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MG MG Y Type - Morris diff in Y-Type

To whom it may be of interest ...
The Morris A-Series diff is good for for TCs and Y-Types because there are seven ratios to choose from (5.375 5.125 4.875 4.55 4.2 3.9 3.7) AND the MG spider gears will fit in place of the 10 spline Morris spiders. This allows the original axles to be used. Unfortunately these diffs in good condition, are becoming scarce. Best to use one which does not have cracks in the carrier. Most have alloy nose-pieces, which are lighter and have a thick flange, which is more suitable for machining.

The spider gears usually fit perfectly if the small, dished Morris washers behind the planetary gears are DISCARDED but the large flat Morris thrust washers behind the sunwheels ARE USED. Rarely the spiders bind a bit and the flat thrust washers have to be thinned down a thou or two. Also ensure that the planetary gears are VERY free on the cross shaft. The sides of the carrier need to be ground out a bit because otherwise the axles are too tight a fit. I go by feel, but guess the axles need at least 15 thou clearance.

The nose piece has to have 3mm machined off the gasket face and re-drilled to suit the TC or Y stud pattern. I fill old holes by threading them and fitting threaded aluminium rod with JB Weld, before having the lathe work done.

Now this is all fairly easy for a TC, but the Y-Type axle banjo is much smaller and so also needs the diameter of the nosepiece (and caps) reduced significantly so it will fit into the banjo! This extra machining results in very little metal being left near the four carrier bearing cap studs. The banjo can be ground out a bit so less has to be machined off the diff. Better still, the metal can be removed using a milling machine or clever, off-centre lathe work, so as to leave more metal near the studs. Then the banjo can be ground out locally at these four points, where (luckily) there are no bolt holes. Cap nuts and washers might need some filing too, depending on the methods used.

TCs need a breather hole drilled in the banjo, but Y-Types have that location as standard. Tail shaft length is no problem because the original sliding splines accommodate. Note that 4.2 and "taller" diffs, have a bit of a complication because some metal "webbing" has to be removed and a filler/level plug must be fitted.

I have been using A series diffs in my road and race TCs for 45 years. I have converted about 45 diffs, but most have been for TCs, one for a Y-Tourer and two for J-Types (similar banjo size to Y-Type).

I hope I haven't missed any details! Cheers from South Australia,
Bob Schapel

R L Schapel


Bob

Thank you for this thread. Sound interesting and tempting. Pity, you live on the other side of the big pond :o)

stay tuned
Anton
Anton Piller

Bob, I have a MM diff sitting on a shelf and a spare rear axle housing (purchased from Tony S) sitting around, just got to get them to a machinist.

I will keep my current diff housing standard. I am hoping to just swap diffs and axle halves when ready to do the change.

I have a copy of Ewan Ward's Jan 2018 "MGY Differential Conversion" 5 page paper (with photo etc), published in the Octagon Bulletin.

When I swap axle housings I am looking at refurbishing the rear end - spring rubbers, bushes, panhard parts etc.

Bob, do you have modern part number(s) for the rear axle bearings.

cheers
Stuart
Stuart Duncan

Stuart

I checked the December/January issue of the Octagon Bulletin, but could not find Ewan Wards article "MG Y Differential conversion"
What page should I look for?

Anton.
Anton Piller

Anton, Sorry Ewan wrote the article in January and it appeared in the September No 564 Bulletin, 2018, as per text across top of pages.

cheers
Stuart
Stuart Duncan

Thanks RL for starting this thread. I'd been asked about my diff conversion to 4.22 and was getting around to writing that up.

About 10 years ago I took the route of having the YA diff housing modified to take the range of A series options. This was a laser-cut and reweld job, meaning that subsequent changes wouldn't need further machining and parts would remain standard, if a bit mixed. MG Workshops in Melbourne did the work. We kept the same Y parts you've described with the same clearance adjustments you've described.

The housing modified, my first experiment was a 4.875 CWP set from a MM. All good but not that much improvement in cruising speed. After some engine work to boost HP a bit, I changed to 4.55. This was better, but 60mph was still a bit above 4,000rpm. With a bit more engine tuning and a warm TD cam, we did the final change to 4.22 final ratio. The car now cruises all day at 60mph @ 3800rpm. Hills are no issue, plenty of torque. I've been running this configuration for 5 years, during which we've done two road trips from Melbourne to QLD. We'll take the YT to Gimpie for the Y rally next year, driving all the way.

As for the CWP set, I was able to buy a high quality new manufactured set from Germany and a good price. They make these for Sprites and Midgets and it's good German precision & quality.

A word is needed about half shafts. I've broken 3 over 15 years. The original shafts aren't strong enough, we all know that. Admittedly, one break was because we didn't leave enough clearance in the diff carrier, but the failed shaft was one of Octogon's new "quality" shafts. It turned out that these new parts are not all they're presented to be. Shaft diameter was a bit different and they're weren't properly heat treated (if at all). I'm back on original shafts temporarily but take delivery next week of 2 new race quality high strength shafts manufactured locally in Melbourne. I'm paying a bit to have just 2 made, but if they perform well a batch job may be organised in due course if there's interest.

Back to the diff ratio, I recommend the A series conversion and 4.22 CWP set. With a little development an XPAG in good order will drive this easily. Certainly for a YT. A saloon is a bit heavier so maybe 3rd gear will be needed on some hills. We drive in modern traffic all the time, no issues with keeping up.

Regards,
Robert
R Ades

Hi Robert, sounds like you have it all under control. Enjoy the YT. They are great cars. My brother and I both had them.
Yes, that German company also does a 3.5 ratio which actually makes it 8 ratios available .... very octagonal choice!!
Re axle breakage ... Quality of material etc is very important, and fatigue takes a part. However, there is another major factor in my opinion. I wrote an article about it in the MG Car Club of South Australia August 2019 magazine. The edges of the sun-wheel splines cut into the axles causing "stress raisers". I always grind the sharp edges off the sun-wheel splines. If I get time I will try to dig out photos and the old article and put it on this forum.
Cheers,
Bob Schapel
R L Schapel

Hi Stuart, Anton and Robert
There are probably a few different ways to fit a taller ratio to a Y-Type. They probably all have advantages and disadvantages. I like options where it is easy to revert to original. I reckon that helps keep the car's value up. With the Morris conversion, spider gears from a TC, Morris 10 and some Morris 8/40s also fit. If spiders can be sourced from somewhere else, the original diff can be kept intact on a shelf in the shed.

I have attached images of what I believe to be a large portion of the cause, and the result regarding axle breakage.
Cheers,
Bob Schapel

P.S. I must one day try to change my profile name on this site. It is now R.L. because I didn't know that was going to be "it". I think the last person who regularly called me Robert was my Latin teacher about 65 years ago! Cheers, Bob





R L Schapel

Hi Bob,

I agree with your sentiment of keeping the option of going back to original. All the mods I've done to our YT and its XPAG are reversible, and all the original parts are in storage. The only exception is the diff housing, but it'd take a committed pedant to climb underneath and measure the differences. The original Y diff will go back in but, honestly, why would you?

I expect the concours guys will throw a tub of cutting paste at me for saying it, but some internal mods are nuts to reverse. Our car is much more drivable and will live much longer between engine rebuilds. YT's are rare enough that a discretely modified diff isn't going to change its value. Not that mine's for sale. We'll keep enjoying it until we can't get the petrol any more.

Regards,
Rob
R Ades

Bob, thank you for the warnings.

I am unsure of the MM diff ratio I have, but it has to be better than the original Y one when driving on open roads or the motorway. The YT's XPAG has a high speed road cam and roller lifters, has been line bored and balanced, so has probably an extra 5-10% in HP.

Rob, when you decide to have a batch of 1/2 shafts made, count me in.

Bob, concerning how to change your 'RL' to 'Bob'. Click on the "Customise" button at top of page menu and edit your details.

cheers
Stuart
Stuart Duncan

Thanks Stuart,
I have changed it to "Bob". It appears that it takes effect on new posts/comments.
Cheers,
Bob
Bob Schapel

A quick postscript to my comments above about the 4.22 Sprite diff (CWP) in my YT...

As part of sporadic COVID-ISO amusements (and running out of projects) I've installed a small digital tacho under the dash to compare against the original Jaeger Chronometric. This proved that the Jaeger is accurate up to 3000rpm but then tends to over-read slightly.

So, using my GPS speedo and digital tacho I can revise the cruising rpm. At 100kph the engine sitting on 3650rpm. This is with the 4.22 diff and 195/65R16 radial tyres. The XPAG engine will cruise all day like this, with headroom to overtake. Plenty of torque in the long-stroking engine to power this gearing.

Regards,
Rob
R Ades

This thread was discussed between 08/12/2021 and 19/01/2022

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