which overdrive for my car ?

[Fizzy]

having just been informed of an overdrive gearbox relatively close and at a very nice price i've got an urge to drop all the other less important expenditure at the moment and get overdrive in my car, as i think its about the most useful thing i could do to it. having said that, i dont know what im looking for. its a mk3 spit with a mkIV engine and i assume gearbox. what are my options ?

can i just fit an overdrive unit to my existing 'box ? if so what type of O/D do i need ? i assume i would also have to somehow sort the switching out somehow.

if i need to buy the whole gearbox and o/d unit together, what am i looking for that will fit to my mkIV engine ?

i know i'll need a shorter propshaft aswell, i'll cross that bridge when i come to it. any other things to look out for? i dont know the condition of the stuff yet, but im more concerned about not missing it while its cheap, i'll fit it as and when money and time allows.

[npanne]

You'll need the gearbox and o/d unit as a combined package, and also the prop shaft (as you mention), and possibly the diff too - depends what you have (I think the MkIII had a 4.11-1 ration diff, where-as the MkIV used either 3.89-1 or 3.63-1 depending on whether it was o/d or not). You'll need to see whether your new o/d box comes with the right angled speedo drive, as these are pricey little gizmos.

You'll also need to either obtain or craft a mounting plate for the gearbox mountings, and the mountings themself are different too.

You might want to consider a new clutch kit whilst you've got it all apart, as they will only cost about £50, and if there is any doubt about your clutch you might as well take the opportunity to change it.

[Fizzy]

thanks. the clutch was only changed about a month ago, so no worries about that. can you tell what diff you have by looking at it in-situ ? i dont know how much of the running gear was changed when the engine was put in. what diff do i want ? 3.89 ?

[Fizzy]

cheers, i should find out some info about it today or tomorrow. i'll keep it mind, though where are you ? im in norfolk.

[Clive]

The diff does not change with the od on spitfires (differing diffs is unique to GT6!)
You will need:
Complete overdrive gearbox with the od
matching propshaft
rubber mount
mounting plate
speedo angle drive
longer speedo cable
switch (MK3 had one on the steering column, mk4/1500 on the gearknob, interchangable)
Be aware that roundtail spits use 5/16 propshaft bolts (1/2" spanner) and later spits use 3/8 (9/16 spanner) so a bit of playing may be required. I have a diff that has an extra set of holes drilled so it can be used with either, a good idea really.
But if it is just a box then if it is cheap and def good then worth buying. As a guide I reckon sub £100 if unknown, £150+ if known good, and £100 approx for all the extras. Of course traders charge somewhat more..............which is fair enough.

[Fizzy]

does j or d type have any bearing on what im looking for ? or is there only one type that fits my needs ?

unrelated, i know my cooling system works perfectly, yet my guage reads at 3/4. i've also been told turning the heating on should ge the engine temp down. on mine it makes the guage go up or stay where it is.

is it possible the guage can be wired in the opposite way to what it should be ? if so, my temp guage would only be reading 1/4 (a seperate reading with a temp guage has my temp at just under 80C) and the needle would then move down with the heating on. which sounds right.

[Richard B]

Wot Clive said.

But.... MkIV box is nicer than a MkI-III box as it has synchro first. (So is recommended!)

If it has to be a package, keep the MkIV engine as a spare, your MkIII engine is better. The clutch is the same between MkIII & MkIV.

[Fizzy]

i already have the mk IV engine, its what it came with unfortunately. not that i know any different, so doesn't bother me.

[Deleted User]

yamaha_fizzy wrote:

i already have the mk IV engine, its what it came with unfortunately. not that i know any different, so doesn't bother me.

Which is the reason that I didn't buy the car when it was for sale last year, I wanted a MKIII with a genuine MKIII engine, not a MKIV engine.

As for the overdrive gearbox, there are three options :

1. A three rail box with a D type overdrive
2. A three rail box with a J type ovedrrive
3. A single rail box with a J type overdrive

On a three rail box, reverse is beside 1st gear
On a single rail box, reverse is beside 3rd gear

Three rail boxes are either 3 synchro or 4 synchro, and singe rail boxes are all 4 synchro.

With a single rail box, you will need a "special" clutch plate to adapt between the flywheel and gearbox as the splies on the input shaft are different - it's well documented here on the forum.

Three synchro three rail boxes are for MKIII Spits and earlier, and will crunch on a down change into 1st gear unless you double declutch.

Four synchro three rail boxes are for MKIV Spits.

You will also need to consider the propshaft, in particular you will need to ensure that you get the correct sized flanges on each end, and on a car that has been "messed around with" you will need to measure things to make sure you get the correct bits, and you could well end up with different flanges on each end of the shaft to match the chosen gearbox/overdrive and the differential.  The difference between the two flange sizes is the PCD and diameter of the bolt holes - the difference is vey subtle.

[rotoflex]

KevinR wrote:

reverse is beside 1st gear

AND THIS IS WHERE REVERSE IS SUPPOSED TO BE

I'm sorry, but I think the fact that it is not said enough is what leads designers into these depraved arrangements with reverse after the top gear.  I believe that members of society have an obligation to loudly evince disapproval when people commit these loathsome acts upon shift patterns.

[beefertoo]

could you get a pic of the box in question? i have got a prop and a 3.89 diff going spare from my mk iv. with o/d was quite good all round,just the lure of a six got the better of me :)

craig

[Fizzy]

looks like i missed out on the 'box :(. gutted, but not the end of the world. at least now i clearly know what to look for, what i need to do the work, and what i should look to spend. now just need to keep an ear open for a suitable O/D set-up box close to me.........