MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Transmission Problem?

I'm uncertain to the source of this sound. My guess its from the gearbox, but before proceeding with removal I'd like some feedback.

The sound (a deep, very easily heard rumble, most pronounced when under load) is only in 3rd gear and first appeared all at once at the present noise level. In all other gears the sound is absent and I can upshift and downshift throughout the gears without problems (including upshifting and downshifting into 3rd.) Could worn universal joints cause this sound? Only 3rd gear makes me think gearbox, but harmonics and virations can produce surprising results.

-Martin Straka
Martin Straka

Does it make the noise sitting still running in neutral? If so does the noise go away when clutch pedal is down and placed in gear? Input shaft bearing can be isolated this way. This noise is common to 1,2 and 3 but not 4 because there is no side load on the bearing in 4.
R J Brown

Only one gear, worse under load? Without more information, I'd guess broken tooth/teeth in the third pair.
D Sjostrom

I agree with D, a sudden noise in only one gear is very likely a chipped or broken tooth on mainshaft 3rd gear or the mating gear on the layshaft.
John H

R J Brown: No noise when the clutch pedal is depressed and moving the selector into 3rd.

D Sjostrom and John H: Seems likely although I can't think of any incident that would have broken gear tooth? Maybe metal fatigue from old age? (that's metal not mental)

What's the quality of gearbox replacement parts from our suppliers such as Moss and Scarborough Faire? Would it be better to purchase a used gearbox to install or use for parts?

-M.S.
Martin Straka

Martin,
hopefully someone with more gearbox experience will pipe in, but I recall reading some years back that mixing new gears with old is asking for trouble due to the wear pattern of the old gears not meshing well with the new gear's teeth. It may have been in my Bugeye factory shop manual. I'll check.
BOL,
Doug
D Sjostrom

Martin,

Good info from the other gents. You could drain the oil and have a look at it and hopefully no big parts come out. I little bit of shiney stuff is probabley normal, but lumps and lots of grey sludge is not that good.

good luck,
Mitch Smith

Was there noise in neutral with foot off clutch?
R J Brown

R J Brown: No noise in neutral with the foot off the clutch. Since posting this thread I've spoken with several persons who have experience with our MGA gearboxes and the similar response I get back is that this problem isolated to 3rd gear is most unusual. A better description of the sound is a constant rattle-rumble both under 3rd gear acceleration and foot off the gas deacceleration when in 3rd. Sounds like a loud small gas engine or portable generator some distance away. Would a broken gear tooth produce this kind of noise? It did not start gradually but happened at once. Its there when the oil is both cold or hot. -M.S.
Martin Straka

Yes, it is almost certainly a broken gear tooth. Been there a few times myself, and have repaires a few for others. A single broken tooth might make no noise at all, so it is more likely two broken teeth on one gear only. Three consecutive broken teeth will make it lock up, not driveable in that gear, been there too.

The noise is a loud clicking similar to dragging a stick against a picket fence but more harsh and metalic in tone. The clatter changes frequency relative to ground speed, and it will be louder with heavier throttle. It will also likely be heard on overrun, but less loud due to less torque applied.

One or two broken teeth usually does no other damage, so replacing the broken gear should suffice. Do not take this condition lightly. If it breaks a third tooth when under full load it could do much more damage.

Repalcing a single gear should be no problem. If that a problem was you would have to replace every gear in the transmission, because they all mate with the laygear.

I have broken a few gears, always a different one, just coincidentally I'm sure. One time 2nd on the laygear, at other times 2nd gear and 3rd gear on the mainshaft. I have always replaced the gears with used parts, and have never had a failure of any of the replacement gears (knock on wood).

I think the broken teeth are related to the number of loaded revolutions, meaning those folks who spend a lot of time racing in 2nd ot 3rd gear are more likely to encounter a broken tooth as the gears accumulate more loaded mileage.

I don't think it has much to do with age of the parts. While I personally put a lot of miles on a gearbox, I have never broken a gear which I had swapped in from a used gearbox. I would attribute this to the high probability that the replacement gears have accumulated far fewer service miles than the ones I had been running incessantly for decades (and may not have been used as harshly).

Considering I have never paid more than $50 for a used gearbox (and often less), I would definitely prefer used to new for the value comparison. I have one extra gearbox at the moment, but currently no spare 3rd gear or laygear.
Barney Gaylord

This thread was discussed between 12/05/2006 and 14/05/2006

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.