Jump to content

Adding Overdrive


Recommended Posts

Took my spitfire for a drive at the weekend, and I noticed that 4 gears is just not enough for the 2000cc engine i got on it, so on speaking with some one today i was told why not adding an overdrive.

So here is the question, if I pay a mechanic, to do this Job is this a big task? tks

Link to comment
Share on other sites

I did this to my Spitfire 1500 last year. Quite a job on my own... :o
I would estimate £500-1000 in parts, depending on condition and where you source them plus 8 hours labour.

You need to change (or add) the following
- Gearbox
- (O/D)
- Propshaft (shorter)
- gearbox/OD mount and plate
- gearlever extension plus reverse and 1/2 gear inhibitor switches
- O/D gearknob and wiring loom
- Solenoid
- Relay (if D Type O/D)
- Speedo angle drive
- longer speedo cable

First off you need to see what box you have. 3-rail boxes mate with D type OD, single rail with J type. The D type requires a relay as the solenoid activator carries more current. I suspect you have a 3-rail GT6 or vitesse box on the back of your 6. The J type OD slips in and out more smoothly and is stronger.

Is it worth it? Its an expensive mod, but it does make the car longer legged (1.2ish  gear ratio). Personally the OD is one of the things I love about these cars. 8)

Link to comment
Share on other sites

Cheaper to do a 5 speed conversion. (unless you go to Jigsaw or other type establishment) Conversion plate gearbox prop, mount etc all set me back £600 and that was for a fully reconditioned box as well. Bigger engine you can use a clutch plate off a few motors, A reasonably easy conversion and no OD to sap the power. Dave at Canleys can Supply the prop and Conversion plate for low cost. He is very helpful .

Link to comment
Share on other sites

bobyspit wrote:
Cheaper to do a 5 speed conversion.

Bought a single rail J type gearbox off Chic Doig last November, bellhousing, gearstick and new Dolomite clutch plate to fit the splines etc all in £175 approx.
Propshaft £3 at TSSC Stafford, shortening and balancing £30.53.
Wires straight into the reversing light circuit, although I added a column switch bought at Stoneleigh some time back. The hardest part to find was an overdrive mounting plate so I had one copied by a local welder, cost me £20 worth of beer.

When I converted the GT6 it was a Dolomite 1850 J type box, cost me £200 second hand, shortened a spare prop but kept the original non O/D diff.
It's not big money, and Stoneleigh in March will probably have all the bits you require.

Link to comment
Share on other sites

I got an overdrive gearbox supplied and fitted with all appropriate parts for around 400 by Max at Totally Triumph near Warrington. The Gbox came with a warranty and I have never had a problem with it. I cannot see how a 5 Speed is a cheaper option and nowhere near as satisfying to drive than an overdrive Gearbox IMHO. Driving through the twsisties in and out of overdrive is a real joy. The best modification I have made to my car.

Cheers, Neil

Link to comment
Share on other sites

My experience was like Paudman's.

Got the transmission & overdrive as an assembled unit.
Collected a used propshaft & had it shortened.
Got an OD transmission mount, & column switch from another salvage yard.
Ordered the switches for the top of the gearbox and overdrive relay from a parts place.

One day, put all that in.  It was pretty much putting tinkertoys together, not brainwork.  I can't see how the modifications & jimmying for a 5-speed could be faster & easier.  

Additional items to make it easy:

Blanking plate for propshaft hump.  You have to cut back the propshaft hump in the car a few inches to clear the rear flange when you install the OD transmission.  Then, you put a plate over the opening.  A salvage yard may have an original Spitfire/GT6 OD plate for that, or you can just cut something from sheet metal.

OD accessory wiring harness:  I did the wiring myself, but later bought a real OD wiring harness after needing to troubleshoot & cursing the fact that I'd used the same color wiring when I put it in.  If you buy one, not only will the colors be correct, but the right connectors will already be on it.  It's only a few wires.

It is easier & faster, like the shop manual says, to remove the engine & transmission as a unit than to squirm on your stomach in the car & attempt to locate bolts via ESP like you must if you try to leave the engine in place while replacing the transmission.  Borrow an engine hoist, or park under something suitable, use a cable hoist, and just disconnect the motor mounts, fuel supply, water hoses, electrical connections (oil & temp sensors), & the bolts on the flange at the rear of the transmission, zoop you're out, you keep your back, knuckles, & sanity.

Link to comment
Share on other sites

rotoflex wrote:
You have to cut back the propshaft hump in the car a few inches to clear the rear flange when you install the OD transmission.  

.......you keep your back, knuckles, & sanity.

The spitfire 1500's all have this plate whether OD or not.

agree +++ re back and knuckles..... ??)

Link to comment
Share on other sites

Simple ole boy,    put a mark on the wheels,  and a mark on the diff input flange, also taking note where
it is at its start point, so really two ,one on diff one on flange

then push car forad,, or have it on axle stands, and rotate the wheels,.

if its a 3.89, then the wheels will have gone round  3.89 times for one rev of the diff input flange

3.27 will be  3.27 imes for one rev of diff input

the diff you got may well be a 3.89,  or a 3.63
if it were a 3.27, then standard car, would feel a little lets say ..no as quick..


Link to comment
Share on other sites

I've run a Mk1 on a non-O/D diff with J-type overdrive for years now, was great for motorway use, but have just upgraded to a refurbished Spitfire 3.36:1 diff and I must admit the difference is amazing. In all honesty if it hadn't been an oil leak from the original diff it would have stayed where it was.

Link to comment
Share on other sites


This topic is now archived and is closed to further replies.

  • Create New...