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MG MG Y Type - MG Y Differentials

Hello ,
Anyone out there that can advise on what diff ratio,s are possibly on a MG Y type being a heavier car than TD 1250 or TF 1500, I have a friend who wishes to go to a 4.55 but I am not sure that it would be compatible to a Y, your thoaghts would be appreciated
T M Clarke

TM,
Which variant is your Y. I believe the Y and YT have a different rear axle to the YB.

Have you looked at the IMGYTR > Hints and Tips page > Higher ratio rear differential item by Tony Slattery. He lists all variants etc and includes additional information. Tony has fitted a Morris Minor 4.55 differential and was able to travel at 100kph (@ 3625rpm) whilst towing a trailer with spares etc.

Remember, the fitting of a different differential will require machining of both the Y housing and the internal body and will need to reuse the sun gears.

Also search the archives.

cheers
Stuart
Stuart Duncan

I fitted a 4.55 Morris diff to my brother's Y-Tourer. It suited the car well, however it did have a 1466cc engine. The Morris diffs are easy to fit to TCs but are a bit more of a squeeze in a Y-Type (or J-Type) because those diff banjo housings are smaller.

Another option would be a 4.875 ratio, but they are harder to find. Taller ratios, such as 4.875 or 4.55 are probably going to be OK unless the car is often driven in hilly areas. The engine would also play a part. A more powerful engine will cope more easily with a taller ratio.

The full range of Morris ratios ... 5.375 5.125 4.875 4.55 4.2 3.9 3.7. New 3.5 Crownwheel/Pinion sets are also available from Germany. I use a 4.55 in my road TC and usually a 4.2 in my race TC (3.7 for circuits with long straights). Some ratios are less common than others. 4.2 and "taller" ratios require a bit of extra work to adapt to our cars because there is extra webbing to cut away and oil filler/level plugs have to be added.
Cheers,
Bob Schapel
Bob Schapel

Thank you both Stuart and Bob,
Friend with the YB has just found a 455 from an MG ZA today so will fitt same, will confirm if all,s well after the rebuild, once again Many Thanks your input.
Terry UK
T M Clarke

Hello Terry. A YB diff is essencially the same as a TD/TF diff. Check the Cederstrand book for all the info. You will need the MGA pinion bearings, need to grind the pinion head spacer down to about .094 from around .125. If you use the pinion bearing spacer from the yb diff you will be close in length but will need to bore out the small end to fit over the larger pinion shaft. Then you need to set the back lash between the chrown wheel and pinion as in the book. One thing I have found is that the gasket for the case shrinks after it is manufactured. The material is .015 thick, you need to wet it so it will stretch without breaking. When you compress the gasket it compresses down to .012 thick so you get the .003 to .004 preload on the carrier bearings. Remember that the carrier bearings need to be placed on in the correct load bearing direction.

Butch
R Taras

I run a 4.22 midget/sprite CWP in my YT. The housing had to be extensively modified. Result is a hybrid but looks the part.
Gearing is matched to the engine tuning, which is somewhat more than factory (XPAG balanced and tuned to stage 3.5). Wheels are 16" radials.
In top gear we cruise at 100kph @ 3650rpm (accurate measure from a digital tacho below the Jaeger Chronometric). Not sure how Tony gets the same rpm on a 4.55 diff.

BTW, I evolved from 5.14 to 4.875 to 4.55 to 4.22 over several years as the engine was progressively developed. Very happy with 4.22 and have no issues with hills.
R Ades

Hi Robert,

My RPM at 100km/h has been checked with an electronic tacho and GPS speedo and correlates to the calculation sheet attached that covers all the possible options of diff ratio, tyre size and rim size.

I think that 4.55 is as low as you want to go with a well tuned XPAG. Your engine performance would allow you to go to 4.22. I also have the 4.55 in our Y-Saloon, and it does have issues with extreme climbs on the single carb.

Safety Fast

Tony

A L SLATTERY

Hi Tony, interesting stuff!

I'm running 195/65R16 radials, dia=660mm, circ=2073mm, revs/km=483. With my 4.22 diff, I'm doing 3650rpm on the digital tacho @ 100kph on the GPS speedo.

Picking 175/80R16 from your table, dia=686mm, circ=2156mm, revs/km=464. The table says 4.22 dif should give 3359rpm at the engine.

Based on tyre size alone, the wheel revs/km difference is 4% higher for my tyres. But with the same 4.22 diff, the engine rpm difference between these two cases is 8.7%. I can't reconcile this, unless there's a calc to translate the wheel revs back through the diff to engine revs.

I've gone with 195/65 tyres to have more rubber on the road, which has helped handling, whilst not being too tall for the wider tyres to foul the guards (particularly when braking into a corner). What radials are you running? It's interesting that we end up with the same engine rpm with different diffs.

Cheers,
Rob

R Ades

This thread was discussed between 30/10/2022 and 19/11/2022

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