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MG MGA - Thoughts on a 4.1 rear axle

I have a 4.3 in my 1600 coupe now, .060 over with 264 degree cam,.414 lift and balanced.I have a complete carrier assembly from a mkII which is 4.1. All I'd have to do is change the gears to match my axles. My question is would it be worth my time to install or would be better to go to 3.9? With my current set up with Continental TS 771 165 SR 15's, at 55mph I turn 3900 rpm at 60,4200 and 65,4600.Never been able to check tach accuracy, but speedo is right on, and engine does sound like it's turning that fast. Any thoughts or comments appreciated. Thanks, Gary
gary starr

With those tires and gears at 3900 RPM you should be running about 70 mph. At 4600 you should be doing about 80 mph. So, either the tach is off or the clutch is slipping.

Regards,
GTF
G T Foster

Gary,

There is something very wrong.
The RPM and speeds that you give equate exactly to a 5.125 rear axle ratio. Although this ratio was a competition item for the mga twin cam, there was never a speedo to match the ratio.
If, as you say, the speedo is accurate you could have a 5.125 Diff and the speedo has been recalibrated. Or the tacho could be wrong.
The first job is to lift one wheel off the ground, turn the tailshft by hand and check the ratio. Remember that with one wheel off the ground the free wheel turns at double the normal speed.

Mick
Mick Anderson

Gary, have you verified your tach? It is easy to do using a dwell/tach meter. Just compare the two readings.

This is a lot easier and quicker than lifting the car etc.


Chuck
Chuck Schaefer

I've owned the car 23yrs and have tried everything except checking with dwell tach. I did try once but my digital meter burnt out when I connected it, don't know if it was coincidence or because I have + ground.Too gun shy to try it again. I also foolishly didn't count the gear teeth when I had the diff apart, who knew at the time.But I have lifted the car as Mick suggested and it is a 4.3,and the carrier is stamped 10/43.My tach is the correct model- 2350/01,that being said it could be something changed in it. But anyway does anyone think it's worth it to change from a 4.3 to a 4.1? Thanks Gary
gary starr

I am working on my 1960 MGA in a box and trying to sort out my mystery engine, gear box and differential.
My engine is a 1622 bolted with the later backing plate to the second generation gear box built prior to 59.
If I understand the info in this thread there are two ways to determine my final gear ratio, one is to pull it a part and read the number stamped on the carrier and two turn the drive shaft and compare the number of rotations of the shaft to the wheel.
I assume and am requesting confirmation or correction that the wheel will turn once and the drive shaft 4. something times, or are both double or do I have it wrong way round?
Can I assume that 0.1 = 36 degrees, 0.3 = 108 degrees if not doubled with one wheel up.
Thanks
Morris
M. E. WADDS

Gary,
I believe that the ratio is stamped into the "pumpkin". Mine is stamped 10 41 and my 3.9 MGB is stamped 11 41 on the small ledge near the top close to the bolt flange that joins the housing. Hope this helps.
Doug
D Sjostrom

Gary,

You still need to determine what is correct on your car before deciding on changes.
In a 45 year old car the axle housing ratio stamp means very little. You say you have physically checked the ratio and it is 4.3 so my opinion is that, with your listed modifications, a 4.1 ratio would be OK.
I believe that the 3.9 ratio should only be used if you go to an MGB engine.
But check that Tacho and the Speedo.

Morris,

As Doug said, the ratio is stamped on the outside of the axle housing, but it means little in a 45 year old car that has probably had several owners.
The best way for one person to measure the ratio is to jack up one rear wheel.
Use a felt tip marker pen to mark the rear universl joint flange and an adjacent point on the rear axle housing. Mark the tyre and an adjacent point on the body or chassis.
Turn the tailshaft until the wheel does exactly one turn.
If the ratio is 4.3 the tailshaft would have turned 2.15 turns or 2 turns and 54 degrees.

Mick

Mick Anderson

Gary

The choice between a 3.9 and a 4.1 depends in part on what you expect from the car. If you want easier high speed touring then the 3.9 is a good choice. My 1500 has a 3.9 and I find that 70 mph on the freeway is about 3700 rpm and is very sustainable for extended periods.

FWIW

Larry
58A
Larry Hallanger

The point about external markings not necessarily being correct regards rear axle ratio is a good one. I have fitted both 3.9 and 4.1 ratios into my 4.3 marked housing on my 1960 MGA-1600. Had to, since I was a teenager in 1962 "Dropping the Clutch" (And often the rear end!)at 4,500-5,750 RPM in "LE GRANDE PRIX DE LA LAMPE CIRCULACION" at the time, running against the dreaded TRIUMPH TR-3s & TR-4s! Only problem I had was DIFFERENT NUMBER OF SPLINES IN THE REAR AXLES! There were axles with FEWER BY FAR SPLINES and the "PROBLEM" axles OFF BY ONLY ONE SPLINE! Another problem was the ONE YEAR ONLY (1960) when MGA had LEFT-HAND THREAD WHEEL STUDS on one side, and RIGHT-HAND THREAD STUDS on the other! The idea was, you always turned the wheel lug wrench towards the front of the car! MG dropped this BAD IDEA only after so-called "MECHANICS" with ELECTRIC & HYDRAULIC WRENCHES managed to SPIN OFF & BREAK THE STUDS! This is how come GHNL-88669 ended up in Japan with an MGB rear axle, but that's another story.
OLD BILL-67

Gary,
This is the USA. We have freeways that we truly use. If you plan to run the car on the road, the difference between 0-60 now and 0-60 with a 3.9 is very minimal.

Vibration
Engine wear
gearbox wear
Fuel economy
Noise

All are decreased. I have had 4.22, 3.9, and 3.7 in my Bugeye with a 1275 and even when autocrossing, I just dont see enough change in the low speed for a motor that has good midrange power. I say the Exact same thing for MGAs and Bs. If they made a 3.7, I would put them in half the MGBs I have owned, and i don't care who disagrees with me. I drive hard and fast, but unless you are Really trying to beat out every car that comes up to you at a stop light, its not worth it. These gears were made in a time where roads were not as well built or as fast, yes you could get your speeds up, but not ass easy as now. Sell your 4.3 and 4.1 gear ratios to an MG T series guy so they can gear there car up better too.

Now those of you with motors that have NOTHING below 4000 RPM and have a redline around 7500 RPM, stick to your low ratio (low speed) high numerical rear ends.

One more thing, by installing a 3.9 and having all of the benefits, you will miss the pure classic experience of driving a classic automobile, but then if your not running bias ply and getting caught/pulled around on every rut in the road, you have already missed the experience!

FWIW,
-BMC.

BMC Brian McCullough

This thread was discussed between 31/10/2005 and 10/11/2005

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