WARNING: This is gonna be _long_!
On Mon, 2 Jan 1995, Bill and Heather Putnam wrote:
> I have an old (1939) MG that I am preparing for vintage
> racing.
Hmmm. MG produced several models in 1939. Based on the rest of your
description, it's either a TA or a TB Midget rather than one of the
Saloons. Right?
[long description of steering box mods deleted]
The mods you described have been used on the TA/TB/TC Bishop Cam steering
box for many years. If done right, it improves steering ease and accuracy
very noticeably. If done wrong, it's a mess.
> My concern is that I know of some parts that have been
> chromed for cosmetic purposes that failed due to hydrogen
> embrittlement. I don't know if this hard-chroming process
> is the same, or if it does not have this problem. Is anyone
> familiar with this, and are there steps that can be taken
> while chroming that reduce the likelihood of hydrogen
> embrittlement? This is very important since if this shaft
> did fail it would likely be fatal.
Yes, it would very likely be fatal! Hydrogen embrittlement is just as
much a danger in hard chroming as in decorative chroming, but in both
cases it can be eliminated if the shop doing the work knows its stuff.
Hard chroming is a commonly used way to build up worn shafts and/or make
them more wear resistant. Since strength is typically of concern in
such applications, any shop that does industrial hard chroming _should_
know how to prevent hydrogen embrittlement. If I remember correctly, it
involves baking the newly-plated part for a fairly long time to drive
out the hydrogen...
I'm less concerned about hydrogen embrittlement than I am about other
factors in the modification.
In the original design, the entire length of the sector shaft bore acts
as a bearing, albeit not a very friction-free one. Most of the wear
takes place at the pitman arm end, but it is spread out somewhat because
of the length of the "bearing". When you bore the housing out and insert
needle bearings, you introduce several potential problems:
(1) The clearance between the gear and the sector shaft is rather small
(notice that the case is cut away between the gear and shaft to provide
clearance, and the cut goes right into the sector shaft bore). In order
not to interfere with the worm gear, you have to use fairly short
bearings, so the load on them and on relatively small portion of the shaft
that is within the bearings is fairly high.
(2) There isn't a whole lot of meat around the sector shaft bore, so you
can't bore it out very far. This limits the outside diameter of the new
bearings, which in turn limits the diameter of the rollers in the
bearing, which in turn limits the strength of the bearing. Since the
bearings (esp. the bottom one) will be under high load, the undersized
bearings may not be up to the job.
(3) The hard chromed shaft will be more resistant to wear than the
original, but if the plating is too thin it may not be very crush
resistant under the localized loads imposed by the needle bearings because
you have a very hard but relatively brittle and crushable skin over a
softer base. If the hard skin is too thin, then the very high pressure
from the relatively small bearing area of those short needle bearings can
crush the chrome skin, ruining the shaft and probably ruining the bearings.
I have seen this modification work very well, and I have seen it fail
miserably. Unfortunately I don't know what caused one to succeed and
another to fail. My own inclination, even though I used to advocate this
mod, is not to do it.
If the shaft is worn, and if you can't get a new or unworn used one, then
by all means cut it down and have it hard chromed. Just be sure the shop
doing the work understands what it is used for and can calculate how
thick the chrome must be to hold up.
If the bore for the shaft is worn, have it bored out oversize and sleeved
back to standard. The sleeve will wear, of course, but can be replaced if
it does.
To reduce some friction in the steering box, install a Tompkins Kit
(still available from Moss Motors, I think). This places a needle thrust
bearing at the top of the sector shaft, eliminating the friction between
the sector arm and the steering box cover. Some say that this is
dangerous because the outer end of the sector arm is no longer braced
against thrust imposed by the worm gear, but thee Tompkins Kit has been
around since the late 1940's and I'm not aware of a failure that can be
attributed to it.
Replace the cam follower (pressed into the sector arm) as a matter of
course. Replace the worm gear (cam) if it is worn. Worm gear wear is
always greatest near the center, since that is where the follower rests
most of the time (assuming the car is driven straight aghead most of the
time). If you try to adjust for minimum play on a worn worm gear in the
straight ahead position, the steering will bind when you move away from
that position. If you adjust to prevent that binding, then the steering
will be too loose in the straight ahead position.
Go over the rest of the steering system to get rid of as much slop and
play as possible. Replace the tie rod balls if they are at all worn.
Replace the king pins and king pin bushings, and the thrust washer
between the axle and the steering knuckles (spindles). Take the car to
an alignment shop that knows how to bend a solid axle to correct
alignment problems. You'll probablt have to go to a shop specializing in
trucks.
DO NOT use needle bearings to replace the king pin bushings, and DO NOT
use a needle thrust bearing to replace the bronze thrust bearing. Doing
so removes a remarkable amount of friction from the steering. However,
unless the front end alignment is perfect, and unless the wheels run
perfectly true and are in perfect balance, and unless there is absolutely
no free play anywhere in the system, then the result will be a car that
is impossible to drive in a straight line. The original bronze kingpin
bushings and thrust washer impose enough friction to act somewhat like a
steering damper, counteracting the tendency of the wheels to go off in
odd directions.
Also DO NOT be tempted to use the caster-adjusting shims used towards the
end of TC production. Reducing the caster does lighten up the steering
somewhat, but at the expense of directional stability.
-----------------------------------------------------------------------
Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO (daily driver)
fold@mail.bcpl.lib.md.us
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