What's involved in converting a 4 speed to a LT77 gearbox?

[march]

Hi,

Does anyone know what is involved with converting a 4 gear car to the LT77 5 gear? The 4 speed one I have has a knackered reverse gear and I have a spare LT77 gearbox and 5 speed rear diff with LSD so could be a bit of a fun car if I ever get it through an MOT.

Thanks

Marc

[busheytrader]

IIRC, in addition to the 5 speed gearbox and rear axle, you'll need to source the different propshaft, rear brake assemblies, clutch and bell housing. There may be some associated items as well.

The Rimmer Bros website shows the 4 and 5 speed diagrams from the original parts book and is a good reference point.

The most involved part would be removing the engine and gearbox to swap the 4 and 5 speed parts over and reinstating them. You need to get the front of the car as low as possible and the rear as high as possible to lift them out with an engine crane. I did this for my V8 conversion. Alternatively drop them from below by dropping the whole subframe and front suspension with them attached.

Adam

[John_C]

Easier and cheaper to repair the reverse gear. Also, 4 speed cars are becoming rare and sought after. But, if you have a whining differential my advice is to see if Dave Pearson of Canley Classics will repair it for you because he's the only person I've ever come across who can do them properly. The end result is a quiet 4 speed car even at high speed (i.e. motorway driving) which not many people have ever enjoyed.

[john 215]

Hi,

Four speed boxes are about as easy as a RWD gearbox gets when it comes to rebuilding, reverse gear normally wears so it chatters against 1st 2nd syncro hub. Have also seen the reverse fork pin drop out and the roll pin slip round on the gear lever selector. Take the top of the box and you may see how things are in there.

If you go the five speed route then you will also need handbrake cable, gear lever gaiter and plate that lives under it in addition to the parts my learned friend Adam mentioned. No need to remove the engine, boxes come out ok from underneath although unless you are called Arnold a tad heavy on your own, I use a transmission jack so possible to do it on your own.

Cheers John

[FI Spyder]

I dropped my transmission out the bottom (would be same with 4 speed) but I had combo jack stands and hydraulic jacks to get about 21" combined lift. Top of bell housing just cleared a natural notch in the side frame. I used a $40 ATV scissor jack on a piece of melamine with 6 rollers. I've not driven a 4 speed but reputedly they are smoother shifting, 1st isn't so bull low, not sure how they are at high way speeds relative to 5 speed (haven't looked at ratios). But if you already have a good LT77 and diff then it might not make sense rebuilding the 4. Some people have used the 4 speed diff to get more relaxed cruising than the 5 speed 3.9 diff, just need some mods to the drive shaft.

[john 215]

Hi,

Very similar setup I have -



Mildred sweared by a five speed box mated to a four speed axle, only thing to consider is the strength of the 4 speed axle and gearbox, ok but not as strong as as five speed set up. Don't forget they where originally from a the Morris Marina.

Cheers John

[saabfast]

It is possible to remove/install a 5 speed on your own without those useful sort of devices, if hard work. I made a simple wood tray to fit the bottom with lipped edges to stop it sliding off. I used two trolley jacks to support it and get it at the right angle. You need the car quite high to get it on/out under the chassis, and so you can get under with it to wangle it into position. It has to be tipped at an angle to come out. I had it up on blocks and axle stands at the front and axle stands at the back. It is VERY heavy for a gearbox. Getting it back in was a matter of sliding it under on the tray and then rolling/lifting it on to the jacks. Not sure I would like to try it again now I'm 5 years older!

[Rich K]

In my experience the 4 speed box is borderline on being up to the job of even a 2 litre in standard tune. When I ran a fixedhead with mild tuning I.e. Carbs, extractor manifold, sports exhaust, I went through 3 gearboxes. I would personally not go down the 4 speed repair route unless originality is your thing. The 5 speed box and axle set-up is much stronger.

[busheytrader]

Ok everyone, you've hooked me about removing the gearbox only from underneath. Unfortunately my garage isn't tall enough to take a 2 post lift.......

How much do you have to raise the car off the floor to remove the gearbox on its own? How easy is it to remove the gearbox from the bellhousing? How easy is it to put it back on again? Is this what's needed to lower and raise the gearbox since I don't have arms like Popeye? http://www.ebay.co.uk/itm/like/15194685 ... noapp=true

Thanks in advance.

Adam

[FI Spyder]

My combo jack stands/hydraulic jacks give a 21" lift (from garage floor to bottom of frame rails were they are holding the car). The top of the bell housing just barely cleared the frame (thank goodness for a notch in it about 2/3 of the way back of the front wheel as I recall). There is the height of the collapsed scissor jack, thickness of the melamine board and the height of the rollers as you can see from the picture in my previous post you can approximate the thickness of all that. That transmission jack on eBay I think is higher than my setup when collapsed so you might have to go a little higher. I wouldn't try and lower it by hand. The scissor jack allowed you to move it back then lower it once the input shaft cleared the flywheel bushing. This involved several steps of lowering and moving back. The fasteners at the top of the bell housing were the hardest to access but doable. Other than these it was pretty easy to remove. Installation was the reverse. I found I did have to chamfer the edge on the input shaft to get it started into the bushing (I had the bearing type). The transmission is firmly attached to the scissor jack by a sheet of stainless steel I had lying around, one edge bolt to the scissor jack and the other edge bolted to the transmission (you can see it in the picture). The jack/lifts I bought at Costco for $36 each (they were clearing them out). Somebody mentioned jack stands on big blocks. You'd have to do a series of lifts front then back slipping in blocks, to get it high enough.

[Hasbeen]

I did my first clutch job on a 7 about 14 years ago, so I was 62, & single handed. I had the car on 12" ramps at the front, & stands at the back. After a couple of hours I found myself on my back under the car, with a surprisingly heavy gearbox on my chest, & in some difficulty getting it off there.

I don't know if the thing is much heavier than the Moss box in my 1962 Morgan +4, [XK 120/150 & saloon Jags], was in my youth, but it sure felt it.

I made up a cradle to hold it on the floor jack, but had to lift the car quite a bit to slide it under there when putting it back. Used the same gear with no real trouble next time I did it, as I knew what to do.

When doing it, just keep telling yourself that the rally team mechanics used to change gearboxes in a 7V8 in under an hour, so it can't really be that hard.

Hasbeen

[john 215]

Hi Adam,

That's what I used, a Draper version of the same tool, worked a treat, lent it to mates to change FWD clutches and worked for them too.The deck the gearbox sits on can be tilted around its axis so can be adjusted to fit the angle the box is sitting at, plus a strap to stop it falling off so a lot safer than jack's, been there done that !

No two poster at home just a decent set of axle stands to get car as high as safely possible. Not had too do a V8 yet at home so not sure how easy that would be with a bigger bell housing.

5 speed is a heavier box than a 4 speed for sure, but not as heavy as automatics I have done in the past on my back or a 3 litre Capri box, cast iron and weighs a ton.

I guess the rally cars had ' quick ' release bell housing's in common with a lot of rally car's ?


Cheers John

[John_C]

Rich, the 4 speed is good enough for up to 200bhp although I wouldn't take it that far. But that's what some extreme Spitfire racers go to when they uprate to the limit. The components used to do this are all from the TR7 4 speed driveline. 200bhp really means about 150bhp at the wheels I suppose. My own car is highly tuned and never had a problem in 30 years.

[dursleyman]

Anyone tried the transmission jack adapter that goes on the normal trolley jack?

[march]

Thanks for the comments guys. I actually have an old TR7 that is going to die for the greater good of 2 others that has had a 5 speed conversion done on it but only has the 4 speed diff so hopefully most of the parts are there to do the job. However having had experience of breaking a 4 speed diff in the dim and distant past I will probably stick with the plan for a 5 speed diff.

As the car needs lots of work and I am already involved with working on another one it will probably have to wait a while until I will have time to take the engine out. Just fancied a quick fix so I could drive it around the farm for a bit of fun.