Transmission housing cracks
A few years ago we were
helping a vintage racer with his TR-3 and I had the opportunity
to race his car on several occasions. On one of these the gearbox
began to crack where the bell housing joins the housing for the
trans. This was the second or third case that had cracked in this
car. The power output of the engine was about 135 to 140 hp, so
it was not an engine that would overtax the chassis. In driving
the car, I noted that the more cornering force that was applied
to the chassis the more the trans. would make nasty noises. I
concluded from this experience that at least part of the problem
with the case failure was probably due to torsional loads transmitted
to the engine and trans. during cornering, caused by the twisting
of the chassis. The engine trans. combination is supported at
two points at the front mounts and essentially two points at the
rear of the trans. At least if the rear trans. mount is a good,
stiff new mount that is so. I would bet that redesigning the rear
mount to have a single central mount under the trans. that would
support and locate the rear of the trans, but at the same time
not resist the twisting torsional loads that would otherwise be
fed into it by the rather flexible chassis , would also solve
For those of you that insist on flogging your TRs, a little TR3
history, and my solution. Quite a few seasons ago I had a series
of races that I successfully completed with the smell of gear
oil. I was cracking the main case, as well as the tail housing.
Because I inspected the gearbox after every track session, it
never caused a DNF, but it was expensive and time consuming. The
balance and truth of the motor parts, gearbox parts, drive shaft,
and rear axle flange were all blueprinted with little improvement.
We tried brand new cases/tail hsgs, and good, used cases/tail
hsgs. We made certain that all important surfaces were flat and
true. Tracks with bumpy, banked corners were the worst. Willow
Springs was one of the worst, but our lap record set with a cracking
rear case held for a amazing number of years, and the 914 finished
I consulted the GCR & PCS and Tech. I rejected the extra
mount at the rear of the motor block because of weight and too
much added time to R & R the motor or gearbox. Friend and
fellow TR3 racer, Dennis Kelly,(C & D Engineering) came up
with some stout aircraft cables and turnbuckles that I fitted
with considerable thought re the gearbox centerline, on each side
of the gearbox. They ran from a special top starter motor nut
and a lower, special gearbox to block fastener, to the tail hsg/rear
mount interface. The cables were tensioned and safety wired, and
the problem went away.
We had learned about the better main cases and used these all
along. It was most convenient for the racer that Triumph never
changed the main case part number on the all synchro box!
Triumph had some brilliant people.
The discussion of bell housing breakage in early TR4s is very
timely for me... I'm ready to put one in a car but have stopped
till I figure out what to do to minimize the possibility of cracking.
Kas has explained about the "gong" device they used to
use, and Nick (at least I think it was Nick) has recommended welding
braces from the bell housing to he tranny. Another "old" TR racer
I talked to today said the problem was probably caused by the
combination of engine and tranny flexing, since they were supported
only by the front motor mounts and the tranny mount at the rear.
Without support in the middle, the flexing would break the bell
housing. He said he used to install motor mounts at the rear of
the engine to eliminate the flex. This seems to make sense, but
I wonder if anyone else has done this or is there some reason
this wouldn't take care of it.
Indeed we had problems with the transmission/bell housing cracking,
but, remember we were using the engine and CAR at the highest
amount of usage, I.E. high revs for at least thirty to forty-five
minute races. This is 6500 revs for a good long time. As you have
surmised there is a method of aiding the situation,the mounts,
and indeed this does help.
We did make mounts that supported the junction of the engine
to the transmission. We used small rubber engine mounts with long
strap iron brackets that came down to the bell housing bolts and
the mounts were on each side of the battery box. This did make
a positive difference. But, we never had this problem until we
started making real power, that is above 150 BHP. We broke at
least ten or fifteen of the gearbox housings until we made the
hangers with rubber mounts . We still cracked the casings , but
the whole business at least held together for the length of the
event. At one time I also welded side plates onto the the gearbox
case but they also cracked. This was probably because they were
very soft after being annealed by the welding. I think I might
have worried a lot of people without cause for most will probably
NEVER use the cars and engines as hard as we did at that time.
I'll try to answer any comments on this.
About the gearbox casing. The early cases did NOT have heavy
ribs along the left side. The later parts DID have very heavy
( wide ) ribs running along the side. If you can find the later
heavy ribbed type you have a winner. We had incredible trouble
with the early types breaking. We finished a couple races with
about an 1" gap between the bell housing and the gearbox case
itself and oil leaking from everywhere. To help this until the
new parts arrived the factory installed a thing we called the
"Gong" under the rear portion of the early types to help with
the vibration problems. If you have one with the "Gong" don't
just throw it away put that darn thing on it does help.) This
"gong" is a weight about 3" in diameter and fitted under the gearbox.
Probably everyone knows this, but it just came to mind.
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