head stud size?

[AlfredHitchcock]

Good point Alan regarding tapping UNF at the top and shortening the top.

 

My easiest option would be to buy a 1/2" unf die this morning and shorten the stud. But if the general consensus is that this will cause a failure I'll not do it.

 

I would prefer to take Mick up on his offer however there is definately something hard in the bottom of the hole that I cannot drill out easily, and I'm getting nervous why. I dont want to drill into something else.

 

I'm getting very frustrated by the whole thing..........I've booked Monday and Tuesday off work to get this bloody thing back in the car - its doing my head in.

[AlanT]

Well you dont have a lot to loose if you cut the thread on the top.

Worst that can happen is that the thread strips and you have to take the head off and do it again.

 

You will have 4in of stud sticking out to get hold of.

 

Getting a Helicoil thread right down to the bottom of the hole won't be all that easy. There is always the risk of breaking the tap.

That will be really hard to get over! Bottom of the hole will be full of swarf.

 

I might take up Micks offer and try to get a little deeper but go very gently.

 

Did someone suggest you can easily buy a stud that has the right thread both ends but is a bit shorter by just the right amount.

Wow thats lucky! And a good idea if you can pull it off.

[Motorsport Mickey]

Nick,

 

Just checked, I have my extended length tap set which will easily reach the deep stud hole length ( a second and bottom tap) but the taps I used are 1/2" UNC the standard thread size and form.

If you do have a Helicoil at the bottom this may not be of any use to you, however the extension set is made up of a square bar with a welded socket with a square drive hole on it which all easily goes down the stud hole. You should be able to use the Helicoil tap in it, or if the drive size is wrong on the tap carefully grind the tap to size to suit the square drive hole which will then allow you to use the tap on the deep hole.

 

I'll PM further details, if you want to have a go with this tool let me know and I'll send by special delivery tomorrow morning and you'll have by lunch Tuesday guaranteed. Recut the thread 15 minutes and on with the rebuild.

 

Mick Richards

[AlfredHitchcock]

Mick I've replied to your pm thanks.

 

This afternoon I have got the head on and torqued down successfully.

 

Basically I ran a 1/2 unf die for another 40mm and cut off the excess height caused by the helicoils being higher up the hole.

 

Hopefully I can paint the block tomorrow and fit the gearbox ready for fitting tuesday.

 

Its been one step back and two forward today.

[RogerH]

If you have torqued it successfully then you have a result - at last

 

Well done.

 

Roger

[AlanT]

This has been a most useful thread which highlights the following:

 

1. do try not the strip the thread in the block, the hole depth makes it hard to fix

 

2. you can sucessfully stack Helicoil inserts

 

3. its hard to get the Helicoil right to the bottom of the hole because the standard taps are too short

 

4. a standard stud may therefore be too long

 

I don't think we have quite discovered the optimum method for fixing this.

I am going to get a Helicoil tap and experiment with extending it, as Mick has done.

 

I wonder if you have to drill out the top part of the newly cut Helicoil sized thread to get the tap extension to work?

[Rob Salisbury]

Hi Nick, I know it's not best engineering practice to cut a thread but sometimes needs must, around 15 years ago my block partially striped the thread adjacent to the oil drilling for the rocker shaft, to get round this problem I drilled the block deeper and ran a 1/2" UNC tap to the bottom of the hole, I repeated this process for each hole in the block and I then got a new set of studs and to the bottom of each I added a further 30mm of thread cut with a 1/2" UNC die using the progressive tapering method previously described (where necessary I lightly relieved the holes on the block face so as not to put any further strain as the studs were screwed in, as I say this happened 15 years ago and each stud has been torqued and re- torqued several times as various iterations of the engine have taken place (86 to 87 to 87.2 to 89mm along with the odd camshaft change). I may just have been lucky but my torque wrench gives a nice firm click @ 105ftlbs every time, so I reckon your cut stud will last.

Cheers rob

[AlanT]

I am encouraged to hear your experiences. The thread in the cast block is more likely to strip than the threads in high grade steel.

 

The risk is fatigue fracture starting at a stess raiser. Irs that little sharp bit where the thread comes to an end thats the problem.

 

Likewise you dont want vice-grip marks on the shaft.

 

Did I read right that you drilled all the holes 30mm deeper. You must like work!

[AlfredHitchcock]

A few more things I've learned from this is that a standard stud is only threaded for 1" top and bottom. However the stud is 5" deep into the block. Therefore there must be 4" of hole doing nothing .

 

Also, yes the helicoils can be stacked but the tangs dont break off very cleanly so you need to get prepared with a hook device as per Rogers advice to pull them back up.

 

Also, the studs are not too hard to be rethreaded but they are quite tough to do and I opened the die right up to do the cutting which seemed to make a nicer fit on my worn nut.

[AlanT]

Hand threading steel at 1/2 dia is never going to be easy and these are probably half-hard.

 

Does anybody know the actual heat-treatment regime used on these.

[ChrisR-4A]

A few more things I've learned from this is that a standard stud is only threaded for 1" top and bottom. However the stud is 5" deep into the block. Therefore there must be 4" of hole doing nothing .

 

Also, yes the helicoils can be stacked but the tangs dont break off very cleanly so you need to get prepared with a hook device as per Rogers advice to pull them back up.

 

Also, the studs are not too hard to be rethreaded but they are quite tough to do and I opened the die right up to do the cutting which seemed to make a nicer fit on my worn nut.

I hope all goes well but I am I bit surprised to see that no one had suggested that using new nuts with the new studs would be adviseable. Probably like myself other posters assumed that new nuts would be used as using 45+ year old nuts is not the best strategy as more torque can be achieved with new nuts and a fuller cut thread. Problem could arise if a nut has to be replaced in the future and a new nut is then tight on a shallow formed thread preventing a true torque being obtained. But having said that as it seems to have worked as is then leave well alone and give it a try.

Edited August 19, 2013 by potts4a

[AlanT]

Because of this topic I decided to buy 1/2in taps and dies in UNC and UNF and a Helicoil tap.

When they came I was struck by just how weedy a 1/2in bolt looked for 110ft-lbs.

 

So I checked the Engineers-handbook and if you have the best grade of bolt

the the max torque is 165ft-lbs but only on the UNF end.

 

On the UNC end its only 115ft-lbs absolute maximum.

 

And this assumes the best grade of bolt with a proper nut and we have these in an iron-casting.

 

This shows that the head studs are being torqued right up to their limit.

So its not surprising we are seeing a few stripped blocks.

 

It also means that a cut thread on the stud at the UNF end has a better safety margin than we had thought.

 

These should really have been 5/8in not 1/2in if you need 110ft-lbs.

[ChrisR-4A]

Or ideally more studs to spread the clamp load over then the torque per stud can be reduced.

Problem is with wet liner engines the liner and its lip take up a lot of space and there's nowhere left to add more studs as in more modern engines with solid blocks. More cylinders for the same capacity also helps to spread the load and maintain the clamping force, hence the evolution to the TR6 . Or looking back to performance cars the reason for the V12 3 ltr Ferrari.

[AlanT]

The fact that we are sucessfully Heli-coiling these suggests there is room for bigger studs.

 

When my Helicoil tap arrives I'll measure it up.

[Motorsport Mickey]

AlanT,

 

To help support your theory I fitted a 16mm stud in place of one of the deep stud holes 48 hours before a race after pulling a stud from the block when final torquing at 110 lb ft, ( latest mind blowing modification had weakened it !).

 

I made and cut a 16mm stud to use in it, same thread both ends ( 48 hours to go, needs must) and drilled the head clearance on it, that engine won 2 consecutive TR Register race championships with 25 consecutive race wins, guess there's more space around those studs than we think !

 

Mick Richards

Edited August 23, 2013 by Motorsport Mickey

[AlanT]

I was actually quite shocked to see the reduction in maximum torque figures for UNC compared to UNF.

 

Tradition has it that you used UNC in castings because the thread was stronger in the weaker material.

I can't quite reconcile this with the maximum torque figures.

 

Metric threads pitch between UNF and UNC and so avoid this dilemma.

 

Interesting that you used as big as 16mm.

[ChrisR-4A]

On modern engines the head studs/ bolts are taken into yield * , to obtain the design clamp force with the minimum sized hardware, which is normally used only once.

 

On the TR engine it is the thread on the stud which causes the problem not the stud itself. The 110 lbs torque puts tension into the stud which exerts the clamp load on the head whilst remaining within its elastic limit* ,larger studs may need even more torque to achimeve the same clamp load. Ideally a waisted stud would achieve the larger threads at the ends whilst still allowing it to stretch and provide the clamp force

 

*

elastic limit - maximum amount a material can be stretched by a force and still return to its original shape

yield point - the point where there is a large permanent change in length with no extra load force

Edited August 24, 2013 by potts4a

[AlanT]

Another thing I turned up when researching this subject is that if you lubricate the nuts you get

more tension in the stud for a given torque.

 

I suspect that this is why a proficient mechanic like Mickey winds up stripping his block thread one day.

The stud had some oil on it and without realising it he put in too much tension.

 

This seems to be a warning for everyone.

[Motorsport Mickey]

You are quite right about an oiled thread giving a misleading torque figure, that's why when I refit any bolt or stud the threads are cleaned by wire brush and sometimes die nut (if I have the size) an thoroughly cleaned with WD40 and then dried to leave a clean thread with a dry finish. I also clean the threads in the block with brake cleaner and allow to flash off so again the threads are dry, and then the studs refitted by hand (no spanner or wrench) until they just nip up tight. This then gives correct torque figures

 

 

No to answer the unasked question the reason the stud stripped was because this was the first engine we'd experimented with, and offset the head leaving a small crescent of material in each stud hole. We got away with it in all the holes but one which stripped upon being retorqued, hence the emergency 16mm stud replacement to get us out of the cack.

 

Mick Richards

Edited August 24, 2013 by Motorsport Mickey

[RogerH]

In my mind (it is an odd place sometimes) I would have thought that the threads in the hole do not need to be dry.

Possibly better from a corrosion point of view to keep them a little oiled.

Once the stud is in place the torquing/rotation is done from the nut end so the wet bottom end does nothing to affect the loading.

 

From a logic point of view only a lubricated nut thread can give a stable repeatable reading - whether it is right or not is neither here nor there.

If it is dry you introduce friction that can be variable so your loading may not be what you want.

 

Having said that I am splitting hairs. The variation in a dry thread is not enormous but does exist.

 

Isn't engineering fun

 

Roger

[AlanT]

I always thought that the effect of having some oil on the thread would be quite small.

But the web-data from bolt manufacturers suggests a 20% reduction in torque required to get the same tension.

 

Since the UNC thread is near the recommended limit at 110ft-lbs you would not want an unsuspected 20% overload on it.

[Motorsport Mickey]

Agree Roger, all I try to do is have the fastenings in the same condition as when they were fitted at the factories and they fitted them dry

 

Having an element of lubrication in place allows too much possible variation, have they lubricated too much or too little ? Yet another element possible of being misapplied and causing a much greater variance than a dry clean fitting.

 

Mick Richards

[ChrisR-4A]

During my years working for FMC and CNH I was involved in the procurement of multi spindle nutrunners usiing low voltage DC motors some of which relied on strain gauges to measure torque and others which took the head fixings to yield. In both cases to obtain a common torque or yield figure the manufacturers emphasised the importance of good quality fasteners made to a closely controlled spec. Fixings were supplied with a light coating of lubricant to prevent corrosion and reduce friction.

The worst situation could occur if a nut was tight on the thread or the head face and washer was rough, either would result in a correct torque figure but undertension in the stud due to friction and insufficient clamp load on the head resulting in gasket failure.

For an engine like the 4 pot TR this means that everything should be clean and free to assemble by hand before torquing and also all studs nuts and washers should be in good condition. A uniform torque across the whole head at say 105 lbs torque is more important than trying for 110 some of which is taken up in friction resulting in an uneven clamp load.