Gearbox swap - any hints and tips?

[Matt UK]

Afternoon all,

I've got a K-series engined car and switching the gearbox from a 6-speed to a 5-speed - I presuming that they will be a straight-forward swap.

I've never really attempted a job as big as this before, so any hints and tips? Other jobs to do whilst it's all in bits? Good build threads from others?

Thanks in advance - will update progress over the next week with pics etc if anyone is interested. And no doubt be popping back with questions!

Cheers.

[Grubbster]

The most straightforward way is to take the engine and gearbox out together, but it can be done by taking the engine out separately, then the gearbox, it is more fiddly to refit as separate items though. Are you changing the diff as well? The final drive ratios are different and using the one meant for the 5 speed box with a 6 speed box will make the engine revs higher for a given speed - not a problem if you're sprinting but could be tiring on the road as 6th is the same ratio as 4th in the 5 speeder.

Edited to remove foolishness!

Edited by Grubbster on Monday 30th June 23:15

[V7SLR]

He's going from a 6 speed to a 5 speed. The gearing might be a little long .... but comfy!

Stu.

[Grubbster]

Ah yes, I should've read it properly! So could make for a very nice touring car then.

[Matt UK]

Thanks guys, yes going from a 6 speed to a 5. Diff is a 3.92 so currently a bit frantic for my liking - I want to do touring more than tracks days. Plus with a 175bhp motor I think it will pull longer gearing ok.

Anything else worth doing in there with engine / gearbox out?

Also looking from some simple steps. I'll be doing it with a chap who knows cars but neither of us have done this job before. So if we can avoid any obvious pitfalls that would be great!

Btw, ad for the 6 speed box going into the classifieds this evening. It's in perfect working order so if anyone is interested let me know.

[Darumvej]

I would look at replacing the clutch at the same time.

[downsman]

Have you got a higher 1st gear (and 2nd?) in the five speed. I wish I'd done that before fitting mine and I've only got 125bhp.

[rubystone]

Matt UK said:

Thanks guys, yes going from a 6 speed to a 5. Diff is a 3.92 so currently a bit frantic for my liking - I want to do touring more than tracks days. Plus with a 175bhp motor I think it will pull longer gearing ok.

Anything else worth doing in there with engine / gearbox out?

Also looking from some simple steps. I'll be doing it with a chap who knows cars but neither of us have done this job before. So if we can avoid any obvious pitfalls that would be great!

Btw, ad for the 6 speed box going into the classifieds this evening. It's in perfect working order so if anyone is interested let me know.

Our of interest, have you thought about fitting a 3.62 'diff. Also, where's peak torque for the engine? That may not suit a type 9 'box. Often the higher bhp KS have a very narrow power band unless they use the VVC kit, in which case you could be OK with the 5 speed (allowing for the useless 1st gear of course!!!!)

[IBDAET]

I have just overcome the same issue by fitting 15" wheels with the 15 x 9 CSR rears shod with 225/50/15 Toyo T1-R. This raises the gearing to give the equivalent of a 3.57:1 diff. I am doing a 2500 mile Eueopean road trip at the end of June and wanted somethibng to make the motorway bits a bit more comfortable.

I can convert back to "mad mode" just by bolting 13" wheels for track days or full on blats.
The Toyos are cheap as chips and have twice the usable tread as CR500's.

Going from 6 to 5 speed is a complex job you may regret as the 5 speed gearbox is heavy, prone to synchro failure on higher revving engines (7500+ RPM), will seriously blunt the performance of your car. You will not belive just how much.

[Matt UK]

Thanks guys, I appreciate the comments. I've thought long and hard and agree that moving to a 5 speed is seen as retrograde by most, but it will suit my needs better.

Also, let's not underestimate the fact that taking cars apart and putting them back together again is fun, right?

Any good step by step guides out there?
Apart from the clutch (which I'll have a spare on standby) any other jobs worth doing while I have the access?

Edited by Matt UK on Saturday 28th June 12:55

[DCL]

It depends how long you have the engine out for, but it's always worth a good clean in the engine bay - power wash it if you can, followed by anti-corrosion treatment of your choice. Flush the engine coolant and radiator through with a hose, replace engine mounts, etc. etc. Generally it is just a good opportunity to check/change things that might otherwise be a bit awkward.

[edit] One thing that springs to mind is to check the input shafts are the same length, I'm not particularly familiar with the 5 speed box but I believe there are variations.

Edited by DCL on Saturday 28th June 15:42

[IBDAET]

"the fact that taking cars apart and putting them back together again is fun, right?"

Not idd you make it a mistake and have to do it twice!


While you have your engine out, make sure the rear crank seal is glued in.

[marcns]

Hi
The alternative could be to change the diff ratio to widen out the 6 speed.

Regards box, I personally prefer do in manageable chunks, and to take the engine out first, and then the gearbox.

With my engine crane (and a single width garage) it only just clears the garage door opening with engine; trying to get the engine and gearbox up in the air and over the radiator to extract wouldn't be possible for me.

It depends on how much space you have.

[V7SLR]

Matt, have the 5 speed modified before you fit it. 1st and 2nd ratios are too low and need raising which will bring it part way to the feel of the 6 speed but still with the top gear that you want. I've run both and in "standard" forms the 6 is a better compromise albeit with a 3.62 diff ratio, but still a pain for touring.

For info ... factory fit 5-speed had a 3.92, factory fit 6-speed had a 3.62. If yours has a 3.92 with a 6 speed one or the other will have previously been changed, 'box or diff.

[DVandrews]

I'd echo the sentiments WRT 5 speed ratios, had my 5 speed converted to a BGH rocket 5 , this had a raised first and second gear and a lowered 5th, superb set of ratios, 5th gear was still and overdrive, but 0.91 rather than 0.82, this made it more of a driving gear than an overdrive.

Dave

[IBDAET]

V78 - Untrue. My car had 6 spd and 3.92 like many others.

One thing to note is that if moving to a std 5 speed - i.e. standard XR4i ratios, the result will be dreadful. The ratios are designed for a heavy car towing a caravan.

[JeffC]

sorry this is not answering your original question but that has been covered already, however from experience if the 5 speed is a standard type 9 I would give it a miss,I have had 3 cars with it fitted and its a woeful box and will ruin the drive , the ratios are close to useless for a 7 type car, as said it was designed for setting away with a caravan on the back of a sierra, BGH close ratio box is better Personally I think you have the best box available but the wrong diff for road use, I would leave the box and fit a 3.3 diff , I have had this combo on 15" wheels and this makes for a less buzzy experience I seem to recall it run at 20mph/1,000 revs in 6th , but if you want a proper cruiser go with a 3.14 .

[rubystone]

JeffC said:

sorry this is not answering your original question but that has been covered already, however from experience if the 5 speed is a standard type 9 I would give it a miss,I have had 3 cars with it fitted and its a woeful box and will ruin the drive , the ratios are close to useless for a 7 type car, as said it was designed for setting away with a caravan on the back of a sierra, BGH close ratio box is better Personally I think you have the best box available but the wrong diff for road use, I would leave the box and fit a 3.3 diff , I have had this combo on 15" wheels and this makes for a less buzzy experience I seem to recall it run at 20mph/1,000 revs in 6th , but if you want a proper cruiser go with a 3.14 .

I think his mind is made up...despite the advice from us previous 5 speeders...so good luck. My sole remaining piece of advice is to keep the 6 speed 'box just In case....

[forest07]

The standard 5 speed has only three useful gears in a Caterham. A BGH conversion is a good box for a Duratec.
Caterham 6 speed is by far the best for a K Series.

[Matt UK]

Thanks guys, I hear you.

As with all things there are compromises regarding time/money/effort. Thanks for your comments though, they are much appreciated. I'll see how it all goes. If this is not the right move then I guess I'll find out