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MG MGA - One for the gearbox experts

I have rebuilt a second gearbox and installed it in the car. It has all new synchros, plus a B second gear and appropriate steel synchro ring. All bearings and seals are new, all gears are in great condition. Layshaft and bearings are also new. All new springs and detente balls to finish.

Most of the time the car will shift smoothly through all gears, and the gearbox makes no unusual noises when driving, but there is always a graunch going from 4th to 3rd. Reverse works just fine.

But the car sometimes will go into second gear and then 20 yards later will jump out of gear and refuse to go back in. In this situation it doesn't like going into third either.

Before I rip it out and stuff the (also now rebuilt) original box back in, any ideas what may be wrong?

I've done about 100km on the new box in case it was just a case of everything needing to bed in, but it's had no effect.

Not really wanting to switch the gbx again...... so grateful for ideas before I do (and in order to rectify what will then be the spare)




dominic clancy

The jump out of second could be caused by interferance at the shift lever from the body or rubber boot. Since a clutch problem would cause other gears to graunch. It may be that while the gear looks good the syncro mating serface is worn under size. There could also be excessive lateral movement of the gears due to worn/whrong thickness thrust washers between gears on the 3rd motion shaft. I think you will have to pull it.
J Heisenfeldt

I drained the oil in the box tonight. As I took the drain plug in my fingers, it was holding a chunk of a brand new synchro ring!

engine is now out, just have to jack up and remove the gearbox support bolt and the speedo cable, and then I can whip out the box.

Fortunately I have already redone the original box, so I should have it running again by lunchtime tomorrow.

It will be very interesting to see what has happened. The synchro appears to have failed immediately. I am wondering if it is a substandard batch. They came from NOS locators on ebay, so be wary if you have also bought a set there.
dominic clancy

Dominic, Was the ring the steel one or a brass ring?
James Johanski

Brass. The ring has lost one of the bosses- it has just sheared off, and the rest of the ring is still sitting where it should be
dominic clancy

here


dominic clancy

Dominic, Don't know if this helps, but here goes.... I bought a 53 TF some years ago, and that jumped out of 2nd. I took the box out and removed the inspection plate for a look. I could see that the baulk ring was engaging just on the very end of the 2nd gear wheel, but couldn't see why. So when the shaft moved a fraction, it would disengage. I thought the wrong gear wheel or baulk ring had been fitted at some stage, so I decided to strip it down and see about getting replacements. It was only when I had it all stripped out on the bench that I realised that what had actually happened was that whoever had assembled the box had overtightened the big nut that holds the drive flange onto the output shaft, thus drawing the shaft, containing the 2nd gear wheel, slightly too far to the rear. The movement of the baulk ring is lmited by the selector rods, hence the reason that it would only grip the very end of the gear wheel. Had I known that, I need only have loosened the drive flange nut, and tapped the shaft back into the box, which I could have done with the box in the car by dropping the front of the prop shaft, and saved myself a lot of work. When I reassembled it, with correct tolerances, it never slipped out of gear again. Different gearbox I know, with a side inspection plate rather than a top one, but you never now, there might just be a simple explanation. Adrian B.
A Bennett

Dominic,
There is an interlocking ring between the 2nd and 3rd gear bushings that also takes thrust and is a spacer. If this is damaged or worn or missing it will cause just what you have described--incomplete engagement and poor synchro performance.
See item 103 here:http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29221#103
Cut and paste the link should work. If not, go to the Moss USA site, Gearbox exploded parts view.
James Johanski

Thanks for the suggestions so far. Car is now running with its original box.

James, the 103 ring is present and showed no sign of wear at all.

Adrian, this is a box with a splined tail, so no nut on the end. However, I didn't use locktabs on the output shaft, used threadlock instead. Maybe the absence of this acting as a pacer could also produce what you describe.

What I don't understand is why a synchro lug has been completely ripped off its ring, and where it was is now just a gap in the synchro ring. The oil was full of little brass twinkles, so there may be more damage than I can see through the side plate.
dominic clancy

I once had an MGA gearbox that would pop out of 2nd gear on overrun, when you take you foot off the throttle at speed, as with engine braking. That turned out to be a 3rd gear thrust washer come loose from the front end of the mainshaft. This would allow the gears on the mainshaft to float forward slightly in response to the thrust load of the helical gears on overrun. This forward movement of the gears was enough to drop out the engagement of the small teeth between 2nd gear and the mating sliding hub.
Barney Gaylord

I have taken the box apart again. No damage apart from the missing lug on the synchro ring, which appears just to have failed - no other damage anywhere. When I took it apart, everything else seemed to be in the right place and mating correctly.
dominic clancy

This thread was discussed between 16/04/2008 and 21/04/2008

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