Tap Trailing Arm Itself or Use Steel Inserts?

[Magz]

Hello all. Preparing to drill out my 1976 TR6 trailing arms to install larger GoodParts 3/8" studs, and looking for a voice of experience - did you drill and tap the aluminum trailing arm to receive the larger studs directly or drill and tap to install steel inserts to receive the studs? IMO the steel will be stronger, but is it needed...

For example: Time-sert inserts

https://time-serts.com/store/3-8-24-thread-repair-kit/

Any advice welcomed!

Edited January 15 by Magz

[Z320]

Are studs ripped out?

[Magz]

Just now, Z320 said:

Are studs ripped out?

No, the 5/16" stock studs are fine, but the new Goodparts CV axles use larger 3/8" studs for strength, so I must drill and tap for the larger studs.

So my question is...install the studs directly into the cast aluminum, or first install steel inserts for more strength?

Goodparts instructions say to install in the aluminum, but I am wondering if inserts might be a better (stronger) solution.

[Z320]

Don’t touch what aint broken

[Dale Moore]

 I would just tap the casting out as suggested by Good parts.  They will need the thicker studs if the hub has been drilled to suit 3/8 th either way. Just make sure you use the correct tapping drill and keep it all square. Drilling out further just risks chances of the holes becoming miss aligned 

Dale

Edited January 15 by Dale  Moore

[trchris]

As Dale said keep it all square l think there’s a guide to use to aid drilling and don’t go too deep when drilling either 

Chris

[RogerH]

If you are increasing the studs to 3/8 then I believe there will not be enough parent metal left to use steel inserts.

If it was me and I had to use the 3/8 studs then I would make sure that the thread into the T/A was 5/16unf with a steel helicoil fitted.

 

Roger

[Z320]

This cast aluminium is + 50 years old, I would not touch it.

The bolts are not the problem, the aluminium is it

[Magz]

Thanks all for the feedback. I think I'll just follow the Goodparts instructions and thread the studs directly into the trailing arm. No need to complicate things.

I need to go to the bigger studs for the CV axles and rear disk brake conversion, so leaving it alone is not an option.

Edited January 15 by Magz

[DRD]

3 hours ago, Z320 said:

Don’t touch what aint broken

+1

[PodOne]

I have CV shafts from CDD and only replaced the studs that had stripped and then fitted a helli coil to again take a 3/8 stud.

After 2 years no issues. After all they are only torqued to 14 ft/lbs to likely to stop ripping them out. After 55 years the remaining originals are still going strong. Fitting larger studs need the same torque and require a lot more meat to be removed from the casting which has marginal excess material to play with and I suspect may make the construct weaker despite using bigger studs. Get it wrong and there's no going back. 

Ps I had a full set of larger UNC studs but decided not to fit them in the end.

Conclusion I'd do the minimum.

Andy

[harrytr5]

I helical and fit 5/16 unf studs (new) back in.  I use a proper kit to drill out and tap which keeps everything in line.

Brilliant piece of kit and made up for me by my departed engineer mate, Stuart bless him.

Regards Harry

[Motorsport Mickey]

5 hours ago, PodOne said:

I have CV shafts from CDD and only replaced the studs that had stripped and then fitted a helli coil to again take a 3/8 stud.

After 2 years no issues. After all they are only torqued to 14 ft/lbs to likely to stop ripping them out. After 55 years the remaining originals are still going strong. Fitting larger studs need the same torque and require a lot more meat to be removed from the casting which has marginal excess material to play with and I suspect may make the construct weaker despite using bigger studs. Get it wrong and there's no going back. 

Ps I had a full set of larger UNC studs but decided not to fit them in the end.

Conclusion I'd do the minimum.

Andy

Andy,

Sorry to depress you, but these trailing arms and helicoils mislead owners into the wrong choice.

We’ve discussed before that these arms had the small blocks on the casting of inner diameter of the hub retaining aperture, placed there by an apprentice of about 3 months training. Unfortunately he was a dipso even at that young age, and his placements of the blocks left a lot to be desired.
 

The standard 5/16 th UNF studs will often only ( just) be retained within the placed block, threatening to break through the side wall, any increase in the hole dia by fitment of the 5/16” UNF Helicoil, requires the hole tapping diameter to be drilled out to .328 thou.
The original 5/16” UNF tapped thread to take the 5/16” UNF studs often degenerate into a grey collection of granuals which spill from the hole when you undo the 5/16” UNF studs leaving you with a neat 5/16” hole in the trailing arm hub surround, totally unencumbered by ANY remaining thread form…whatsoever.
The fitment of the aforesaid 5/16” UNF Helicoil requires this 5/16” sized hole of .312 to be opened up to the tapping size of .328 thou removing a further 16 thou from the wall of the trailing arm. The Helicoil is a thread of 5/16” UNF sat within a larger thread which is tapped into the larger .328 thou hole in the trailing arm, this often breaks through the material in the blocks of the interior hub aperture in the trailing arm.

WHEREAS… the 5/16” stripped hole left by the corroded alloy as it drops from the trailing arm…has an overall dia of .312 thou at it’s stripped dia, which co incidentally is the tapping size for…3/8” UNC thread form. You don’t even have to drill out the holes of 5/16” dia that originally were tapped 5/16” UNF to take a 3/8” UNC tap ! just bang the 3/8” UNC tap straight in there. This means…

The hole has 16 thou more material left in it than a helicoiled hole, giving more material ( not much) retaining the hub.

The hole ( if you so wish) can be deepened by drilling a 5/16” dia hole another 3 mm into virgin trailing arm material at the bottom of the hole, increasing the thread depth and increasing the stud retention capability against stripping.

The original trailing arm holes can be used WITHOUT drilling, their stripped dia is the correct tapping size to take the 3/8” UNC tap, this means a jig is not required to accurate drill the holes to take the studs.

The 3/8” thread form has more material across the throat of the thread ( it’s a bigger thread) resisting stripping better.

As detailed in technical journals

In an alloy, a 

UNC (Unified Coarse) thread form is generally stronger than a UNF (Unified Fine) thread form of the same size, particularly in the context of the female thread (the tapped hole in the alloy material). 

Detailed Breakdown

Shear Strength: UNC threads have a deeper thread profile and more material in the cross-section of each thread, which provides better shear strength in softer materials like aluminum or other alloys, making it more resistant to stripping.

Tensile Strength (Bolt): For the male fastener (the bolt itself), a UNF thread typically has a slightly larger minor diameter, resulting in a greater tensile stress area at its core, which makes the bolt itself marginally stronger in tension.

Engagement: The overall strength of the joint, however, often depends on the engagement in the female (alloy) material. UNC's coarser pitch is less likely to strip the threads in the alloy, providing a more robust connection in this specific material.

Vibration Resistance: UNF threads offer better resistance to vibration loosening due to their smaller helix angle and more threads per inch, creating better self-locking characteristics and a tighter fit.

Application: UNC is typically used for general-purpose applications in softer materials where quick assembly is needed and thread damage or corrosion is possible. UNF is preferred in hard materials and for precision assemblies, such as in aerospace or automotive engines, where high clamping force and vibration resistance are critical. 

Therefore, when working specifically in an alloy material, the UNC thread provides a more reliable and stronger connection in terms of preventing the threads from stripping out of the alloy itself.

Also in this application…

The holes on this 6 stud configuration in the hub wall on the trailing arm require their centres to be drilled within fine limits, hence the application of a “jig” to drill through, to ensure the drill bit is square in two planes,  avoiding the drill bit leaning over and moving the stud centre away from it’s intended position.

This movement of stud centre will prevent the hub sliding over the studs, and require hole diameters to be increased in the hub to allow it to slide over the studs.

And lastly, as a bonus you can in place of the studs use 3/8” UNC cap head set bolts to retain the hubs. This means the hub can be leaned over enabling any slight misalignment to be “ juggled” into place as you wind the bolts through the hub retaining holes. If you  need any recommendation for this, it’s what many of the racers with IRS suspension cars use to keep their rear trailing arms on.

Mick Richards

 

 

Edited January 16 by Motorsport Mickey

[Magz]

2 hours ago, Motorsport Mickey said:

Andy,

Sorry to depress you, but these trailing arms and helicoils mislead owners into the wrong choice.

We’ve discussed before that these arms had the small blocks on the casting of inner diameter of the hub retaining aperture, placed there by an apprentice of about 3 months training. Unfortunately he was a dipso even at that young age, and his placements of the blocks left a lot to be desired.
 

The standard 5/16 th UNF studs will often only ( just) be retained within the placed block, threatening to break through the side wall, any increase in the hole dia by fitment of the 5/16” UNF Helicoil, requires the hole tapping diameter to be drilled out to .328 thou.
The original 5/16” UNF tapped thread to take the 5/16” UNF studs often degenerate into a grey collection of granuals which spill from the hole when you undo the 5/16” UNF studs leaving you with a neat 5/16” hole in the trailing arm hub surround, totally unencumbered by ANY remaining thread form…whatsoever.
The fitment of the aforesaid 5/16” UNF Helicoil requires this 5/16” sized hole of .312 to be opened up to the tapping size of .328 thou removing a further 16 thou from the wall of the trailing arm. The Helicoil is a thread of 5/16” UNF sat within a larger thread which is tapped into the larger .328 thou hole in the trailing arm, this often breaks through the material in the blocks of the interior hub aperture in the trailing arm.

WHEREAS… the 5/16” stripped hole left by the corroded alloy as it drops from the trailing arm…has an overall dia of .312 thou at it’s stripped dia, which co incidentally is the tapping size for…3/8” UNC thread form. You don’t even have to drill out the holes of 5/16” dia that originally were tapped 5/16” UNF to take a 3/8” UNC tap ! just bang the 3/8” UNC tap straight in there. This means…

The hole has 16 thou more material left in it than a helicoiled hole, giving more material ( not much) retaining the hub.

The hole ( if you so wish) can be deepened by drilling a 5/16” dia hole another 3 mm into virgin trailing arm material at the bottom of the hole, increasing the thread depth and increasing the stud retention capability against stripping.

The original trailing arm holes can be used WITHOUT drilling, their stripped dia is the correct tapping size to take the 3/8” UNC tap, this means a jig is not required to accurate drill the holes to take the studs.

The 3/8” thread form has more material across the throat of the thread ( it’s a bigger thread) resisting stripping better.

As detailed in technical journals

In an alloy, a 

UNC (Unified Coarse) thread form is generally stronger than a UNF (Unified Fine) thread form of the same size, particularly in the context of the female thread (the tapped hole in the alloy material). 

Detailed Breakdown

Shear Strength: UNC threads have a deeper thread profile and more material in the cross-section of each thread, which provides better shear strength in softer materials like aluminum or other alloys, making it more resistant to stripping.

Tensile Strength (Bolt): For the male fastener (the bolt itself), a UNF thread typically has a slightly larger minor diameter, resulting in a greater tensile stress area at its core, which makes the bolt itself marginally stronger in tension.

Engagement: The overall strength of the joint, however, often depends on the engagement in the female (alloy) material. UNC's coarser pitch is less likely to strip the threads in the alloy, providing a more robust connection in this specific material.

Vibration Resistance: UNF threads offer better resistance to vibration loosening due to their smaller helix angle and more threads per inch, creating better self-locking characteristics and a tighter fit.

Application: UNC is typically used for general-purpose applications in softer materials where quick assembly is needed and thread damage or corrosion is possible. UNF is preferred in hard materials and for precision assemblies, such as in aerospace or automotive engines, where high clamping force and vibration resistance are critical. 

Therefore, when working specifically in an alloy material, the UNC thread provides a more reliable and stronger connection in terms of preventing the threads from stripping out of the alloy itself.

Also in this application…

The holes on this 6 stud configuration in the hub wall on the trailing arm require their centres to be drilled within fine limits, hence the application of a “jig” to drill through, to ensure the drill bit is square in two planes,  avoiding the drill bit leaning over and moving the stud centre away from it’s intended position.

This movement of stud centre will prevent the hub sliding over the studs, and require hole diameters to be increased in the hub to allow it to slide over the studs.

And lastly, as a bonus you can in place of the studs use 3/8” UNC cap head set bolts to retain the hubs. This means the hub can be leaned over enabling any slight misalignment to be “ juggled” into place as you wind the bolts through the hub retaining holes. If you  need any recommendation for this, it’s what many of the racers with IRS suspension cars use to keep their rear trailing arms on.

Mick Richards

 

 

So if I may summarize, Mick, you are telling me that I can simply remove the existing 5/16" studs, tap with a 3/8" UNC tap, then thread the new 3/8" studs into the trailing arm?

That would certainly ensure I don't screw up the drilling, as there IS no drilling!

[JohnC]

1 hour ago, Magz said:

can simply remove the existing 5/16" studs, tap with a 3/8" UNC tap

Only if the threads in the TA are already stripped (as Mick described).

[Z320]

13 hours ago, Magz said:

Thanks all for the feedback. I think I'll just follow the Goodparts instructions and thread the studs directly into the trailing arm. No need to complicate things.

I need to go to the bigger studs for the CV axles and rear disk brake conversion, so leaving it alone is not an option.

Andy,

how does Richard Good explain the need für 3/8“ studs?

I fitted his u-joint axles more than 10 years ago without any modification.

Is the flange of his CV axles drilled 3/8“? Or is this needed for the disc brake conversion?

Just to clear: IMO, what ever reason you have been told,

if the threads are not rippid out, don’t touch them.

That reminds me to a friend, but that is a poor story, you don’t want to read it…..

Ciao, Marco

[Motorsport Mickey]

7 hours ago, Magz said:

So if I may summarize, Mick, you are telling me that I can simply remove the existing 5/16" studs, tap with a 3/8" UNC tap, then thread the new 3/8" studs into the trailing arm?

That would certainly ensure I don't screw up the drilling, as there IS no drilling!

As JohnC says above, and as I said in my prose, if the hole is stripped it’s o/d is the correct tapping size for 3/8” UNC to take the 3/8” UNC studs without drilling further.

For any threaded holes remaining, they thread form for the 5/16” UNF studs are very shallow, and being formed in alloy very much wants to drag your 5/16” dia drill into it. However I should caution against NOT using a jig to drill out the piddling remaining alloy threads. You do have to hold the drill at 90 degrees in 2 planes as you drill, and risk having studs leaning at angles if the drill is not controlled closely. 
 

Mick Richards

Edited January 16 by Motorsport Mickey

[Magz]

4 hours ago, Z320 said:

Andy,

how does Richard Good explain the need für 3/8“ studs?

I fitted his u-joint axles more than 10 years ago without any modification.

Is the flange of his CV axles drilled 3/8“? Or is this needed for the disc brake conversion?

Just to clear: IMO, what ever reason you have been told,

if the threads are not rippid out, don’t touch them.

That reminds me to a friend, but that is a poor story, you don’t want to read it…..

Ciao, Marco

The disk brake conversion requires 3/8" studs. The flange of the axles are supplied with 3/8" holes when you are also doing the disk brake conversion along with the CV axle conversion.

[Magz]

2 hours ago, Motorsport Mickey said:

As JohnC says above, and as I said in my prose, if the hole is stripped it’s o/d is the correct tapping size for 3/8” UNC to take the 3/8” UNC studs without drilling further.

For any threaded holes remaining, they thread form for the 5/16” UNF studs are very shallow, and being formed in alloy very much wants to drag your 5/16” dia drill into it. However I should caution against NOT using a jig to drill out the piddling remaining alloy threads. You do have to hold the drill at 90 degrees in 2 planes as you drill, and risk having studs leaning at angles if the drill is not controlled closely. 
 

Mick Richards

OK, got it. I need to drill with a jig.

Thanks for sharing your experience. I'm learning a lot of new things from this forum!

[Dale Moore]

Yeah as Mick says drill and tap 3/8 unc  It will give a better bite into the aluminium.  Cr** idea in the first place to tap unf into an alloy casting

Dale

[RogerH]

Standard Triumph were very good engineers. They really did know what they were doing.

So why were they so damn stupid fitting UNF thereads into Light alloy.

The simple answer is because you can if the conditions call for it.

The material is light Alloy. Ther loading on the studs is a shear load of quite small value.   So a small torque ir required to tighten down the nuts to hold it all in place.

If you used UNC threads you would need a higher torque load to keep the nuts held down as the UNC thread angle is such that it could actually undo with vibration.

Because the UNF thread has a shallower thread angle it is naturally locking itself into the structure with a lower torque.

You can't have a large torque load so you must work around it.

 

Roger

[TRTOM2498PI]

21 hours ago, Z320 said:

Don’t touch what aint broken

This, is very sound advice ! Agreed.

[Kenrow]

Richard Good is really good (😉) about answering questions about his products, suggest sending him an inquiry…

[Dave McDonald]

Mags, the photo shows the “blocks” that Mick referred to which some of the studs are threaded in to. As you can see, they are very marginal for wall thickness if drilled out further. Take great care. 
Dave McD

[tim hunt]

Isn't the answer to use 3/8UNC-5/16UNF studs, thereby avoiding the need to enlarge the holes in the hub. CCD supply such studs.

Tim