Series 3 Gearbox, Overdrive and Useability

Hi All,

I'm currently in the situation where I've stripped my Series 3 109 down to a bare chassis and am starting to put her back together. I was going to re-use the existing stock gearbox to ensure originality, however I have had a though....

I'd like my 109 to be a little less forgiving as a weekend daily driver. Would anybody be able to advised on the following:

1. Is there a way of keeping the stock gearbox and adjust the gearing ratios internally to allow for a 5th gear?
2. I believe there are kits out there to fit LT77 or R380 gearboxes. Can this be done with keeping the differential and engine in the stock mounting locations?
3. I've come across Ashcroft Transmission's high ration transfer case kit. Could this assist and is it worth the investment?

I'm looking to try and keep as much originality as possible whilst bring a wee bit of modernism.

P.S. - She's a four cylinder diesel 109, 1979 manufactured and 1980 registered.

Thanks for your help in advance

JB.

I don't know if it is possible to fit a fifth gear to a series box, I doubt it or I would have heard about a few. The Ascroft conversion is popular because it extends the ratio to give more speed but they are more suited to petrols I believe. Some owners change their diffs for Range Rover ones and another option is to fit overdrive. For any of these to work well, the engine and gearbox needs to be in good condition.

Col

Fit an overdrive. Less hassle

https://www.ashcroft-transmissions.co.uk/the-series-vehicle/high-ratio-transfer-case-kit.html

This is a solution cheaper than 2nd hand overdrive of unknown provenance

Sounds a good project put some pics up, while GB out may as well overhaul that, did it make any funny noises or leaks when driven

Good thing he lives in Lincolnshire where it is pretty flat and a series 109 diesel could cope with some higher gearing.

What I did to my 88 and 109 was fit early defender or Range Rover diff's (109) salisbury out of defender
Diff's can be picked up for as little as £20
Both are 2.25 diesel with 7.50 16 tyres and the gearing is not a problem now.

Ashcroft HRT in my SWB but 200di engine good and I like it , but 2.25 diesel in a 109 it would be over geared you would rareley get out of third.
Also depends on what size tyres you are running mine is nippy as anything on the 205's I use during the winter but I have a set of 7.50's that I fit in the summer for off road and laning I end up in 3rd gear on any steep hills.

Depends on budget, low budget - 750 / 235 tyres are worth close on 10% higher gearing over 205s, second to that 3.54 diffs, at least that much. If money not an issue get a romerdrive as its a true bolt on 5th gear. The Ashcroft is good but you still only have 4 gears and it gives a very large 3 to 4 change, OK with a petrol because it can rev in 3rd but a big jump for a diesel. I do quite a lot of mileage in my S2a and I use 3, 3/OD, 4 and 4OD a lot as these are all syncho changes.

my understanding as said is that the Ashcroft transfer box whilst better than an od is better with a more powerful engine, a 109 std diesel would struggle

You can use this calculator to work out your options:

https://www.ashcroft-transmissions.co.uk/calc/ratio_calc.html

Though as others have said, its how 'fit' your engine is that's going to be crucial here.

Welcome JB. Always good to have more of the Lincolnshire massive represented on here. We are fortunate to have a very knowledgeable chap on this forum called JamesMartin. His is just near Louth and is also known as Aztec Engineering. Great fella and a walking land rover encyclopedia. He has helped me out more times than I can remember. Depending on which end of the county you are in but could be useful to know. Good luck with the project. Steve.

Only real downside to the ashcroft box I can see (as long as you have a healthy engine) is the big gap between 3rd and 4th. Leaves a bit of a hole around 50mph where 4th is a bit high but 3rd is too low. Maybe on the flat you can plod through it but laden or towing and on a hill and you could find it limiting.

An overdrive gives you the option to go to standard gearing and you can do 3rd/3rdOD/4th/4thOD to narrow the jump between gears.