Caterham 6 Speed Gearbox
Posted: Wed Nov 04, 2009 11:52 am
I had a standard Type 9 box in my 4AGE 20v Striker and the ratios were quite frankly rubbish. 5th was geared for an utterly useless 180 odd MPH and 4th for 145ish and that's before I even thought about the huge spacing between 2nd, 3rd and 4th and the stupidly short 1st gear.
So destroying the synchros on 3rd gave me the perfect opportunity to upgrade.
My car is mainly used on track these days and I wanted to end up with a set of ratios with a better spread and significantly shorter overall gearing - even cruising on the motorway 4th gear was just fine. Firstly I looked at the BGH and other close ratio Type 9 options, but these all suffer from a 4th gear that's a 1:1 ratio and can't be changed - so the close ratio sets are all clustered around a 145mph 4th gear. Making 2nd and 3rd gear longer to gain a nicer spread of ratios was exactly the opposite of what I wanted! I could use a shorter diff to bring the overall gearing down, but I've already got a 3.92:1 diff - the shortest standard ratio. There are shorter aftermarket CWPs about for the diff, but they're pretty impossible to find and expensive too.
So I started looking at the Caterham 6 speed box. The ratios looked good, with 2nd a little shorter than my standard Type 9 and then a nice spread of ratios to a 1:1 6th. A bit of investigation showed that Caterham had designed the box as a drop in replacement for their Type 9 equipped cars, so it sounded promising. Those that know Type 9s will know that there are 2 common lengths for the input shaft. The box from 4 cylinder cars has a short input shaft and the one from V6s has a longer shaft (and a longer 1st gear). Caterham in their K-series powered cars at least use the long input shaft box, but with the spigot on the end shortened so the spigot is the same length as it is on the short input shaft version. Overall length of the input shaft on the Caterham 6 speed box is therefore something in between the 2 common versions! Otherwise externally the boxes are similar, same splines on the input and output shafts, same overall length and similar dimensions.
At this point I was confident I could make it work, so I went ahead and bought a Caterham 6 speed box from a K-series powered car. I got the engine and box out of my car and started measuring up. I decided that a 20mm spacer between box and bellhousing was just what I needed to mate the Caterham box with my engine, clutch and flywheel combo (standard RAW 20v 4AGE stuff). This would mean that the propshaft needed shortening 20mm and the gearstick and gearbox mount would move back by the same amount.
I looked around for a suitable spacer and found something at RWDMotorsport. I talked to them and found that the spacer they make is 18mm thick. This puts the ends of the splines on a long input shaft gearbox input shaft at exactly the same place they would be on the short input shaft version. The only problem with that is the Caterham box has a longer nose and I was worried about the clutch plate hitting the end of the nose of the box. I went ahead and ordered the spacer anyway, thinking I could always 'adjust' the nose of the box if necessary.
When the spacer arrived I found I had another slight issue. The Caterham box has a plate on the front of the box below the nose (held on by the blue bolts in the picture)
My spacer didn't have a recess for that plate (some kind of bearing housing I think). Modifying it so it did was relatively straightfoward though
Then I hit what should have been a fairly minor snag - the 'longer' M12x1.5 socket head bolts supplied with the spacer weren't 'longer' enough for my bellhousing. I needed 3 that were 75mm at least and could make use of 1 of the 50mm ones supplied. Finding those proved a lot trickier than I'd thought and meant it was another week before I could mate the gearbox with the engine.
I got it all bolted together and to my surprise found I could still operate the clutch. I couldn't tell if it was hitting the nose of the gearbox or not though, so decided to make an additional spacer out of some 1.5mm ali sheet I had knocking about. I simply didn't have time to put it all together and risk it not working - by now it was Saturday morning and the car was due out on a track day at Oulton Park on the Friday.
Getting the engine and box back in the car only showed up one other problem. The Caterham box has a breather on the top, which hits the transmission tunnel top if you try and put it in the same place as the normal box. I spaced the gearbox mount about 15mm to drop the box and get some clearance here.
Next job was to get some oil in the box. The filler is on the opposite side to the Type-9, so the hole I'd previously made in the passenger side of the transmission tunnel (the old box used to leak a bit) was useless. New hole made, but my 10mm drain key wouldn't fit, it was fractionally too big. It was late Sunday by now, so I didn't have a chance to get anything else. Turns out that the filler plug SHOULD take a 10mm key, but some of them are 3/8" instead
Anyway, by Wednesday evening I had the car running again and even quick test drive in the dark and in traffic showed that the new box was a huge improvement.
On Friday at Oulton it was a revelation (despite some teething troubles from me rushing getting the engine back in) - the car is completely transformed and it's so easy to keep above 6000RPM all the time now, which is where the engine really comes to life. I was arriving so much quicker at corners than on previous visits to Oulton that for the first couple of sessions I kept almost outbraking myself and tiptoeing round the outside of corners... The gearchange is fantastic now too, a 2nd only to my oh's S2000 (which has the best gearchange of anything I've ever driven)
In summary to do this you need
Caterham 6 speed box
Bellhousing spacer from www.rwdmotorsport.com
Shorten propshaft 20mm
Possibly 1.5mm ali sheet to make extra spacer for gearbox / shorten gearbox nose by the same amount (I currently run with the flywheel exposed to the elements, making a cover for this out of 1.5mm sheet that goes between block and bellhousing would probably be a good plan and solve this clearance problem too)
M12x1.5 capheads - 75mm long.
Drop gearbox ~15mm by spacing mount.
New transmission tunnel panel as gear lever moves back.
It ended up being a lot more straightforward than I'd feared and has made a huge difference to the way the car drives.
Hope I haven't bored everyone too much
(I've got lots more geeky stuff about ratios to do that with 
)
So destroying the synchros on 3rd gave me the perfect opportunity to upgrade.
My car is mainly used on track these days and I wanted to end up with a set of ratios with a better spread and significantly shorter overall gearing - even cruising on the motorway 4th gear was just fine. Firstly I looked at the BGH and other close ratio Type 9 options, but these all suffer from a 4th gear that's a 1:1 ratio and can't be changed - so the close ratio sets are all clustered around a 145mph 4th gear. Making 2nd and 3rd gear longer to gain a nicer spread of ratios was exactly the opposite of what I wanted! I could use a shorter diff to bring the overall gearing down, but I've already got a 3.92:1 diff - the shortest standard ratio. There are shorter aftermarket CWPs about for the diff, but they're pretty impossible to find and expensive too.
So I started looking at the Caterham 6 speed box. The ratios looked good, with 2nd a little shorter than my standard Type 9 and then a nice spread of ratios to a 1:1 6th. A bit of investigation showed that Caterham had designed the box as a drop in replacement for their Type 9 equipped cars, so it sounded promising. Those that know Type 9s will know that there are 2 common lengths for the input shaft. The box from 4 cylinder cars has a short input shaft and the one from V6s has a longer shaft (and a longer 1st gear). Caterham in their K-series powered cars at least use the long input shaft box, but with the spigot on the end shortened so the spigot is the same length as it is on the short input shaft version. Overall length of the input shaft on the Caterham 6 speed box is therefore something in between the 2 common versions! Otherwise externally the boxes are similar, same splines on the input and output shafts, same overall length and similar dimensions.
At this point I was confident I could make it work, so I went ahead and bought a Caterham 6 speed box from a K-series powered car. I got the engine and box out of my car and started measuring up. I decided that a 20mm spacer between box and bellhousing was just what I needed to mate the Caterham box with my engine, clutch and flywheel combo (standard RAW 20v 4AGE stuff). This would mean that the propshaft needed shortening 20mm and the gearstick and gearbox mount would move back by the same amount.
I looked around for a suitable spacer and found something at RWDMotorsport. I talked to them and found that the spacer they make is 18mm thick. This puts the ends of the splines on a long input shaft gearbox input shaft at exactly the same place they would be on the short input shaft version. The only problem with that is the Caterham box has a longer nose and I was worried about the clutch plate hitting the end of the nose of the box. I went ahead and ordered the spacer anyway, thinking I could always 'adjust' the nose of the box if necessary.
When the spacer arrived I found I had another slight issue. The Caterham box has a plate on the front of the box below the nose (held on by the blue bolts in the picture)
My spacer didn't have a recess for that plate (some kind of bearing housing I think). Modifying it so it did was relatively straightfoward though
Then I hit what should have been a fairly minor snag - the 'longer' M12x1.5 socket head bolts supplied with the spacer weren't 'longer' enough for my bellhousing. I needed 3 that were 75mm at least and could make use of 1 of the 50mm ones supplied. Finding those proved a lot trickier than I'd thought and meant it was another week before I could mate the gearbox with the engine.
I got it all bolted together and to my surprise found I could still operate the clutch. I couldn't tell if it was hitting the nose of the gearbox or not though, so decided to make an additional spacer out of some 1.5mm ali sheet I had knocking about. I simply didn't have time to put it all together and risk it not working - by now it was Saturday morning and the car was due out on a track day at Oulton Park on the Friday.
Getting the engine and box back in the car only showed up one other problem. The Caterham box has a breather on the top, which hits the transmission tunnel top if you try and put it in the same place as the normal box. I spaced the gearbox mount about 15mm to drop the box and get some clearance here.
Next job was to get some oil in the box. The filler is on the opposite side to the Type-9, so the hole I'd previously made in the passenger side of the transmission tunnel (the old box used to leak a bit) was useless. New hole made, but my 10mm drain key wouldn't fit, it was fractionally too big. It was late Sunday by now, so I didn't have a chance to get anything else. Turns out that the filler plug SHOULD take a 10mm key, but some of them are 3/8" instead
Anyway, by Wednesday evening I had the car running again and even quick test drive in the dark and in traffic showed that the new box was a huge improvement.
On Friday at Oulton it was a revelation (despite some teething troubles from me rushing getting the engine back in) - the car is completely transformed and it's so easy to keep above 6000RPM all the time now, which is where the engine really comes to life. I was arriving so much quicker at corners than on previous visits to Oulton that for the first couple of sessions I kept almost outbraking myself and tiptoeing round the outside of corners... The gearchange is fantastic now too, a 2nd only to my oh's S2000 (which has the best gearchange of anything I've ever driven)
In summary to do this you need
Caterham 6 speed box
Bellhousing spacer from www.rwdmotorsport.com
Shorten propshaft 20mm
Possibly 1.5mm ali sheet to make extra spacer for gearbox / shorten gearbox nose by the same amount (I currently run with the flywheel exposed to the elements, making a cover for this out of 1.5mm sheet that goes between block and bellhousing would probably be a good plan and solve this clearance problem too)
M12x1.5 capheads - 75mm long.
Drop gearbox ~15mm by spacing mount.
New transmission tunnel panel as gear lever moves back.
It ended up being a lot more straightforward than I'd feared and has made a huge difference to the way the car drives.
Hope I haven't bored everyone too much
(I've got lots more geeky stuff about ratios to do that with )
, which meant at somewhere like Oulton I was using 2nd and 3rd pretty much all the time and just hooking 4th on the longer straights.