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MG MGB Technical - MGB Mk1 gearbox failure

Renton's racecar gearbox went bang the other day - here is what we found. Has anybody alse had this happen?
Garth

Garth Bagnall

I've heard of it happening to factory V8s, I belive mine has a modified 4-cylinder shaft (lead-foot PO) as the originals weren't available at the time.
Paul Hunt 2010

It was the weak link in the factory V8 transmissions. There were several reports of drivers pulling away from stop lights only to loose all power to the rear wheels. The input shafts were suspect from the beginning of manufacture and failed due to the increased torque of the V8 engine. RAY
rjm RAY

That is a 3 synchro mainshaft - the V8 comments don't apply to these as the V8 had a 4 synchro box.

But yes this does happen to the 4 synchro mainshaft too - usually they snap just behind the front securing nut.

in this case it has snapped in the lock ring securing groove.

I don't think these failures are power related - in both cases the broken off bit takes no power! It is likely to be a vibration issue as the input shaft needle roller bearing runs on the nose of the mainshaft and any wear in the needles or input shaft bearing will cause high frequency radial flexion of the shaft which, being induction hardened, eventually snaps off.

In your case I can see what looks like surface breakdown on the rear part of the tip and I would bet there is similar breakdown on the rear of the needles.
Chris at Octarine Services

Chris -
Gotta argue on this one, the busted bit takes all drive in 3 & 4, through the 3/4 synch hub.
Failure at the ring groove is (as you say) a fatigue failure, but only minimally and secondarily due to needle wear. The needles carry little to no load IF all the other bearings are good. Discoloration in the bush section indicates a lot of heat, and the spigot bearing wear indicates out of alignment running under load. That's exactly what I'd expect in a box with bad bearings and/or being run hard at and above design limits. Or, a homourable death for a racing box, RIP.

FRM
Fletcher R Millmore

Hey Fletcher, whoops! yes you are right re the 3 synch but not on the 4 synch shafts that snap in front of the 3/4 hub.

I wasn't suggesting the needles do carry load - but a bad bearing on the input shaft will allow off centre pressure on the needles, the break up is a result not a cause and it is the flexing that snaps the shaft - not the power going through it.



Chris at Octarine Services

Just to add - the blueing in the bush area is odd - the bronze bush is keyed to the shaft and doesn't rotate on it - the gears rotate on the outside of the bush. To create enough heat to blue the shaft under the bushing would surely have melted the bronze!
Chris at Octarine Services

Blue is around 650-700F, bronze goes way over that, The heat was from the interlock thrust washer being extremely heavily loaded as things were going all goofy.
I've done a thousand 3 synch boxes, but I've actually only had one 4 synch gearbox apart, way back when they were new, like 1973. They are good enough that they lasted well past when the cars were actually being used as cars here. No factory V8s here either, and the usual swap uses Rover boxes. Now when they break, parts are impossible here, and gearboxes are all over the place as doorstops etc, so I can't imagine trying to rebuild one!

FRM
Fletcher R Millmore

Well here is what happens to the 4 synch shafts - I have seen 3 of these in a 100 boxes or so - the number of 3 synch boxes I have seen number fewer than 10 - they are hens teeth here too!

Chris at Octarine Services

Chris -
Interesting. They managed to screw up a perfectly good design. Take all the bad features of all known examples and combine them = recipe for extinction.
This is a simple stress concentration through design error, probably exacerbated by poor control of the zonal induction heat treatment for thread and bearing areas. The cracking is from stress at the thread root in too hard material, and is mainly a function of initial torque load. Any misalignment stress would certainly add to it. I'd guess that failure is more common on OD boxes, since Non OD have the support of the rear bearing, but OD get the same short shaft/wobbly bearing as the transverse cars.

FRM
Fletcher R Millmore

Actually the OD 4synch mainshaft is probably better supported than the non OD one - the OD box has a bigger double bearing in the centre, then the shaft is supported by the cone clutch which runs on a large diameter bearing, then the OD sun gear, then the sprag clutch assy and finally it is bushed inside the annulus.

All failures I have seen are in OD boxes - but then Non OD boxes are even rarer over here than the 3 synchs. I have a guy offering me 2 OD boxes complete in exchange for one non OD box - I bet you wish you could get that deal over there!
Chris at Octarine Services

I'll take that trade! I have a spare 4 synchro non od trans and a 3 synchro non od trans in storage, just in case. RAY
rjm RAY

Well, all those bearings and bushes still add up to slack, but it sounds good. I'd don't have a good cross section to study. Quite likely that it is solely a machining/heat treat error. Do you have a pic of an unbroken shaft?

And here I am tripping over non OD 4 synchs, and I've seen people just scrap piles of them!

FRM
Fletcher R Millmore

Only have a pic of the front end -

Chris at Octarine Services

- and the back end! Lol

Chris at Octarine Services

Hi all,
The box had been apart 3 meets before to have a new set of close ratio gears fitted - made by John Needham, Melbourne - along with anew set of bearings. The over heating seen was caused by the car being driven in 2nd gear back to the pits & on to the trailer. This also pushed the broken off piece into the 1st motion shaft which caused the damage seen to the bearing area of the mainshaft.
Garth
Garth Bagnall

Garth -
Since I assume it was being driven somewhat gingerly in 2nd, and for a short time, I suspect that it was already blue, and from what I can see in the pic the bearing damage looks like severe misalignment failure, if it was not spalled on rebuild. But since neither of us was inside observing as it died, I expect we'll never know!

FRM
Fletcher R Millmore

Owner/Driver comment. Yes it was driven very gingerly back to pits in 2nd gear. Tongue in cheek of course, but I think it's Peter Burgess's fault. Too much power from his head. Just to clarify..it was a 3 synchro non OD box. Freshly rebuilt by Garth.It actually managed 9 meetings before giving up. Question now is should I change to 4 synchro...I have the box...with or without OD.
R F Murray

I'd change to the 4 synchro. It is a much more robust unit with many improvements over the 3 synchro. I've been running a 4 synchro/od trans in my '67 for over 25 years now with no problems. For the last 7 years I've been running a supercharger. The original 3 synchro trans failed after only 43,000 miles. RAY
rjm RAY

This thread was discussed between 20/04/2010 and 25/04/2010

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