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6 speed to 5 speed K


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My K Series 1600 Supersport is fitted with a CC 6 speed box, cruising is not very comfortable as the revs are quite high, higher than that of the type 9 five speed due to the 'overdrive ' nature of the fifth gear of the type 9 box, I believe? My question is:- Q: what is entailed swapping the 6 speed out, for a type 9?

Q: if a basic type 9 is sourced from say ebay, would the first motion shafts' splines be compatible I,.e. a straight swap - Ford box to Rover clutch splines?

Q: clutch cable connection set up?

Q:which Bell housing is used?

Q: gear lever position the same?

Q: any trick bits required?

Q: does anyone have a 5 speed they are not using?? (Pls)!

Assistance greatfully accepted,

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The six speed box is paired with a 3.6 rear diff. 6th is 1:1 so final drive is 3.62 compared with .82(5th) times 3.92(rear diff ration for a 5 speeder) = 3.21 so a bit lower by 12% but not hugely

i think the cc 6 speed is loosely based on the ford box and they are a straight swap (good news) but you do need to change the rear diff ratio (bad news). You should be able to cheaply source a 5 speed box but consider upgrading with a BGH gear  kit which raises 1st gear to make it more useful (standard 5 speed 1st gear is quite low)

You will recover a decent amount for the 6 speed box and diff to cover the costs


good luck



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Check your diff ratio, some came with a 3.9 ratio which gives a very low top gear. I changed to a 3.6 ratio, much better. If you have a Supersport the standard 5 speed ratios are totally unsuitable for a Caterham, also the higher ratio of the 5 speed will kill your top gear acceleration with a Supersport 1.6, the 1.8 is much better as it is less peaky.

The 6 speed box was specially designed for the 1.4 Supersport, do you have 13 inch wheels? this will give very low gearing with 60 profile tyres. I am running 14 inch wheels 60 profile tyres a 3.6 diff and a 1.8 this gives around 18mph per 1000rpm and around 16 mph with the 3.9 diff. The 1.6 Supersport is quite a peaky engine and you may find the 5 speed box will ruin the character of the car

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Dont do it - check you have a 3.6 diff first (sure another post intimated it wasn’t). The 6 speed and supersport cams are a perfect combination.

I read elsewhere caterham have stopped making the 6 speed so if you do remove it maybe worth storing for a few years as bound to go up in value (as the 5 speeds seem to be doing)

Also, the standard 5 speed is a compromise as the first gear ratio is wrong for the car so will ideally need changing and before you know it you’ll be into a £2k rebuild with quaiffe or Tracsport semi helical gear kit!

Keep the six and sort the diff out!



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Jack up a rear wheel or two and turn the propshaft or wheel and count the relative number of turns. It's easier to count the turns if you mark the wheel and shaft.

It's slightly different between open and locking diffs, and you need to remember the factor of two if only one wheel is turning.


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Quaiffe do/did a type 9 seven speed replacement, so a 6 with an overdrive 5th type gear. 

Was expensive, not looked lately. 

Or indeed change the car? For those longe motorway/arterial roads I use a trailer.

I’m planning a 6 speed with open 4:1 diff in my BDR, (if that’s ever finished.) So that should be fun. However,, I am keeping the 5 speed and LSD for when the fun stops.


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Yes I changed from a 3.9 to a 3.6 and my speedo reads low by about 10% of the acurate speed! so I have to be careful not to speed. You can alter the the speedo drive ratio or have the speedo re- calibrated or just leave it and remember it reads low.

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I run a 1999 1.6k with a 5 speed box but with DVA work to the head, verniers and 633 cams, kit 13A if I recall. I got my car from another buyer then got the performance mods done. I did not know till later it had a 3.62 ratio differential, whereas the 3.92 is normal. With the performance mods. and a powerspeed 4-2-1 exhaust the car is still quite quick for road work, 60mph comes up in 5.5 seconds but the higher speed cruising has the lower revs. This works for me, but is not usual. I guess I am losing a bit of sprinting capability for the trade of lower revolutions, each to their own. 

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