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Late TR6/J overdrive gearbox rebuild - previous rebuilders bodge/mistakes!


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Hi Guys. 

I purchased a late TR6 gearbox fitted with a 'J' overdrive to fit into my TR4A, serial number CC76461, always  intending to strip and rebuild.

Initial inspection by removing the top box indicated that it had definitely been rebuilt before as evidenced by the amount of surplus silicone sealer oozing from all the joints, but the gears seem to be good. The oil thrower on the Input shaft was however badly worn and loose. The magnetic drainplug had lots of shards of metal attached to it, clearly from this oil thrower.  

When splitting the overdrive from the gearbox, it became clear why the gearbox had been removed from the donner car as of the 4 nylock nuts holding the two Piston Bridge pieces, 3 were hand tight and one was in the bottom of the box (see Photo).This had allowed one of the operating pistons to push out of the bore past the O ring! However a total strip down of the J overdrive revealed no damage and it was able to be rebuilt with new seals etc.

Next I stripped down the gearbox. The first thing  I noticed was that the spacer behind the Circlip holding the gear onto the first motion shaft was missing. This was the reason why the oil thrower had been destroyed as it was not held onto its location step on the shaft!

Next pull the main shaft and Lay shaft and gears. Amazingly all the gears were in excellent condition as was the layshaft.  The main shaft was then stripped of it's gears for inspection. All seem to be in good shape - initially!

Next check the end float between 2nd and 3rd gears by temporarily fitting the two steel bushes and the existing adjustment washer and securing with the reversed splined securing washer and old clip, the end float was 0.063 thou. Should have been 0.003 - 0.009 thou. What! However when assembling with the gears, 2nd gear was too tight, no clearance!

After much trial and error I realised  that the previous rebuilder had substituted the 2nd gear for an earlier one. The later gearboxes use steel bushes, the earlier gearbox's brass bushes. The bush lengths are different, the steel bush is shorter not not protrude through the earlier 2nd gear therefore the 0.062 clearance found. It had been assembled using  a thinner earlier adjustment washer that was bearing on the counterbore in the incorrect 2nd gear, not the steel bush. 

Whilst it could be made to function like this by reducing the thickness  of the earlier adjustment washer, not sure how it would effect the life of the synchro rings?

I therefore need a late a late 2nd gear compatible with the later steel bush. so if anybody has one and can help that would solve my problem.

Thanks 

Rich

 

 

 

 

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Missing photo of J overdrive when removed.

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Hi Guys, Third paragraph of the original script, 'gear' should read 'bearing'. Rich

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Hi Guys,

Having had to bite the bullet and buy a new 2nd  gear from Moss at £107 + postage.

I now have an excellent pre 1974 TR6  2nd gear available to sell to offset the cost (picture above).  According to those in the know, good original 2nd gears are getting difficult to find as are the later gear, as I have found out.

Currently these are on backorder at Moss and sell for £137. 

PM me if your interested.

Thanks

Rich

 

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Hi Rich,

I may be poking my two-pennorth in unnecessarily, but be careful when interchanging gears and steel bushes. It's a few years ago now so I don't remember which fits with what, but the oil grooves in the steel bushes only suit one of the two types of gear. Some gears have an oil drilling from the actual gear teeth through to the bore, and some gears have a drilling from the spigot through to the bore. The grooves in the steel bush only line up with one type of gear, and sadly I don't remember which it is. I do remember that I replaced a brass bush with a steel one which was claimed to be suitable, and the bush seized up on the mainshaft. Fortunately it didn't lock up the rear wheels but stripped all the teeth off third gear. I had to replace the mainshaft; the third gear and the bush, with a brass one. All has been fine since over around six years and fifteen thousand miles.

Cheers,

Deryck

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Hi Deryck,

Thanks for your concern, but all I am doing is replacing an incorrectly fitted 2nd gear which is  only suitable for use with the earlier brass bush and should not have been used in this gearbox by the previous rebuilder! The later style 2nd gear, post 1974 that I have purchased after discussion with Pete Cox will put the gearbox back to the correct fitment of a steel bush, only suitable with the later 2nd gear, and thicker adjustment washer. All should be good.

Thanks 

Rich

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It does seem that rebuild quality is variable..

I had a 'box rebuilt by a well known supplier about 5 years ago. lasted about 15,000 miles before dieing during the RBRR. Got home safe but very crunchy gearbox. Had it rebuilt by Mike Papworth and he discovered that the main shaft had not been replaced in the previous rebuild. Some of the bearing surfaces had been machined and sleaves fitted but the remaining surfaces just broke up.. He rebuilt the 'box with new shafts etc and fingers crossed it is OK (survived the 10CR and supercharger unlike the halfshaft UJs :-)

Tim

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