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MG MGA - Steering rack/pinion - case hardening?

I have a repro RHD steering rack bought many years ago (the restoration has been on & off for some time). Should the rack and pinion be case hardened? This one does not seem to be and I vaguely remember reading that they should be.
C J Bond

CJ. Depends on the material the new rack is made from. Case hardening (carburizing) is a process which allows low carbon steel to be surface hardened. In certain applications, this is a very desirable method. In other cases, there might be an advantage to using a higher quality steel and heat treating it through some other process. Unless you know the hardness of the new rack and what steel it is made out of, then, can compare its projected performance to the performance of a steering rack made of low carbon steel that has been carburized, I would not worry about what you have. Your not going to be able to figure out what you have, and whether it is better suited, equally well suited, or less well suited to the job than the original design unless you have access to a metals lab, sophisticated test equipment and a well qualifed materials science engineer.

I do not think I would worry about it myself.

Les
Les Bengtson

Hi, Chris - don't know if this info will cross over, but I suspect it will. The center three or four teeth on the earlier midget racks were "flame hardened", and can be seen when the rack itself is removed from the tube and cleaned well. The theory is that the pinion gear contacts these few teeth 95% of the time in normal driving, and thus, they bear the brunt of the wear.

One of the procedures to reinstall the rack assembly calls for you to make sure the unit is centered, both to use these teeth, and to avoid "bump steering".

Best of luck - - Alec
Alec Darnall

Nice picture of a flame hardened MGA steering rack here: http://www.chicagolandmgclub.com/photos/a_rack/4118.jpg
The flamed area covers just 6 to 8 teeth in the center of the stering range where it spends most of its life.
Barney Gaylord

Alec and Barney. You are both correct when discussing original racks. CJ, however, is discussing a "repro" rack, which I understand to be a rack made by someone other than the original company which produced the steering racks for BMC.

BMC developed the steering racks to be made using the materials, the machines and the heat treatment processess available to them at the time of development. "Flame hardening" is not, in itself, a heat treatment. It is one mechanism for heating a part as part of the heat treatment process. (You can heat the part in an open flame, followed by quenching, or you can heat the part in an oven, followed by quenching.) The only advantage, in this case, to the use of an open flame, rather than an oven, would be economy. It would be quicker to heat one small area, using an open flame, then quench the piece, than to use an oven to heat the entire piece and quench it. This would result in a hard area where it had been heated, tapering off to a less hard area where the heat boundry was and an even less hard area beyond the heat boundries. All of this is old technology.

Any modern part can differ, both in construction materials and in the heat treatment used, from an original piece. Much better steels are available today than there were 50+ years ago and the heat treatment of these steels is significantly different than was used in the past. Thus, perfectly possible to produce a superior item to the original by using more modern steels, heat treatments and machining processes.

As to the exact object under discussion, all we can say is that it differs from the factory design. A decision as to whether it is equal to the original, inferior to the original, or superior to the orignal cannot be made without knowing what it is made of and what heat treatment process was used in the manufacture.

Les
Les Bengtson

One way to get an idea is to take an original rack and the replacement to a machine shop with a Rockwell hardness tester. This would allow you to get an idea of how the repro compairs to an original.
John H

John. Not quite true. I would only be a true comparison if the parts were made of the same material and if you knew what the hardness range was for that design within that application.

A number of years ago, I tested two machine parts, both of which were used for the same application. One went 35 on the Rockwell C scale, the other went 50 on the Rockwell C scale. The first part was made of 3.5% nickel steel, the second was made of low carbon steel with a double heat treatment. Both parts showed no wear and had been in service for over 50 years without problems.

Hardness testing is only of use when you know what the material being tested is made of and what the hardness requirements were.

Here, we have a de facto question of "is this safe to use?". The answer is "none of us know, but, should you contact the manufacturer, he can tell you--maybe". This is not the type of question which can be examined, without access to the part, nor knowing whom has manufactured the part, that can be answered on the internet.

As to the concept of "flame hardening", may I offer up the concept of "flame softening"? Having thought about this subject today, keeping in mind that I hold a degree in a related area, along with an instructor's certificate, I suspect that the "flame hardening" that others have posted about was a "drawing" process which rendered that area of the steering gear less hard rather than more hard.

Les
Les Bengtson

OK, Les - I'll bite - why would any engineer specify a process that would leave the a part in a critical wear area less hard? Unless - that is - we are talking about two different types of hardness - wear vs. inherent strength.

The drawing process you describe would certainly make the rack less brittle, which I can see as an advantage in the crucial center teeth, where a broken tooth could lock up the steering at the worst possible time!But the manufacturing process would be much more complex, involving rough tooling, heat treating, final grinding, then toughening by drawing.

Wouldn't it be easier to use a low-carbon alloy, then do all the tooling at once, and finally use a carbon-rich flame to add some surface enhancement to the part? More like case hardening a hammer or trigger from an 1851 Colt, as opposed to building a mainspring from high-carbon steel (which involves the "black magic" - to me - of hardening and drawing).

I wish we had someone from the OEM who could 'splain what their thinking was at the time.

Your idea of questioning the repro manufacturer has merit, but if the part was "bought many years ago", I'll bet that venue is now closed. I see it as more of a, "You PAID yer money, NOW ya takes yer chances." <grin>

Best of luck, CJ - - Alec

Alec Darnall

Wow!

I had know idea this query would start such involved discussions.

Thanks for all you input, the one thing I that I have noticed is there is a blue heat mark area in the centre of the rack - which appears to be good news.

But as you rightly say I've paid my money so I'll take my chances (keeping an eye out for any play in the steering)

Thanks again

Chris.
C J Bond

FYI

Case hardening is any heat treating process that selectivly hardens the surface of the piece in question relative to the core. The objective in all cases is to have a hard surface for wear resistance and a soft core for impact resistance. Gears and some bearings are clasic example of case hardening.

The two most common types of case hardening.

1. Carburizing. In this process a low carbon (not necessarly plain carbon steel it could have Cr, Ni, Mo etc) is heated in a furnace with a high carbon potential (either due to the gas mixture or in a carbon packing) to defuse carbon atoms into the surface of the piece in question. Now you have a piece that upon hardening and tempering will have a soft core and hard surface.

2. Flame hardening or now induction surface hardening. Here the part in question (made out of a higher carbon grade of steel than for carburizing) is quickly heated with a torch to only heat the surface of the piece (and potentially only a select portion of the surface). The same thing can now be produced by selectivly heating the surface or a selected part of the surface with an electric induction coil. In both cases the part is then quenched and tempered. This will leave a hardened surface and a soft core. (the core didn't get hot enough to transform to austenite) Flame hardening is no longer a common practice, it has been replace by induction.

Hope this helps

Russell
R Egge

In at least some of the shop manuals - I am looking at a Magnette one - it explicitly states that the teeth in question are hardened. Sometimes you find a case where the heated area was drawn or softened, as in gearbox shafts which are hard all over, except where the threaded ends have been drawn via induction heating,
FRM
FR Millmore

This thread was discussed between 16/02/2006 and 18/02/2006

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