left, then OK, but it’s for a vice on a saw.

Pete/Noel – I do appreciate the offer and would have tried it – but I’m in North West.

The Time-Sert looks the quick ‘n’good solution but the cost is prohibitive, given that taps/tool are also required.

I found/ordered a couple of these self-tapping inserts – as they are low-cost and obviously avoid the cost of a large tap that I may never use again.

Just out of interest really, I will try it out in a lump of scrap first – see if the self-tapping aspect is reliable. I’d be interested in what the Helicoil users think of these.

If I’m not 100% I will buy a large tap/die and go with the (belt and braces approach) headed threaded bush and Loctite in place. I see that an M16 tap/die set is the cheapest approx £15.

cheers

 

[noel shelleyParticipant @noelshelley55608]

Are you sure there for use in metal ? I have seen something like this but only for wood ? Noel.

[Danni BurnsParticipant @danniburns84841]

Hi Noel

High quality Inserts made of Case Hardened Steel and to AN 335 standards.

Here is the eBay item number:253838305966

It says:

Application:

– Light alloys
– Cast iron
– Brass
– Plastics

– Laminates
– Hardwoods

[Danni BurnsParticipant @danniburns84841]

Guys – I found this video of a chap doing a great job repairing tread inserts (done out in the field – so probably excusable).

Obviously, this is a far bigger (mission-critical) job and he has all the gear in a workshop, but his process of plugging with a larger bolt and then moving the clamping hole half/half (nice if you have the option I guess) has given me food for thought – for future fixes anyway.

So, my takeaways – go with the ‘headed threaded bush’ but I’ll make a drilling jig to ensure 90-degree holes.

Have a great day

[Author]

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