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MG MGA - 5/8'x18 tie rod thread

I'm installing MGB front suspension, discs and calipers on my modified MG TF 1800. I'm using an MGA tie rod to connect the TF rack to the MGB steering lever. I need to cut 1/2" off the end of the tie rod and continue the thread up the tie rod by 1/2" or so. I just bought a Craftsman large diameter tap and die set from Sears that includes the 5/8"x18 die. I'm having difficulty running the die up the tie rod thread which is 5/8"x18 in order to extend the thread. It will thread on about 1/2" and then gets tight as though it's binding on the existing thread. Could the new die (made in China) be the problem? Or is the something special about the thread?


Andrew Preston

Andy - You are probably experiencing the difference between rolled threads and cut threads. I don't know how much difference this makes on the tie rods, but cut threads cause stress risers in a piece of rod. Perhaps someone like Bob Grunau can comment on this. Cheers - Dave
David DuBois

Andy. What type of threading die do you have? There are two basic styles, one of which is designed to clean up existing threads and one which is designed to cut new threads. The former is most commonly hexagonal in shape and is solid. The second type is commonly round in shape, has a split on one side, and a set screw that can be turned (right to tighten the screw and expand the die, left to loosen the screw and allow the die to spring closer together) to set the exact size thread being cut. There are tolerances and fits involved with any thread form and the die is adjustable to allow them to be produced.

But, I would suggest you invest in Carroll Smith's book, "Nuts, Bolts, Fasteners and Plumbing Handbook" before you do anything else. Then, read it very carefully. Especially the section where Smith, on page 42, first full paragraph, notes: "One of the most unpopular proclamations that I made in Prepare to Win was that either die cutting or lathe cutting threads onto a bolt is a crime against nature.".

If, after reading and understanding the potentially life threatening nature of the modification you are considering, you decide to go ahead with the modification, at least you will be making a decision based on some level of knowledge.

But, I would not perform this modification myself and I would, most especially, not recommend it be attempted by someone who does not even know how to operate a die without asking. Yes, I am trained as a machinist and gunsmith and have years of experience in this topic.

Les Bengtson

Andy, If your die is around 1/2" thick it could be the last thread on the die which isn't formed so try turning it round to see if it goes on by the same 1/2".
If it does then suspect the tie rod thread.
Alternatively you could go to your local hardware shop and buy a 5/8 screw to test the die.
But whatever you do don't cut any new threads with a suspect die.........................Mike
m.j. moore

Les, what modification would you not attempt - extending a thread or fitting MGB suspension to a TF? Surely with your training the thread should be trivial.
Andy, the dies have a front and a back side. Also, maybe the OD of the bare rod is too large. Check the thread tables.
Rolled vs cut threads. Maybe rolled threads are stronger, but you can't roll the internal threads to my knowledge, so nuts are no stronger.
Art Pearse

Thanks for the help guys. I just got a 5/8"x18 nut and bolt from the hardware store. The nut threads onto the tie rod perfectly smooth and also threads onto the new bolt. The die does not thread onto the new bolt more than 1/2" before it binds and seems to be trying to cut the threads wider, so it appears that the die is faulty. I'm back to Sears to return the tap and die set and then where can I buy a decent die or should I take the tie rods to a machine shop and have them extend the threads?

Installing MGB disc brakes is a fairly common conversion on both the TD/TF and As, but it does require shortening the tie rod and extending the threads up the tie rod. According to the archives many people have done this and so it should be easier than I'm finding it.

55 MG TF
60 MGA
67 MGB
Andrew Preston

Andrew, someone has already mentioned that the threads might be rolled. This means that the O/D of the thread is bigger than the rod.
If that is the case, you will not be able to cut a deep enough new thread into the rod.

Sounds dangerous to me!!!

Colin Parkinson

"what modification would you not attempt - extending a thread or fitting MGB suspension to a TF? Surely with your training the thread should be trivial."

What Les is saying is that cutting threads into a rod that is subjected to high stress is a really good way to get the rod to break. I mentioned that cut threads act as stress raisers in a rod - that means that any stress applied to the rod through the threads is multiplied by the cuts and will cause the rod to fail at a much lower stress level than what it is specified for.

As Les suggests, if you don't understand stress factors and how they effect the strength of a high tensile strength material, then Carroll Smith's book, "Nuts, Bolts, Fasteners and Plumbing Handbook" should be purchased and read before doing any modifications to steering and suspension components - your life depends on understanding this subject before undertaking modifications. It is Les' training that makes him realize that such a thread is anything but trivial and his advice should be heeded. Cheers - Dave
David DuBois

Once again thanks for your advice. I have returned the bad tap and die set to Sears and will be taking the tie rods to my local machine shop in the morning and see what they say. I don't know what rolled threads are but the diameter of the threads is the same size as the tie rod diameter.

Andrew Preston

That would indicate that the threads are cut rather than rolled; thread rolling for such comes later, but not even on MGB. You might notice that MGB/MGA tie rods are quite large compared to their modern kin. In thread rolling, the rollers squeeze the metal to form the thread, making low grooves and high peaks, so the rolled thread diameter is always greater than the starting material diameter. Your machine shop should be able to tell instantly.

It's been a while, but I do not recall ever having to cut more threads when fitting MGB suspension to MGA; I imagine that your TF job would be the same. It is necessary to cut a little off the threaded end of the tie rod, about 1/4-3/8", to get correct toe-in adjustment. The lock nuts should be 1/2 height, but sometimes people have replaced them with full height nuts.

Art -
Nuts are much larger at the critical root diameter where the stress concentration happens, and they are not subject to bending stresses, so they are inherently both stronger and less stressed.

FR Millmore

Wasn't there some discussion recently about newer replacement tie rod ends being too long, and people needing to either shorten the tie rods or the ends to get them to work together? Is it possible that is what is happening here, that you are trying to use the new off-spec tie rod ends? If you are using new parts, maybe you could measure an original to compare with?
Del Rawlins

So, if the threads are cut to begin with I don't see anything wrong in extending them. Also, there is no bending stress on a tie rod. It is just push or pull along the axis. excepting if the ball joint has seized.
Art Pearse

FRM, I believe you have to do this to fit the MGB suspension on the MGA too.

Del, that's another problem, there is no need to cut the tie/track rod for those "off spec" ends. Either shortening the rod end or cutting more thread is enough. I guess there are a lot of cars out there with threads cut further up the tie/track rods!
Neil McGurk

I didn't mean to imply that cutting the rods themselves was a preferable solution, only that it was one of two possible remedies. It's more or less academic to me, since I still have a set of NIB replacement rod ends that my dad purchased 20 some years ago.
Del Rawlins

Once again guys many thanks for your wealth of information and help. In order to complete this thread topic I took the tie rods to the machine shop yesterday. The machinist carefully extended the threads on a lathe using the existing ones as a guide and then removed 1/2" from the ends. He advised me that the only way to extend the threads was by cutting them on a lathe and NOT to use a die.

My new tie rod ends are made by Beck Arnley (made in England) and have a threaded depth of 1 1/2". The ones are got from Moss were rubbish and had a very loose and sloopy fit so I returned them with a letter to their quality control director.

I read all of the archive infomation about fitting MGB suspension and discs to the TD/TF and most people only had to shorten the tie rods by 1/4". I measured mine several times before I had them shortened by 1/2", I hope that when it's all back together they're not too short.

many thanks

Andrew Preston

Measure 3 times and cut once!
Art Pearse

This thread was discussed between 15/12/2010 and 17/12/2010

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