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MG TD TF 1500 - Rear Axle Ratio Preference

I am intending to convert the rear axle from standard 5.125:1 to either 4.5 or 4.3:1.

The engine has a Stage 2 upgrade with a fast road cam.

I live in South Devon UK where we have mainly single carriageway A roads and what is best described as "undulating hills", but also intend to take the car over to France. Hence my dithering.

Would really appreciate some learned advice on which would be the most suitable option.

Thanks in anticipation.
John Winzer

My TF has a similar state of tune. I am running a Ford Type 9 box and a 4.55:1 diff. The combination works well. Fourth gear is the same 1:1 ratio as top with the standard MG box. Anything above 35-40mph and I'm into 5th.
Dave H
Dave Hill

I agree with Dave. I have a five speed Datsun 210 transmission and roller lifters with a modern cam. I was careful not to lower my ratio too much and thus lose power in fifth gear. I also selected the 4.55 rear end with the new hardened axles.
I am assuming you are doing this to a TD just as I did. You will love it.
Mort
Mort Resnicoff

I run a 4.1 in my 1500 TF, with a very strong engine. I hardly ever have to shift down. PJ
PJ Jennings

Thanks guys.

Should have said I am running a TD.

I was advised by the MGCC Technical Advisor to get a genuine BMC Crownwheel and Pinion set if possible.

Been able to find someone who can do the conversion and has both the 4.55 and 4.3 available.

At the moment I have reserved the 4.3 but would obviously like to find out the views from those who have done this mod. Rgds John W
John Winzer

Hello John. I'm running a 4.3. I can hold 3500 rpm on 6% grades, but if someone is slow in front of me and I loose rpm I have to shift down to pass them. I've attached a scan from Carl's manual of road speed vs rpm.

Butch

R Taras

I had John Twists shop convert mine. They felt the 4.5 was overall good for the TD.

As pointed out 4.3 will give you lower RPM, but will require more frequent downshifting to keep RPM up.

The 4.5 allows me to cruise at 60/65. As Butch's chart shows. 65 gives me about 4000 rpm
Bruce Cunha

I would say it probably depends on where you spend most of your time driving.

If you are going to be on multi-lane highways then I would go 4.3

If you drive mostly on two lane roads with lots of elevation changes then I would go 4.55.

This of course coming from a person who still drives in a 5.12, but rarely on multi lane highways.
Christopher Couper

Hello John,
I converted my stage 2 TD few years ago to 4.3. 4 speed gearbox. And no regret at all. I can drive in top along "nationales" or "périphériques" at 50 mph for long periods at 3000 RPM. Quiet and the engine is quite happy.
I also can drive at 70 mph on highways and even reach speed of 75 mph when passing. Once I had a top speed of 82 mph(GPS) when I decided to slow down.

I also drive in Pyrénées mountains. The car is not so lively that it used to be for these steepy winding roads but it is still a great pleasure to drive with a little more shiftings.

Fuel consumption dropped down from 30 mpg to 35 mpg for a mountain trip and even 38 mpg for flat roads at 80 km/h

Laurent.



LC Laurent31

I would advise the 4.3. have a 3.9 on a mildly tuned TF. This gives around 20 MPH per 1000 revs about the same as a Ford T9 conversion but without the advantage of another gear. On my local roads all is OK but when I venture into Wales I sometimes have to cog down too much as the ratios are too far apart on the standard box.
Ray TF2884.
Ray Lee

I went with a 4.55, two reasons, I found a brand new in box CWP and it keeps it within original specs for classic racing.
B W Wood

My TD is standard and I am running a 4:3 which I really like. I have NO issues and thoroughly enjoy driving the car locally or extended trips (2000 miles)
Regards, Tom
tm peterson

I have driven thousands of miles with the stock 4.875 , a 4.3 and now a 4:55. I have run the car at 70 mph all day long in each. Should mention I also have a 5 speed along with a modern supercharger. I stepped back to the 4:55 as I did not like the steps between the gearing when driving in the mountains around me. .Just my two pence.
L E D LaVerne

My TD is fairly stock with .060 overbore and slightly milled head/block face. While our area is very flat, the 4.5 I changed to is still too low. Wish I had done 4.3. George
George Butz III

Thanks to everyone for your well intended and helpful replies.

Hope I haven't opened a can of worms between the 3.9's, the 4.1's, the 4.3's, the 4.55's, the 4.875's and the 5.125's!!

So, what am I going to do?

Well, I am going to opt for the 4.3, which will hopefully giving me the balance of flexibility on local roads here in rural South Devon as well as motoring in Europe.

So once we have come out of "lockdown" here in UK, I will take the axle to the transmission specialist who I have been told is experienced and has done many of these conversions.

Thanks again to you all. I will report back.....

John W


John Winzer

Thread update!
I have now got a 4.3:1 rear axle and have just put the car back on the road.
The car is so much nicer to drive and really gets down the road. Really pleased with the result.

The only fly in the ointment is gear stick chatter. It was originally present, and on stripping the remote housing it was badly worn.
I had the remote gearstick housing line bored and bushed in way of the gear selector shaft and refitted the unit back on the gearbox using a 1.5mm rubber gasket. But it's quite noisy in second, third, and top. It only stops when I hold the stick.

The gearbox has been stripped and rebuilt with new bearings, thrusts, new third gear syncro hub, and doesn't slip out of gear.
I have read suggestions about retarding the ignition timing. I am loath to do this. The engine is timed to 5 degrees BTDC and runs really well.

Is it a case of "they all do that Sir", and should I have to live with it.

Would appreciate any suggestions.

Regards

John W
John Winzer

Should have mentioned the gearstick chatter happens when the car is under load.
John W
John Winzer

Hi John,
To reduce gear stick vibration, I fitted a gasket made from thin rubber sheet (approx 2mm thick) between gear shift remote and gearbox. I found this to be quite effective, as have others as reported in the archive.

I also fitted a thin rubber pad and washer under each of the four mounting bolt heads to provide additional metal-to-metal isolation of the remote body from the gearbox housing.

The existing paper gasket can be used as a template for cutting the rubber sheet.

It's worth a try before being forced to other more complex measures!

David
David Padgett

John,

Further to my last, if you propose to fit the rubber pad/washers under the mounting bolts (recommended), check first that the bolts are long enough. I used longer ones and also included a drop of thread-locker.

David
David Padgett

John, B&G do an ‘updated’ gear lever spring to help wit the rattle. I’ve had one in my TF for 10 years or more and it completely eliminated the noise -

https://www.ukmgparts.com/product/td-midcat-1-subtd9-gearbox-mounts/gear-lever-spring-uprated-mgb-mga-td-f-11g3144

David
David Wardell

Gear stick chatter can also be caused by the timing being too far advanced.

Peter
P G Gilvarry

I second what Peter is saying. Check your timing. It can make all the difference in the world.

Regards, Tom
tm peterson

Gear Stick rattle is usually caused by the remote rod wearing its guidance hole in the aluminium remote and so it rattles.

It could be that the damper (that large bronze nut on top of the remote) has worn. Adjustment maybe possible or a rebushing.

Fitting another damper nearer the gear lever can also reduce/eliminate the rattle. I did this some years ago (10/20) with success.

To do either of the above you have to remote the long rod which involves removing the gear lever from the casing. There are two pins which retain the gear lever. These do not go through the gear lever so no amount of hammering will get them out. The answer is drilling one of them which requires care and using a pedestal drill. Once one pin is out the gear lever can be removed.

I replaced this pin with a suitable bolt (with a modified end to fit the groove in the gear lever) so that should I need to strip the mechanism it is or will be a matter of undoing the bolt.

Good luck
R J Marshall

Thanks to you all for your valued comments. They are much appreciated and will I work my way through them.

New uprated gear lever spring as well as a new brass plunger and spring have been ordered. Will fit these in the next few days.

The remote operating shaft housing has recently been bored out and rebushed and is already jointed on a 1.5mm rubber gasket. I will make some rubber pads to go under the heads of the securing bolts.

Will check the ignition timing and report back the results of my efforts.
Regards
John W




John Winzer

Hi John,
See the check list that worked for my car.
https://mg-cars.org.uk/cgi-bin/or17?runprog=mgbbs&access=&mode=archiveth&subject=8&subjectar=8&thread=2018093016460425002

Laurent.
LC Laurent31

Many thanks Laurent.
John Winzer

LaVerne, 70?? me thinks you was going faster than than on occasion. LOL
TRM Maine

This thread was discussed between 25/01/2021 and 23/04/2021

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