Overdrive units
 

Given that you all seem to be so knowledgeable and helpful on here, I wondered if anyone can help me with another query.

My daily driver is a 122s with a B20 lump and 4 speed box. As I do a fair amount of miles for work, and rebuilding the SUs didn't seem to make much of a dent in the (quite hideous) fuel consumption, we sourced an earlier 120 series which has an overdrive box and the intention was to put that gearbox in the daily. However, the one we've picked up has a B18 engine in it and from what I've read, the B18 and B20 overdrive units were different (hydraulic pressure differs i think?).

Does anybody know whether it's still possible to mate the B18 box with overdrive to the b20 lump that's in my daily driver or is it a case of needing to swap the b20 and 4 speed for the b18 and 4 speed with overdrive i.e. complete transplant? (assuming possibly need to swap prop shafts over too) And would I really notice the difference between a well set up b18 and the current b20?

My other 122s is a b20 with overdrive (but don't really want to pull that apart as this is a customised one which is really starting to come together) and it went to beaulieu and back from Cornwall on 45 quids worth of fuel. However, after a mild improvement in consumption on the daily, it's back to returning about 25 to the gallon, 30 if I'm lucky (figuring perhaps my right foot is an issue or I'm paying the price of no overdrive?).

Any thoughts/advice would be gratefully received :notworthy:

Not sure what model amazon you have but if it is a original B20 engined car then it is going to be a 131. If it is a 122s with a B20 then that makes things easy for you.

Late cars 68 on (B20 engined) have a differnt prop that is basicaly the same as a 140. If your car is a later one then you will need a prop off that age car or have your front half shortened.

If your car was originaly B18 powered then you should be fine but B18 gearboxes have two types of flanges. Very eary cars were secured with U bolts and later ones with 4 bolts.

As to the gearboxes it does not matter if they came off a B18 or B20 for use.


I would suggest a good going over with your engine to sort the fuel consumption. You should be getting close to 30MPG on normal driving in a 131 and not much worse in a 122s

For city driving, I would say 25 and occasionally 30 is about right for a twin SU car. For economy runs (60-ish up and down the motorway) you should be closer to the 30s. But then spirited bursts (to show the **** on your tail that you're not some dozy out for a dawdle) will bring this down.

Make sure your choke is going right back, this can cause bad mpg. Ron Kwas' site has a good article on carbs and choke showing everything to check. Are your carbs a tad rich? But rich is better than lean which can cause expensive (burned valves etc).

Good luck

Tom

As Dai says there is quite a bit of mix and matching that has to be done. You already have a car with OD so you can check and compare with that. Late B20 Amazons never came with OD as standard. They also have a cable clutch which complicates things. J type OD came in with the B20 but although the box is still an M41 the OD's don't interchange. I'm sure somebody might query that but there are some differences. You mostly buy the box/OD as a pair so best to keep them that way. If you use a hydraulic clutch/bell housing the later ones bolt in. Sorting out the shorter front prop and matching flanges usually the the only problem.

Thanks for that guys. Re the daily driver, from all the evidence, it is a 1969 122s with b20 engine and 4 speed box (which means it has the later prop?), the donor is a b18 powered '66 120 series (no idea which one as no evidence on car or log book and don't know enough to identify). sounds like the gearbox/overdrive mountings may be different, hence me wondering if it's just easier to swap the engine and box to get overdrive and better consumption rather than faffing and trying to make the wrong stuff mate up. Thanks for the tips, will have a look at the article you suggest too. Much appreciated:thumbs_up:

Thanks Tom, I'll check the article and that the choke is going right back, and also check myself for the spirited bursts as we tend to have a few of those!! It seems worse on a 60ish run on motorway and i figured that may be due to lack of overdrive as my other one is ridiculously more economical. Will do some investigating as part of the learning curve

The bell housing bolt spacing didn't change. You simply fit an early gearbox that started life with an hydraulic clutch bell housing onto the cable clutch bell housing the car will already have.