Morris Minor Owners Club

please can anyone help me. i am looking for a diff for my morris which i have put a midget engine in it.i have been offered a diff which sais on it 9/41 4-55 ratio i would like to know if its the right one .

Probably hundreds out there ready to offload a spare 4.55 onto you!!
If your Moggy is a standard saloon/traveller later than about 1961-62 then it will in all probability have that fitted as the standard! For a 1275 it will be fine, but a bit undergeared. A 3.9 (Wolseley 1500 and some Midgets) will be more relaxed all round by about 10 percent! If someone offers you a 3.7 diff(Riley 1.5), then you should prick your ears up, and don't tell anyone else about it or they/I will outbid you!!

many thanks for the advice

A 4.55 will be far too high a ratio - you definitely want at least a 3.9, although you could 'manage' for a while with a 4.22 until you find a 3.9 or even better - the elusive 3.7.

^^^^^^Ooops, I did it again!! 4.55 is indeed farrr too LOW (not high!) a ratio! If you are set on one then I can probably give you one free. From around 1962 the ratio was 4.22. As the Master says; 3.9 good, 3.7 double plus good!

No -4.55 is indeed a HIGH ratio...... it needs a HIGHER number of turns of the prop shaft for one turn of the wheels...... While 3.7 is a (relatively) LOW ratio - it needs a LOWER number of prop turns for one turn of the wheels.......

It dosen't normally work like that... Convention would normally term a 4.55 ratio as being a lower ratio than a 4.22 or 3.9 etc. Yes a higher number of propshaft turns are required for a turn of the wheels but the distance travelled is less for a number of turns. Its the same convention that refers to changing 'down' and lower gears in the gear box. High gear is normally called top.
Land Rover transfer boxes are refered to as LOW ratio for eg.
A 3.9 or 3.7 ratio equipped Minor would be high geared. Tractors are low geared!

And it just shows how wrong 'convention' can be. For me 4.55 is a higher number/ratio than 4.22 etc - it also requires higher engine revs for the same speed.......

Just two ways of looking at it, but it can be confusing for people when they think they want a high ratio diff...
Engineering convention isn't always logical. Take swg and BA thread sizes for eg.

I agree - the discussion can go on for ever........

in two minds on this subject. i used to have an 803 with a 5.1 diff. i then changed to 948 and a 4.5. slightly better i'd say. then i fitted a 3.9 and it wouldn't pull away to quick.

so i came to the conclusion that i'm happy with a 948 and a 4.2.

i still have the 3.9 diff though. someone will want it one day i guess.

I would look for a 3.9 and drop a 4.2 in until one turns up. I had a 4.5 diff fitted with my 1275 motor and it was terrible, switched to a 3.7 which was too far the other way, no pulling power (good on Mways tho). Have a look at this thread for more info http://www.mmoc.org.uk/viewtopic.php?f=4&t=42137