QuestionI have rebuild several TD gear boxes in the past and now want to improve on an already good thing. One thing I notice common to every trans I've gone through is that the lay shaft is usually well worn, especially where the rollers touch the oil cut out area of the shaft. I was wondering if you have any recommended modifications or advice on improving this area?
It seems reduced wear in this area would greatly aid in quietening the 1st gear noise.
AnswerHi Richard. You are correct. The slots in the layshaft slightly overlap the ends of the needle rollers, which seems to cause premature wear in the shaft. Perhaps a shaft without slots would be an improvement. After all, oil can easily run between the needles.
(Note that the original needles are imperial, 5/32 x 1 inches. They are NOT metric. Using 4x25 metric needles is a common mistake, but they are slightly oversize and this will cause the layshaft to fail.)
Personally I don't think the layshaft on the TD is strong enough to cope with the power of the engine. If you look at a TC gearbox you will see that the layshaft is much larger in diameter, and TC gearboxes last MUCH longer than TD gearboxes.
Also, the needle rollers themselves are too large in diameter and they rub against one another.
If I could re-engineer the gearbox, I would fit a larger diameter layshaft and delete the lubrication slots. Using cages would solve the problem of rollers rubbing against their neighbours and the slots between the rollers would allow even more oil to get through.
If the cages had 3mm needles, then the layshaft could be increased in diameter by about 2mm. The gearbox casing would need to be line-bored to accept the larger diameter layshaft.
By the way, the reason that first gear (and reverse) are noisy is because these gears are straight cut.
