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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 the problem.

Regards,
Greg Solow

 

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 second.

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.

Harvy Printice

 

FOTers:

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.

Thanks...
Don Marshall

 

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.

Kas Kastner

 

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.

Kas Kastner

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