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MG TD TF 1500 - TD gearbox rebuild

I rebuilt my late TD gearbox as part of my restoration. I replaced the layshaft (not the laygear), 1st gear and sliding hub, and all bearings, bushings, shims, and seals. It is still jumping out of 1st gear under load. The small gear on the laygear was deemed worn but serviceable by a mechanic but now I have doubts. Another suggested that the new 1st gear may not mesh as well with the worn laygear as the worn one did. Finally, the new sliding hub (the inner part of the 1st gear on the splined shaft) is slightly shorter on the shaft than the old one, allowing it to slide nearly 1/8" back and forth on the splined shaft. The old one had some movement but not as much. Is this part of the reason for jumping out of 1st gear? What is the factory spec length for this inner hub? Any ideas will be welcome.
B Roseberry

Did you replaced the " spigot bushing on the flywheel ?

A worn bushing could be the source of the problems you discribed .
Gerard Hengeveld

Gerald,

How does a worn pilot (or spigot) bushing effect the trans in such a way to cause it to pop out of first gear?

Alex
Alex Waugh

Was it jumping out of 1st before you did the rebuild? If so, it is hard to see that the little extra movement in the new sliding hub could be the cause this problem. Since there must be some lateral force on the gears to make them slide out of mesh I suspect that the small gear on the cluster has teeth that are tapered along their length. Unfortunately it is impossible to have a look at the cluster unless the gearbox it totally dismantled.
Cheers, Hugh
H.D. Pite

Alex, Hugh gave you almost the answer or better possebilety's severe play on the gears ( first motion shaft )COULD be the culprit . But as Hugh stated you have to take the gearbox apart to investigate.

Gerard
Gerard Hengeveld

B, are you sure the gear mechanism is fully engaging the gears? i.e. all the slop in the remote shifter, shifting rails and forks is taken up? is there significant shift lever movement when it pops out of first? regards, tom
tm peterson

The remote shifter was rebushed and seems smooth. The selector forks seem pretty snug in the striking dogs. The 1-2selector rod does seem to want to move into neutral but only when the top detent ball is not under pressure. When under pressure from the spring, it seems reasonably fixed. The bushing at the back of the crankshaft was replaced during the earlier rebuild. The TD has only several hundred miles since then. So far, the only obvious things to me seem to be the fore/aft movement of the inner 1-2 slider on the third motion shaft splines or wear on the small gear of the laygear. I did find that the nut on the flange which connects to the driveshaft was a little loose, allowing some minor fore/aft movement of the flange at the back of the gearbox. Again, thanks for your ideas.
B Roseberry

The shift lever does move toward neutral when it jumps out of gear.
B Roseberry

This is a very common problem. My car did the same with new cluster and first gear for years under hard acceleration. Make sure that the detent ball/spring under the rear of the top cover is engaged and centered in the notch in the 1/2 shift rail when in first. If not, something in the mainshaft is mis-shimmed or out of position. If it is, then the "fix" is to add an extra detent ball/spring. Search archives, details in there somewhere. There is a slotted screw in the rear tranny housing, I think on the right side and adjacent to the row of detent balls and springs. Remove the screw, shift into first, use a thin punch and make a dimple in the 1/2 shifter rail. Then remove the rail (don't let the detent ball bearings fall out into the bottom of the tranny!), and file/grind a detent notch into the side of the rail (match the contour of the top ones). Reassemble , put ball bearing into the hole, and fit a detent spring (you will have to shorten it a bit) and adjust, put the screw back in. This give first an extra detent in first only, and cures the popping out. I think John Twist does this on all of the TD trannys he rebuilds. George
George Butz


Is the shifter fork worn?

Ed
ECS Stanfield

George, I'll look for that slotted screw tomorrow when I get back on it. I believe I understand the concept but it will be clearer when I'm actually looking at it. I do think the existing top detent is correctly located under the spring & ball.

Ed, when I wiggle the selector shafts fore & aft, there isn't any noticeable play in the forks. I think the forks fit the striking dogs within tolerances.
Thanks again for the suggestions.
Bill
B Roseberry

I second the diagnostic from Hugh? That there is wear on the first gear cog on the layshaft cluster. If the teeth are worn to a taper then you will get a component of the force pushing the first gear forward. The only thing preventing the assembly from sliding apart is the detent ball and spring, if the teath are worn then the ball and spring are not strong enough to keep the gears from separating. If the teeth are pefectly parallel then the separating force will be near zero, in that case the detent ball and spring is capable of holding the two gears together.

John

J Scragg

Thanks for all the great suggestions. I'll be tearing into it again tomorrow and be back here when I have the laygear out.
Bill
B Roseberry

Note my comment: mine would come out of gear with a brand new cluster and first gear, so that had absolutely to do with wear on the teeth! Really search the archives for this, as it has been thrashed around before in detail (which I don't remember). Adding the extra detent and spring may take an hour, not a bad idea to try that first IMHO. George
George Butz

George,
I pulled the 1-2selector rod and added the described 1st gear detent. I had spare ball for the application but I need to get a spring. Looks right and makes sense. Guess I'll find out if it solves the problem when things go back together. Back on this site when I know.
Thanks all.
Bill
B Roseberry

Back again after getting everything out of the gearbox. The first motion shaft and bearing came out cleanly by tapping on the back of the third motion shaft. Laygear had been lowered to the bottom of the box to clear. I was at a loss to get the third motion shaft and it's bearing out. It was tight in the case and snug on the shaft. I Finally figured that what worked for the first motion shaft should work in reverse so I gently tapped on the front of the third motion shaft w/ a brass punch and lead hammer and the bearing gradually came out of the back of the case. It was still stuck on the shaft so I offset it from it's hole in the back of the case and tapped on the rear of the shaft until it popped loose. I supported the front of the shaft while doing this and there was no contact with the laygear. It seemed a lot easier than trying to pry it out with all the gears in the way.
Attached is a picture of the laygear. These are the worst teeth, most are similar to the lower 2. Is it salvageable or am I screwed?
Bill

B Roseberry

Do not hesitate to give John Twist a call. He answers questions during their lunch break. He does transmission rebuild workshops and would most likelygive you exactly what you need to figure this out.
Bruce Cunha

john has a gearbox rebuild seminar coming up in the next few weeks i believe. regards, tom
tm peterson

I've worked on a lot of TD/TF gearboxes, as an amateur. The odd thing is that the TC box was sought after by dirt track racers for V8's, while later boxes are lucky to stand up to ordinary use. My idea, for what it's worth, is that the long rear shaft is the problem - it flails around. So, I'd recommend close attention to all the bearings that keep the two - piece shaft in line.

David.
D A Provan

This thread was discussed between 16/02/2014 and 24/02/2014

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