tr250 Helicoil feedback

[Motorsport Mickey]

On the rear swinging arms on the TR4a to 6 the rear hubs have a 5/16 unf thread in the swinging arm which often strips.

 

If you drill out to take a 5/16th UNC helicoil the drill size is .328 and almost certainly will break through the wall thickness in the hole wall material at certain places, it being fairly thin. Those engineers who have often used them can possibly recall what Helicoil advise, In this position can you rely on a Helicoil or will the breaking through compromise the helicoil security and clamping affect ?

 

Mick Richards

[stuart]

Very often if the stud has pulled out you will find the hole edge is powdery with corrosion and it wont take drilling. The helicoil tap will work straight in without drilling provided you use a jig to ensure it goes in straight. There should still be enough wall thickness provided the drilling depth is only enough to take the length of the helicoil. Trailing arms do seem to be getting worse as the years go on. I came across one last year that had one stud drilled right through and bolted through with a nut on the back!

Stuart.

[Motorsport Mickey]

Is there anybody else with information regarding if it is ok to fit a helicoil where the helicoil will just break through the wall on one side, or will it compromise the helicoil fixing in the trailing arm ?

 

Mick Richards

[AlanT]

Might be something interesting on here:

 

http://www.repairengineering.com/

 

Knew I'd seen this:

 

http://www.repairengineering.com/differential-housing-repair.html

Edited April 20, 2014 by AlanT

[Motorsport Mickey]

Thanks Alan.

 

Mick Richards

[Peter Cobbold]

Build up the wall with Technoweld before tapping?

http://www.techno-weld.co.uk/product.html

[AlanT]

The drilling wont break out. You are only taking 5-thou per side. The tapping might do.

 

This really calls for the solid type of insert. These will be fine even if they do break out.

 

I actually think a spring type insert will wok. But this is a wheel.

 

Presumably you are using non-standard studs if you are switching from UNF to UNC.

 

Consider what happens if you use M8 which is v.close to 5/16in in diameter and pitch.

Be easier to get the inserts perhaps.

Edited April 21, 2014 by AlanT

[Motorsport Mickey]

I think the 5/16th UNC Helicoil calls for a .328 drill size which is 16 thou more than the current tapped hole to drill out for a core dia.

 

I'm considering using some 3/8th UNC x 5/16th UNF studs available instead of a Helicoil. That only needs a .312 thou drill for the 3/8th UNC core dia, 16 thou less dia than for the Helicoil although as you say even then I think the threads will break through on the wall thickness.

 

I found a Helicoil advice sheet which says the wall thickness should be .375 x Dhc ( overall thread dia of the Helicoil), which I make out to be about .144 wall thickness minimum which the Helicoil in the trailing arm can't achieve.

 

Mick Richards

[oldtuckunder]

Hi Mick

 

Take a look at the following inserts, I don't know if they have anything in the size you need, but I have used them a couple of time over the past 12 months and they were a joy to install compared to helicoil, and as they have their own wall breaking through may not be an issue.

 

They are fairly inexpensive, and they have always arrived next day when I have ordered any

 

Regards

Alan

 

PS. When I say I have used them I don't mean in TR suspension arms, but have used them into Alloy and Steel

Edited April 21, 2014 by oldtuckunder

[roy53]

Alan i am looking

 

but they are invisible / vanished.

 

Roy

[oldtuckunder]

Alan i am looking

 

but they are invisible / vanished.

 

Roy

Whoops sorry

 

http://www.memfast.co.uk/shop/Vprod2.asp?cat=2250004326

[stuart]

Unfortunately they are all in Metric.

Stuart.

[AlanT]

As I explained above an M8 is so close to 5/16in in diameter and pitch that you will find it hard

to measure any difference and decide which is which.

 

8mm is 0.31496 in and 5/16 is 0.3125in.

 

In a 1/2in length of the UNC thread there are 9 turns and in the metric thread 8.47 turns.

 

I put a thread gauge into a bolt and could not tell it did not fit perfectly unless I used x8 magnification.

The M8 nuts will go on the 5/16in threads.

[Motorsport Mickey]

Hmmmmmmm... bit worrying a M8 insert needs a 11mm to 11.2mm drill size, over .400 thou dia and almost certainly going to break through the trailing arm alloy sidewalls. It helps that the inserts are self supporting and don't rely on any spring action to lock them in place but even so I estimate almost 20% of the insert will be sticking out on the side with lesser wall thickness.

 

Mick Richards

[Ghianightmare]

I just helicoiled my trailing arm - on Sunday. I used Baer coil and the Patton machine jig. Worked a treat. Use plenty of oil/ cutting fluid or it will just chip away the aluminium. Torque up to 16 foot pounds no problem. Plenty of wall thickness remaining. The drill bit in the kit is 8.2mm. I am sure you can find in the uk but the irish supplier is in the link below. The coils spread the load differently to the original threads. Plenty of tech info in the second link

 

http://www.jgengineering.ie/collections/unf-helicoil-kits-ireland/products/unf-5-16-x-24-baercoil-kit-helicoil

 

http://www.baercoil.com/en/baercoil_technical_info.html

[Motorsport Mickey]

Thanks for that info.

The drill size at 8.2 mm is I think ,328 thou and then the thread form to be tapped into the arms. I think the Patton jig is excellent if a little expensive but certainly will allow drilling and keeping the axis at 90 deg in both planes.

The Baercoil does say it relies upon the "sprung coil " being retained by the tapped hole which is where the possibility for failure may occur if the tappings of the outside threads for the insert break through the hole wall, especially so with the insert not being a one piece thin wall design but a coil. This may be a case of failure on a case by case basis depending upon castings used.

I have seen two other sets of trailing arms with Helicoiled 5/16th UNC stud holes where the Helicoil has broken through on the thinner inner wall, although tellingly neither of these other installations was done with a jig to ensure no run off.

 

Mick Richards

[AlanT]

This is a really useful topic. These studs are about the only thing on my 4A that I broke during the resto.

Plenty of stuff broken by somebody else of course.

 

They are small, the thread is wrong originally and the wall is thin.

Plenty of room here for your avearge owner to mess up with this.

 

I reckon that avoiding inserts is a good idea. 3/8in studs that reduce to 5/16in seem good to me.

I could make these easily.

[Ghianightmare]

AlanT,

 

Forgive me if you know this but -

 

Helicoils are used in the manufacturing industry to solve the probems we have with the trailing arms. Similar example is VW originally tapped the alloy aircooled engine cases directly where the cylinders attached, but due to the bolts pulling, the later cases used wire inserts/ heli coils. Problem solved - even with similar thin wall issues.

 

The function of helicoils and similar is not primarily to repair threads, it is to permit the use of smaller fastening in certain matrials while providing the tensile strenght of a larger fastener. Not sure exactly how the physics works, but it does.

 

Therefore, based on industry practices, the helicoil seems to be a better option over the 3/8 stud.

 

There was very liittle material removed by drilling out to 8.2mm considering the threads were all stripped on mine. Tapping removed only a little material too. Put it this way, I was not concerned with wall thickness afterwards.

[Motorsport Mickey]

However there is still the Helicoil recommendation we can't achieve,

 

"I found a Helicoil advice sheet which says the wall thickness should be .375 x Dhc ( overall thread dia of the Helicoil), which I make out to be about .144 wall thickness minimum which the Helicoil in the trailing arm can't achieve"

 

I suppose another benefit of using the 3/8ths UNC stud is the threaded length can be deeper than the original holes, going into Virgin non drilled alloy a further .300 thou and giving a proportionate increase in thread strength purely because there are more threads available ( I estimate about 30%). As far as I know nobody has a problem with shearing the original 5/16th studs only pulling the UNF thread from the trailing arm, it's useful to maintain the the same UNF thread and 5/16th size on the hub side.

 

Mick Richards

[AlanT]

Sure, but we ain't talking factory fitting here. Its owners drilling by hand with a pistol-drill.

In these circumstances I prefer removing as little metal as possible.

 

It so happens that the tap-drill size for 3/8UNC is 5/16in. So if you have pulled out the thread then you can cut 3/8 UNC without really drilling anything out at all. All you are doing is cleaning out any remaining thread.

 

However like you I used a Helicoil without problems with the walls.

 

I'd prefer we all used the kind of insert that is not made like a spring but has one thread inside the other but is actually solid.

Unfortunately these are hard to get unless, like a lot of useful things, you are in the USA.

[Ghianightmare]

AlanT,

 

Agree with doangers of someone drilling without a guide - hence I used the Patton Machine jig, which is excellent.

 

Baer do the inserts you are referring to

 

http://www.baercoil.com/en/products_bf_inserts.html

[AlanT]

Do they do the synchronised thread inserts like this:

 

[oldtuckunder]

Do they do the synchronised thread inserts like this:

 

 

The MEMFAST ones I linked to are very similar

[harrytr5]

I have done this job quite a few times and been very successful.I used a proper jig my mate machined up for me to locate the drill and taping keeping everything lined up. Helecoils worked for me.

Regards Harry.

[AndrewMAshton]

Hello, Roger Williams recommends Timeserts in his excellent book and they are available from Wurth, Cheers, Andrew