Correct Overdrive in GT6 Mk3?

[sbarc]

I've got a 1973 without overdrive  GT6......which O/D should be fitted to a GT6 Mk3?  J Type?

I'm pondering switching up to a O/D for my car.  From what I've heard and read an O/D is very much worth getting.

[Jason]

I think the answer is both - late GT6 had J type, early had D type, it may be related to the switch from Rotoflex back to Swingspring but I'm no expert here. Overdrive may be very expensive to find over in the USA, it's not cheap here and you'll need an overdrive gearbox as well, it's not just a case of find and overdrive unit and bolt it onto what you already have :-(

[Straight Six]

I'm not sure, but didn't the non overdrive GT6 have a higher ratio diff fitted? If so, another thing to consider before converting to overdrive.  

[Clive]

Disagree. All GT6 had d type, but some have been retro fitted with j types, mainly from early dolly 18/50 or as uprated units from various suppliers. Also seem single rail j type from later dolly 18/50 fitted, needs shorter prop than std od and a bit of fiddling with the bellhousing. Or of course the popular ford 5 speed conversion. This may be the best option? probably more reliable, and if the type 9 box is easily available the conversion parts should be fairly easy to source.
Diffs, certainly UK non od have the 3.27 against the od with 3.89. In fact that makes them give the same final drive. 20mph/1000rpm.
Clive

[sbarc]

Thanks guys.  Spitbits does stock rebuilt D type O/D's as well as rebuilt O/D gearboxes....plus O/D prop shafts etc.  I'd need to ponder the other bits and pieces.  I figure I'd be lucky to do it for $2000 (£1000)...which is quite a bit considering I don't plan to do many highway trips in the car.   ;D

I doubt I'd change the diff as  a long legged  car would suit me fine.

But it would be cool to have.....hummm.....

I've rebuilt transmissions int he past with great success.  How hard is it to rebuild an overdive unit?  

[Llessur]

Quote:

I've rebuilt transmissions int he past with great success.  How hard is it to rebuild an overdive unit?  

From what I've heard it's like staring into the pits of hell....

[don cook1]

You will need a fair number of parts. In addition to what has been already pointed out you will need a mounting plate and actual mount, you will need to cut back the tunnel to get at the prop bolts, a tunnel extention (Spit 1500) to cover up what you just cut off. A good diff to get is the 3:63: lower ratio in the gears and a bit higher in overdrive. All these bits add up!!! Provided you get a good reliable drive train then it's really worth it.

I have Ford 5 speed Type 9 fitted and only did it because I was fed up with Triumph gearboxs coming apart. Not cheap but reliable so far.  

[Clive]

IIRC canleys did/do an export o/d conversion kit, I think its and o/d, adaptor plate, wiring, mounts etc and the o/d mainshaft. You then fit this into/onto the non od box and get your prop shortened. May be a cheaper way?
Still think I would be tempted by type 9 conversion. Stronger, similar cost.
Clive

[thescrapman]

cliftyhanger wrote:

Disagree. All GT6 had d type, but some have been retro fitted with j types, mainly from early dolly 18/50 or as uprated units from various suppliers. Also seem single rail j type from later dolly 18/50 fitted, needs shorter prop than std od and a bit of fiddling with the bellhousing.

Disagree with your Disagree!

Some of the very last GT6's left the factory with J-types fitted.

I beleive they were just using up spare stock.

Cheers

Colin

[Clive]

Fair do's then. Just never seen them listed anywhere, and all the swingspings MK3's I have seen have had d types.
I realise some late mk4 spits had j types fitted with the 3 rail box, maybe the change was at the same time?

[sbarc]

Llessur wrote:

From what I've heard it's like staring into the pits of hell....

That pretty well puts it into a perspective I can respect.   ;D


My production date is Oct 72 and it is a non-rotoflex car.  I'll asume D-Type is the correct one unless anyone can rpove differently.  

I can partially justify the cost by the fact my current non-O/D trans needs to be rebuilt.  So my only actaul extra costs for the O/D would be the O/D unit and bit and bobs as the trans has to be rebuilt anyways regardless of what I do.  

Going with a 5 speed isn't an optiuon as I would being going against my goal of trying to keep the car somewhat Triumph original.

Which O/D unit is better and last longer etc?  D or J type?

[don cook1]

J Type....but I don't know why.

Hey, our Chairman has given us St George's Day off, he calls it a Bank Holiday. Now then, GT6, motorbike, lawn mower.......?????

[heraldcoupe]

sbarc wrote:

Which O/D unit is better and last longer etc?  D or J type?

For a 6-cylinder fitment the J-type is a much better bet. The D-type is OK on a 4-cylinder car, but it really is on the limit with a 6-cylinder.
Having previously travelled in a Triumph Vitesse with overdrive and a 3.27:1 diff, I would really recommend against the combination - you're unlikely to be able to pull at all in top gear with overdrive. Go for a 3.63:1 diff with the overdrive and you'll have the best of all worlds,
Cheers,
Bill.

[Nick Jones]

heraldcoupe wrote:

Go for a 3.63:1 diff with the overdrive and you'll have the best of all worlds,
Cheers,
Bill.

This is true, I had this combination (now 5 speed g/box with 0.78 5th gear which is even better) and it gives relaxed cruising but still allows some acceleration even with a 2L engine.

Nick

[sbarc]

J-Type with 3.63 does sound like a nice combination.  I don't want to sacrifice the in-town acceleration.  I'll have to ponder if it is all worth it.

[sbarc]

heraldcoupe wrote:

For a 6-cylinder fitment the J-type is a much better bet. The D-type is OK on a 4-cylinder car, but it really is on the limit with a 6-cylinder.
Having previously travelled in a Triumph Vitesse with overdrive and a 3.27:1 diff, I would really recommend against the combination - you're unlikely to be able to pull at all in top gear with overdrive. Go for a 3.63:1 diff with the overdrive and you'll have the best of all worlds,
Cheers,
Bill.

I forgot to ask Bill......I'l assuming I can get all the bits etc to make the speedo work correctly if I mix and match diff ratios with a J-Type O/D on my GT6 Mk 3?

[heraldcoupe]

sbarc wrote:

.I'l assuming I can get all the bits etc to make the speedo work correctly if I mix and match diff ratios with a J-Type O/D on my GT6 Mk 3?

It's pretty unlikely to be accurate with all the factory fitted bits, it may get better or worse by changing bits ::)
Seriously, it should be possible to find a combination of speedo drive gears and/or an appropriately calibrated speedo. Sorry I don't have the raw information on what does and doesn't work on the late GT6s, but I've no doubt that someone else will....
Cheers,
Bill.

[Jason]

cliftyhanger wrote:

Disagree. All GT6 had d type,

You can't disagree, I really do think that :-)

Apparently the parts books show a J type on a GT6

[sbarc]

Jason wrote:

Apparently the parts books show a J type on a GT6

REALLY?!?!?!  I looked through my GT6 MK3 parts book and it does show an overdrive(s) but it doesn't say D-Type or J-Type.......at least I couldn't see it mentioned?  I can't tell which is which just by looking at the photos.

Can you give me further info on this?

[Gt6s]

sbarc wrote:

REALLY?!?!?!  I looked through my GT6 MK3 parts book and it does show an overdrive(s) but it doesn't say D-Type or J-Type.......at least I couldn't see it mentioned?  I can't tell which is which just by looking at the photos.

Can you give me further info on this?

J type has its solonoid on left hand side, On a D type it is fitted on the right hand side.

[Richard B]

I seem to recall reading that the J'Type was planned for the GT6 MkIII; but production was stopped before it happened.

Seeing as the 1850 had a very similar box and that got the J'type you can see it was going to happen....