Changing BJ7 Differential Ratio

[Riquet1963]

Hello,

I live in France and I have a BJ7. I plan to go to the European Healey Meeting in Norway next June, 6000 km by road.
To prepare for this trip I plan to replace my "CROWN WHEEL & PINION" to change the differential ratio.

Have you some advice about this operation? Some guys here speak about the risk of noise after this exchange.
Have you experience with that?

BR

Eric G

 

[Bob McElwee]

I put 3.54 gears in a BT7 and BJ8 and didn't experience an increase in noise.

 

[Bob_Spidell]

My father and I changed my BJ8's differential from the stock 3.9:1 ratio to a 3.45; this mod was the most 'extreme' one I've made to either of my Healeys, as far as changing the whole 'Healey Experience.' Since much of our driving in the USA has to be made on 'freeways,' unfortunately, I feel this change was almost required, as spending hours on end at 75MPH+ with the shorter rear-end is fatiguing to the point of torture. The taller 3.45 is an 13% reduction in RPMs at every speed, and the 300+ RPM reduction is significant. Yes, you'll sacrifice a little 'off-the-line' acceleration, but with a stout engine it's not a big deal and Healeys are not great dragsters to begin with (a modern Corolla with an automatic will beat you off the line).

As for installation, we used dial calipers and I think we did a decent job, as my father was an excellent mechanic but, yes, I have a bit of a whine which is usually only noticeable with the hood up. I've heard from others who've had 'pros' do the job and still experienced the whine, but some who had professional installation say theirs are quiet (it's what you could call a 'crap shoot'). My dad was a factory rep for Ford when the Mustang debuted, and he said a lot of those cars were brought to dealers with 'noisy rear-ends,' so it's a not uncommon issue. About the time I made the change I experienced some judder from the drive train on launch, but later when I rebuilt the gearbox I found a couple broken damper springs on the clutch disk, since that was remedied no more judder (but perhaps a slight bit more slipping is required to get a smooth launch).

There is a requirement for various spacers to set offset and backlash--we were able to reuse mine--which may be difficult to source, though the Healey (MOWOG?) differential is said to be very similar to contemporary American units. Backlash is set with a spacer instead of a crush washer, if less spacing is required the spacer can be milled down, but if more is required a larger spacer would be required. Of course, you can replace the bearings while you're at it, but mine were still serviceable at 100K+ miles. I like and use synthetic oils in both my Healeys' gearboxes and rear-ends. There is a specific break-in procedure for differentials; usually, it's up to 500 miles of increasingly longer drives, with a mandatory cool-down period in between. Your gearset should have included instructions; they may specify not to use synthetic oils for break-in, and you'll want to change the oil after break-in).

Overall, a very good mod, IMO, esp. if you can't take backroads everywhere you go. Some even recommend the 28% O/D as found in the 100s used with the taller rear-end; that would make an excellent grand tourer.

 

[Healey Nut]

If it’s installed and set up correctly there will be no increase in noise but you will get a significant drop in rpm in OD and it makes 1st gear actually useful and third gear becomes a great pulling up to speed gear .You will also be able to talk to your passenger at 70mph without having to yell at them .

 

[Rob Glasgow]

Changing to 3.54 diff. Was the single best improvement I made to my BT7 in 60 years of ownership. Hands down. And it is not noisy.

 

[twas_brillig]

I purchased a 3.54 diff a few years ago but it had been incorrectly set up and it takes very few miles to develop a wear pattern that results in a permanent whine. It`s currently sitting in a bucket, and we`ll try installing it again for next summer`s driving season and see if the noise is excessively irritating. The 3.54 was originally sold in Healeys that didn`t have the overdrive, so the factory thought that was an acceptable ratio. We also have the Toyota 5 speed conversion with a synchromesh first gear which make it easier to live with the lower ratios. I suggest that you have a specialist shop properly set the gears up.

 

[RDKeysor]

I purchased a 3.5 diff gear set from SC Parts in the UK several years ago. A fellow member the St. Johns Austin Healey Club provided me with a donor differential "pumpkin." I took this to Gear Works in Clearwater, FL, for installation. Though that dragged on a bit, they did the work, installing only new seals but not bearings in the process. Subsequently we removed the old gear set and installed the new one from SC parts in my '60 BN7 at a club work session that took only part of one morning. I have zero gear noise from this conversion. The reduction in engine RPMs, as observed by others, made driving the car much more satisfying. This car came into my hands with a "Smitty"Toyota five speed. My impression is that it does not have the most desirable gearing, likely not from one of the most recommended boxes from Supras, etc. I think I'm at about 60 mph at 3000 rpm. But still a wonderful transmission, every bit as good as the Getrag six-speed in my Porsche 968. I haven't driven a Healey with the original Moss gear box since I sold the BT7 I owned back in 1964 and have no recollection to compare how that transmission worked. I can look up my costs for the new gears and professional rebuild if anyone wants them, but now you have to beware of shipping costs from the UK.

 

[Bob McElwee]

I put 3.54 rear end gears in a BJ8 with a standard Healey 4 speed w/overdrive tranny and BT7 with a Toyota 5 speed. In the BJ8 it made the car much better to drive around in town. Before I would leave it in O/D most of the time to keep the RPMs down.
In the BT7 it made it a road warrior. We drove to many regional/national events, two lane and interstate. With the Toyota I was doing 70 mph at 3000 rpm.
I had a pro set both of the rear ends up. The BJ8 was our daughters car and don't remember her complaining about the noise. I think the BT7 was a little more noisy that before but not enough to really bother me.

 

[roscoe]

I'm sorry I'm not sure how to post a direct link to a previous post but if you search for one of my posts in this forum titled;

Rear end crown and pinion gear swap. 4.11:1 to 3.54:1 [long read]​

I hope you will find all you need. It is not a hard job for someone who has not done it but it will certainly help if you've used a dial indicator and calipers/ micrometer set before. You will need a torque wrench in the 0 to 50 inch pound range and a larger one for the pinion nuts and crown gear bolts. The factory shop manual is a pretty good guide but does leave out a little detail. The shims Bob referred to are in fact hard or impossible to find but if you can't use what is in the differential you have, you can either have some made or scavenge from another differential. I was also able to use what I had. There are other shims for setting the pinion preload that are readily available in many thicknesses. I did buy some of those. I did mine with no problems and it has been on the road for about 500 miles. I had never done this job before but I had rebuilt numerous gearboxes for helicopters, so the concept of gear mesh patterns and pinion preload were old hat to me. Youtube has many videos which cover the basics that apply to most differentials. Bonne chance.

 

[vette]

Hello Eric, as Bob Spidell said, it can be a crap shoot. I believe early gear sets can be a better quality. I have installed two sets of 3.54:1 gears in my BJ7 and both are noisy. The first set I couldn’t live with that’s why I bought a second set. The first set came from a highly regarded UK Austin Healey supplier and I had them installed by a retired NASCAR team member who spent years setting up rear end gears. He told me then that those gears will never run quite. The UK supplier would not take them back because I had them on the shelf for over a year before I used them. There was no deterioration to them during that time. I was very dissatisfied. I was able then to get another set of gears from Mike Lambert in the states. Mike would on occasion have a batch made then just sell them casually. I had them set up by a professional shop and different from my first installer just in case there was an issue with the first guys method. This second set also whined. But it is bearable and can really only be noticed with the top up. Even tho I miss the quietness of the original gears I am glad I have the higher gear ratio. It makes first gear much more usable. You can still start out with 2nd gear if you want to. I do not know which Overdrive ratio I have. My car was modified for rally purposes when I bought 20 years ago and at that time I never verified the OD ratio. With GPS my car is doing 62 mph at 2800 rpms in OD. I have 165/15 tires.

 

[Jack T]

Hello,

I live in France and I have a BJ7. I plan to go to the European Healey Meeting in Norway next June, 6000 km by road.
To prepare for this trip I plan to replace my "CROWN WHEEL & PINION" to change the differential ratio.

Have you some advice about this operation? Some guys here speak about the risk of noise after this exchange.
Have you experience with that?

BR

Eric G

Eric,

The 3.54 gears completely transform the car on the road, making it a much more relaxed cruiser at highway speeds. It is probably one of the best possible upgrades you can make to a big Healey.

While I am a reasonably competent wrench turner, I opted to have my gears set up by a shop who services a lot of our local vintage racers. I felt the cost was worth the peace of mind of not possibly damaging the gears and / or having a whining final drive.

Jack