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MG Midget and Sprite Technical - Helicoil for SU carb.

Has anyone tried a helicoil for stripped filter bolts on an HS2 , is that a relatively straightforward task I have never used one ? I have found a cap head bolt floating around in the K&N filter when I took it off I am hoping there wasn’t a spring washer underneath as I can’t find one ! . I seem to remember this was discussed not so long ago by Colin Mee but can’t find the thread in the archive ? I have tried a longer bolt with a nyloc 1/2 nut on the back but cannot get a spanner on it at all .
Mike Fairclough

Hi Mike

I have successfully heli-coiled a carb. You do need to shorten the insert for the carb flange, as it is too long.

Are you around on Thursday, by any chance? I have to go to Birmingham. I have a few spare inserts.
Dave O'Neill 2

Mike, it was indeed me who wrote on this subject recently. Before you do anything, get hold of an endoscope and check that there is no spring washer, or bits of one, inside any of the cylinders.

My problem was due to 'butchered' bolt holes on the outer flanges of my carbs. I only noticed the un-round holes when I came to mount them on the manifold. I can't remember whether I used the Sprite's original mounting bolts or used slightly longer ones but I managed to fit a plain nut behind the flange. This arrangement worked fine for over a year until I had the engine out again last winter when I decided to try what I thought would be a better remedy. I managed to get a spanner on both outer nuts but, as the threads on the inner flanges were both fine the bolts fit and can be tightened sufficiently, I left them alone.

My disaster occurred after I filled the holes with plastic metal and drilled and tapped them. The holes were too deformed to Helicoil. One of my 'fillings' failed, the thread stripped and the bolt fell out to rattle around in the air filter. Unfortunately, the engine sucked the spring washer into cylinder 4, smashed it to pieces and spat them out with cylinder 3 swallowing some of them.

I'm sorry I can't be more helpful.


Colin
C Mee

Thanks Dave not around on Thursday but you’re welcome to drop by anytime.not sure what you mean by shortening the flange. You seem to be able to buy a 5/16 unc helicoil kit for about £15. Thanks Colin I will have a look with an endoscope but I think I would have heard it by now but you never know. I have feeling I knew it wasn’t a great fit and left the spring washer off I have noticed another one is going that way too and won’t quite tighten up enough .
Mike Fairclough

Mike, I believe Dave is referring to the fitted length of the Helicoil. For reference they are specified in multiples of the nominal diameter, so for example a 6mm x 1.5D coil will have an installed length of 9mm.
If your chosen size is going to be longer than the depth of the hole it is easy enough to nip off a turn or two before installing the coil.
Bear in mind that the fitted length is greater than the unfitted "relaxed" length of the coil because when it is installed it is squeezed into the hole which in effect reduces the diameter. This causes the coil to "grow" a bit in length.
I have seen 1xD,1.5xD, 2xD and 2.5xD coils, where D = nominal fitted diameter. I doubt there is any practical need for coils any longer than 2.5xD.

HTH.
(Edited for typos).
Greybeard

Bit of an overkill but there's a 'full' explanation of the process here---

https://www.sw-em.com/SU_Carb_Adding_Threaded_Inserts_for_Air_Cleaner.htm

Also--on a couple of 2" su's i had here a while back, they had a lightly splined steel insert that pushed in from the manifold side of the carb with a step on them to stop them pulling through--neat as ,but don't know if original- the carbs were off an old Jag.They looked a bit like these only shorter-

https://bullantperformance.com.au/store/5-16-UNC-Threaded-Insert-20-Pack-p116249047

IF you've got a Harley Davidson outlet local, they have an double threaded steel adapter 1/2"od x 5/16"id that screws into a 1/2" threaded hole and takes a 5/16" bolt for the breather on Evo Harleys----You'd need to tap the hole out to 1/2" thread and loctite the adapters in

willy
William Revit

Forgot i still had the Jag. carbs here---here's some pics front/rear ---- 420G Jag.-- don't know if the inserts are available separately.





William Revit

That's a great sollution Willy. Better than helicoil I reckon.

When I stripped a couple of my threads, I just drilled through for a bolt with a nut. Works perfectly. I've reverted to my single hif44 and don't use my twins anymore. But if I strip the threads on the hif, I'll look for the inserts Willy put up.


anamnesis

Thanks all . I will try a nut again first if I can get a spanner on it. I will look into Helicoil too and your insert options to Willy. They look like a Rivnut .
Mike Fairclough

Mike,

I think what Willy posted a link to is a flanged rivnut, he noted they look like that but shorter.
David Billington

Don't need to try and get a spanner on the nut, as the flat wedges against the carb body. And if it does turn, a screwdriver blade will lock it.
anamnesis

I agree with Anam, a flat blade screwdriver should hold it - doesn't have to be mega-tight.
Bill Bretherton

Willy, those threaded inserts are indeed better than Helicoils. I have made a few on the lathe. I have a Helicoil kit but rarely find they work. If you happen to have rivnuts the right size they will save a lot of work.
Les Rose

All sorted thank you all for the input . Went for a plain nut rather than a nyloc and mixture of small open end spanner and screwdriver blade got it tight . Next time I am due to get the carbs off I will look into. Rivnut/ thread insert solution .
Mike Fairclough

Is there not enough meat for you to tap the hole with a larger coarse (whit) thread,

Jan T
J Targosz

It may seem obvious, but be sure to use new bolts once the insert or Helicoil is in place.

I recently had my carbs rebuilt by Joe Curto and both throttle bodies had to be replaced due to thread damage and helicoiling I had done over the years. Having the carbs back in spec has made an enormous difference.
Glenn Mallory

This thread was discussed between 09/07/2023 and 22/07/2023

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.