Fitting a Spitfire Overdrive
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Fitting a Spitfire Overdrive
My car is a 1300 mk iv 1975 spitfire with standard gearing and has no overdrive. In order to make life a little easier on my ears and my pocket I decided to fit a second hand overdrive unit from a donor car (one owner from new and had covered only 68000 miles!). Just in case you don't know and overdrive unit sits between the gearbox and the driven wheels and effectively adds 2 additional gears to reduce revs in 3rd and 4th. The unit is activated typically by a switch on the gear stick. Given the nature of our modern roads and the higher average speeds an overdrive unit is a very worth while addition and makes life in a Spitfire a whole lot more comfortable.
I wanted to identify the diff ratio, to do this I thought I'd put a chalk mark on the prop shaft and on the road wheel turn one ten times and see what the other one does. This was a good idea but it didn't work, ten turns at the prop gave 5.25 turns of the road wheel which does'nt seem to be right. So I checked the number on the bottom of the differential it starts with FH which indicates that it's a 3.89 standard differential.
The first big job was to get the overdrive out of the 1972 donor car, the easiest way to do this was to remove the gearbox and overdrive complete ,it a speceilist job to fit an overdrive to a gear box . First off I removed the H frame by the gear lever and tunnel . It is worth noting that the passenger seat base removes very easily and helps in terms of easy access for this job. Then undo the bell housing bolts ( don't miss any). Next is the prop shaft ( mark the flange at the front end ) then one big pull and its out, into the interior of the car the next job is to remove the over drive electrics from the donor car , there are wires from the gear lever to the inhibitor switch and then on to the relay which is bolted to the back of the horn relay and lastly on to the ignition switch.
Donor Gearbox Including Overdrive Removed (see above picture) The overdrive is the section at the end with the blue plate. I was getting the new overdrive ready for the car the first move was to degrease the whole assembly in an old paddling pool then drain the o/drive and gear box ( two drain plugs ) next take of the cover plate opposite side to blue plate (photos) the oil that came out (1.25 L ) was new clean oil but the filter needed cleaning (see below). Refill with ep 90, next a static check on the wires , when i do any electrical work i do my own diagram so that i understand and keep all the info in my head while working , it does not have to be pretty but you have to understand it , if you don't get some help! Here is my diagram ( opposite ) you turn the ignition on and power goes to the relay W1 ,C1 . when driving if you select 3rd or 4th gear this closes the inhibitor switch and you are ready to engage over drive by clicking the gear lever switch on ,this creates a path to earth from W2 and lets power from the ignition switch to W1 closing the relay as the relay closes power goes to C1 then to C2 and on to the solinoid which opens a hydraulic valve and in turn engages the tapered cone clutch , routing power throught the epicyclic gear train to raise the output gearing up from 1 to 1 .
i checked out each item seperately, first the relay , it goes clunk clunk as you earth it .ok then the 3rd 4th inhibitor ,it switches a test light on and off , ok .then the solinoid , first i checked with canleys that its safe to check it statically , they said ok .it checked out . lastly the switch . so all the bits are ok . next i tried it all with the original wiring ,no good . so next job is to remake the wires . the unit is now ready for the swop over ,check thrust bearing and fork pivot on both units before refitting . also make sure that the peg on the clutch fork is pointing the right way , if its not you have to take it all out again !!.
the last job before fitting was to set the soleinoid lever , there is a 3/16" hole in the lever which must line up with a hole in the casing when the solenoid is working. finally check the power the solenoid is using if its 2amps appx its ok if its 20amps appx it will burn out soon. and is u/s.