Helicoiling

[gav]

Morning all.

I want to reduce a thread from 3/8 UNF to 5/16 UNF. They are both 24 tpi.

Does anyone know whether a 5/16 UNF helocoil will fit staight into a 3/8 UNF thread please?

Thanks
Gavin

[alaric]

Hi. When I helicoiled my gearbox bell housing the kit included a special tap which has a different cross section to a standard thread. So, if you're fitting helicoils I believe you should tap the hole with the appropriate thread before fitting the helicoil. I ended up with the same size bolts as original, but had to drill out each of the holes and re-tap before fitting the helicoils. So, maybe checking the 5/16" helicoil drill requirement will answer your question?

Sean.

[gerrym]

Gav, you don't say what the component is you are trying to helicoil: is it irreplaceable or expensive?

Presuming you have the full 5/16" helicoil kit, i.e. correct size drill and special tap, run these through your existing 3/8" threaded hole. It should then be set for accepting the 5/16" helicoil.

Regards

[CBUEB1771]

Does anyone know whether a 5/16 UNF helocoil will fit staight into a 3/8 UNF thread please?

According to my chart the tap drill for a 5/16-24 Helicoil is 21/64, or 0.328" while the tap drill for a standard 3/8-24 is Q, or 0.332". The Helicoil, will be a tad loose in the standard 3/8-24 but the Helicoils are compressed quite a bit when they are installed. It just might be fine. Is this a heavily loaded attachment or one that has safety implications? Unfortunately I only have coarse thread Helicoils in the shop, otherwise I would just run one into a 3/8-24 nut and see what it feels like.

[GrUmPyBoDgEr]

Does anyone know whether a 5/16 UNF helocoil will fit staight into a 3/8 UNF thread please?

According to my chart the tap drill for a 5/16-24 Helicoil is 21/64, or 0.328" while the tap drill for a standard 3/8-24 is Q, or 0.332". The Helicoil, will be a tad loose in the standard 3/8-24 but the Helicoils are compressed quite a bit when they are installed. It just might be fine. Is this a heavily loaded attachment or one that has safety implications? Unfortunately I only have coarse thread Helicoils in the shop, otherwise I would just run one into a 3/8-24 nut and see what it feels like.

That was "Top" answer Russ; I was searching my tiny Brain. It had to come from someone like you!
This Forum is formidable.

John

[gav]

Thanks for all your help so far.

I need to reduce the thread size for the bolts holding my harnesses which is currently oversized by 1/16th.

I have seen an ad for thread adapters that can be screwed in and loctited in place but they only seem to be made in metric not UNF - unless of course someone knows different...

Thanks
Gavin

[elanman999]

Gavin,
Have I got this right? You say "bolts holding my harnesses"? Do you mean selt belts? You want to reduce those to 5/16? I hope I've got the wrong end of the stick but if you do on your head be it.
1, Normal seat belt bolts are 7/16 UNF.
2, You do not use loctite on a helicoil.
3, UNF are available see http://www.engineering-supplies.com/sub ... e-inserts/
Cheers
John
Unless this whole thread was a windup?

[gav]

John

No wind up...

This is a very non standard racing elan and I am fitting a Willans 6 point single seater harness.
The cage is (very securely) bolted to the chassis (probably over engineered) and within the cage are welded inserts for the harness points.
Thing is - I stupidly didnt check the bolt size on the harness fixings and assumed that they take a 3/8 x 24 high tensile bolt. Thats the thread we used for the inserts - you've guessed the rest - the harnesses take 5/16 bolts and I dont want to drill out the mounting holes.
I dont really want to use cast eyes and harnesses that clip to the eyes but will if there is no other solution - hence the thread.

Hope this makes a little more sense

All the best
Gavin

[gerrym]

Like John, I'm missing something here. 5/16" sounds too small for a harness anchor. Are the bolts in shear, how many share the load?.

Regards

Gerry

[gav]

Gerry

5/16 is the correct fixing size for Willans single seater harnesses. You can check on their website (silverstone 6 point).
One bolt per belt and yes the bolts are in shear.

Whereas I suppose I could drill out the harness mounting plate to 3/8, it won't leave much meat and I don't think thats a place I should visit.

Thanks
Gvain

[CBUEB1771]

That was "Top" answer Russ;

John, I thought more about this later and realized that my answer fell rather short of "top". I just measured the cutting diameter of my 3/8-16 Helicoil tap and find it is 0.387" or 0.012" bigger than 3/8, so it creates a non-standard thread. That combined with the smaller tap drill tells me that the Helicoil system creates a nearly full thread form instead of the truncated root area you would go for in normal production tapping. This gives the insert the strongest possible fit in the tapped hole. What this means is that installing a 5/16-24 Helicoil in a 3/8-24 tapped hole will "compress" the insert more tightly than normal and a standard 5/16-24 fastener might not want to start in. This is all based on my assumption that Helicoil applies the same design principles to coarse and fine thread repair kits, again I only have coarse thread kits to evaluate. More to the point, most racing harness systems I have seen are through-bolted at roll cage mounting locations and done up with nuts. This give much better load capacity in tension and shear. Gav, if you want to go this route you may have to get a 5/16-24 Helicoil tap but I would look for a way to use longer 5/16 bolts and nuts if you have access to both sides of the weld fitting.

[gerrym]

Gavin yes Willans website clearly shows 5/16" size. I guess amongst the 6 attachments there is some load sharing. As you said, the harness points have very little metal around the holes, so I guess you are wise not to modify the harness by opening out the holes. Best bet would be to test one of the captive fasteners on your bench in the vise. Drill out the existing thread to the size recommended in the helicoil kit. Tap it out with the special helicoil tap. Check there is enough meat in the captive fastener. As already mentioned, this has a special profile to suit the helicoil. Run in the helicoil insert. I'm assuming the existing threaded holes are not blind if they are anchored in a tubular frame. If OK with the bench test, drill and tap all 12 holes if you are fitting two harnesses. Make sure you use the recommended grade of fastener in terms of strength.
Regards
Gerry