> Are the serial numbers different for OD's?
Yes. The S/N on the OD is for the OD, has nothing to do with the S/N on the
gearbox.
> I have just taken
> possession of a
> transmission with an overdrive. It came out of a junkyard many
> years ago and
> sat at a friends house. The tag the dealer put on it said it came from a
> TR4a with 20,000 miles on it. The serial number is on a plate on the OD
> unit, says Laycock and has the numbers 22/61753/ 009353. On the trans
> unit are casting numbers 306812SM and V2947.
According to the magic decoder ring in the Moss catalog, model 61753 is from
a TR4A IRS. It will engage softer than the stock TR3 OD, but should be
otherwise equivalent.
> There are three switches on the top plate of the transmission, can anyone
> tell me which one is for what?
The positions varied over the years, I don't recall which one is which
offhand. However a continuity tester or DVM should quickly identify them.
One will be closed only in reverse, it's for backup lights. One will be
closed only in 2nd gear, it's an OD lockout. The remaining switch will be
closed in 3rd or 4th, it's the other OD lockout. If you're not replacing
the switches, I'd recommend testing them under load before you install the
tunnel.
> And most importantly, will I have any trouble installing it to my
> TR3A with
> a bullet nose starter?
The 4-synchro gearboxes are a bit longer than the TR3 crash box, you will
need to drill new holes in the rear crossmember to avoid straining the motor
mounts. Also if your car is pre-TS50K (in other words the bullet nose
starter is original), then you will need to do some sheetmetal work on the
trans tunnel, so it will clear the larger starter bulge on the later tranny.
If memory serves, the TR4A had the later 'diaphragm' clutch, which took a
shorter front housing on the gearbox. Although possibly not strictly
necessary, I would suggest changing the front cover to the longer one from
your old gearbox.
Also compare the thickness of the flange at the top of the bellhousing. It
got thicker over the years, although again I don't recall what the change
points were offhand. If the new one is thicker than the old one, you may
need to change the 3 studs in the engine. Might also need longer bolts.
You'll probably want to use the shift lever from your old gearbox, now is
the perfect time to be sure the spring and plunger at the tip are in good
shape.
You may also want to consider the loss of the dipstick ... the TR4A box will
have a plug on the side to check the oil level. It is possible to rebuild
the early TR3 top cover with the TR4A shift rails, forks & OD switches. Up
to you of course, but that's what I did. Might also want to check the
synchro rings for wear, which can be done by just removing the top cover.
I would also suggest replacing the pilot bearing in the end of the crank,
and making sure the alignment pins are present and in good shape.
Randall
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