MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MG Y Type - Axle ratio change

I have decided to uprate the rear axle ratio on the YB, by fitting a 9/41 crown wheel and pinion set from an MG ZB, to give me a 4.55:1 ratio. Part nos ATB 7147/ATB 7148. Has anybody done this and can give me any practical tips to help me make the task easier. I shall be using the original YB Diff, but with new bearings and have new bearings for the ZB pinion. Should the ZB pinion face be set at the same point as the original YB.Many thanks in advance.
Keith D Herkes

Keith, I fitted a 4.55 to TD rear end(same as YB) many years ago upgrading from a 4.8. I can tell you this exercise is a piece of cake. The crown wheel on these two ratos is identical only the pinion is larger so you you will need the larger pinion bearings, can't remember about the pinion bearings spacer, and you will need the later fine spline drive flange and different front seal I think. Not sure what you mean about the position of the pinion face. If you are talking about the drive flange, a few mm difference will not matter as the tailshaft is telescopic. When I did my mod I converted the carrier bearings to tapers but that is a much bigger job. On that topic make sure you get thrust bearings of angular contact not standard gutter ball races, they will wear very quickly. The original bearing from memory was 19LTJ35 but there is also a non thrust 19LJ35.

As a matter of interest the MGA 4.3 and the MGB 3.9 banjo type axle also fit with a little extra work. Our TD now runs a 3.9 and although you have to hold through the gears a little longer the lower revs at cruise are brilliant, 3500@70mph!!! With our engines producing more power and better petrol they can handle it and the TD is definitely no slouch. I would go for 4.3, this would be a good compromise in the heavier saloon, you will be disappointed with 4.55. I wrote a detailed article on my mod years ago I will have to find it and submit it to the webmaster for a tech topic. Fell free to fire any questions back. Good luck, I lied when said it was a piece of cake, these split back axles are a prick of a design.
Richard Prior

Just a note re the bearings if you do go ahead with the conversion... 19/LJT35 and the likes were/are purely automotive bearings specifically designed for more modern vehicles, invariably using spacers, or inner races being extended and acting as spacers. Each digit on the prefix denotes one modification away from standard ie 19/ was nineteen mods from a standard LJT35, now known by the way as a 7205 (LJT35 is like talking in old money... Hoffman, R&M and others). If you're starting from scratch let me know your ideal dimensions and I'll select a more standard industrial quality bearing accordingly.
Richard is quite correct, LJ bearings (Light Journal) are purely radial load bearings (though they were typically used on wheel bearings including the rear hubs of the YA as I found out during a recent rear hub seal change). LJT designs (Light Journal Thrust)take radial and thrust loads and are therefore better suited. As I have worked for the largest bearing distributor in Europe, BSL, for the past 27 years I can gladly supply mint RHP/SKF bearings to suit.... just drop me a note.

Regards,

Ken
Ken Jones

Sorry that bearing conversion should have read 7207 (last two digits x 5 = 35mm bore.
Ken Jones

Richard and Ken.
Thanks for the quick response. I am not an engineer, so all this information is all part of the learning process for me. The standard ratio on the YB is only 5.125 so whilst I take Richard's point about the TD I felt the jump for the YB would be too big for the type of driving in the UK. On that basis I got hold of a ZB CW&P with the 4.55 ratio.
Ken, perhaps I should have posted my request before I went out and bought the new thrust bearings for the Pinion and the Differential from one of our major MG parts Stockists!(at considerable expense)The bearings for the Pinion are manufactured in Japan and carry the markings NTN 4T 31888 on the outer bearing and NTN 4T 151005 on the inner bearing.

My question re the Pinion face was the setting of the pinion by the use of the appropriate thrust washer, relative to the crown wheel. Other articles I have read normally relate to the fitting of MGA CW&Ps and seem to quote specific calculation/measurements for this setting.
Richard, good to see you are coming to UK with the car in June.
Keith D Herkes

Hi: as has been pointed out, the TD and YB rear axle are essentially the same. To fit any of the later crown wheels and pinions from the later MG,s, Austins ect. is not an afternoon's job. Some fairly accurate measurements need to be made and a lot assembly and dis-assembly to get the head of the pinion in the right location, the pre-load correct and the back-lash as specified. The "Ring and Pinion Interchanges for the MG TD/TF" by Carl Cederstrand is almost a necessity to do the job without a lot of wasted time and frustration. It is available from MOSS.
Regards, Hugh
Hugh Pite

HI, the head of the ZA pinion should be .018" further from the axis of the ring gear than the original Y/TD/TF pinionis.
Regards, Hugh
Hugh Pite

Hi again, the tapered pinion bearings will be a mixed set if you do the conversion. That is, for each of the pinion bearings the inner cone (and roller assembly) will need to be from the ZA and the outer cup from the "Y". These bearings are availabe at any bearing supply house and are much cheaper than buying from the MG suppliers. The ZA preload spacer will also need to be used.
Regards, Hugh
Hugh Pite

This thread was discussed between 22/01/2003 and 23/01/2003

MG MG Y Type index

This thread is from the archives. Join the live MG MG Y Type BBS now