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BN1 3 Speed to BN2 4 Speed


Jedi Hopeful
I am sure this post is here somewhere, but I searched and did not come with anything? I am upgrading my BN1 3 speed transmission to the more reliable BN2 4 speed transmission. Is there a good link or post to the process? I am told I will have to weld the rear mounting brackets to the frame and unbolt the BN1 mounts that there now. Could someone shed some light. Thank you in advance!



Jedi Trainee

The BN1 has a center member that is just in front of the cross of the two diagonal members. This has to be removed in order to make room for the O/D unit on the 4 speed. The BN1 mounts use this member and the BN2 mounts to the sides into the main rails if I recall correctly. Other than that and a new tunnel I think it's pretty straight forward swap.

One option which I have considered myself at times is to use the Smitty 5 Speed conversion. Toyota 5 speed trans which is one of the best around and mounts right in. As I understand it no cutting or modification required so you are maintaining the chassis 100% if that's important.

The previous owner of the BN1 that I have now did the same thing and I just reversed it to stock 3 speed....

Hope this helps.


Country flag
I have not done it but looked into it to when I had a BN1, but ultimately got my BN1 box rebuilt, the 3 speed tends to be leaky and even when rebuilt the synchros were slow, though I never had a major failure with mine despite driving it hard.

I kind of liked the unique nature of the reverse pattern and it was nice to have synchros on first.

Anyhow back to the cnversion, in addition to the mounts you will need a new or modified tranny cover, which will probably mean a new carpet for the clutch cover or the whole care if you can't match it as well.


Country flag

I have modified my BN1 to fit a 4 speed gearbox and BN2 overdrive. The support for the overdrive had to be cut out and a new one welded in. In the photo you can see its not as wide and sits back a bit than the original. Hovever getting hold of a BN2 overdrive in Australia is proving difficult.



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Senior Member
Country flag
I performed this switch when I first got my BN1 with a bad box in 1975.
The BN2 box was available at the local scrapyard, but I put my original overdrive on the new box for some reason--there probably was a separate cost for the overdrive. I believe you may have to change the clutch--check on this.
Somehow I got everything back together and the car drove again.
I never had any welding done to accomplish this.
Make sure that you will end up with the gearbox eye that the engine tierod attaches to.

I didn't notice that I did not have the eye till years later; probably caused by using the later gearbox with the early overdrive. The gearbox is firmly mounted despite not having the tierod.

Dave Russell

Yoda - R.I.P
Also you will need to add the BN2 bell housing & it's associated clutch & release parts. The housing was integral with the BN1 transmission but is a separate part on the BN2. This housing is getting hard to find. Repro housings are available but at a steep price. Here is one source:

With all of the expensive changes required it's no wonder that the Toyota conversion is popular. I would personally do the four speed with OD conversion instead of Toyota just for the sake of more originality.


What order were the B series transmissions used in AH...? My '57 Wol has, I understand, the same transmission, only it doesn't have 1st gear locked out?
The engine is an early 2 port and side linkage!

Dave Russell

Yoda - R.I.P
gblawson said:
What order were the B series transmissions used in AH...? My '57 Wol has, I understand, the same transmission, only it doesn't have 1st gear locked out?
The engine is an early 2 port and side linkage!
As I understand the evolution, the BN1 originally had the early four speed. When the BN1 was actually road tested it was found that first gear was so low as to be useless in the lighter car. The quick fix was to block out first gear & add an overdrive.

A few folks experimented with removing the first gear blocking but found as the factory did, that first was too low to be useful & was also very fragile.

This evolved to the newer? four speed with OD for the BN2 & up. A much more robust transmission. It can be argued that the four speed first gear is still too low to be useful. A change from the BN2 axle ratio of 4.11 to the 3.55 axle ratio makes a great combination for the light weight, high torque BN2 & actually makes first gear somewhat more usable.

Healey 100

Jedi Warrior

I understand the reason for blocking out of the first gear on the BN1 slightly differently. I read somewhere that it was done because the early spiral bevel rear axle could not handle the full throttle torque in that first gear. Most folks have removed the block and there should be no problem with doing that provided you don't use it for stump pulling or burning out. I use mine occasionally for crawling in traffic, it is handy for that.

I agree with Dave Russell that the Toyota transmission makes a lot more more sense if you find it necessary to ditch your BN1 transmission. I find the BN1 transmission very pleasant, with its funky shift pattern, tall first (second) gear and quiet, syrchronized operation. True, spare parts are hard to find and supposedly some of the gears are weak. But the BN2 transmission is hardly a jewel -- with its notchy shifter, whiney first gear, and the need to cut up your frame and transmission tunnel to use it. And BN2 transmissions aren't all that easy to find anyway.

Bill S

Dave Russell

Yoda - R.I.P
Hi Bill,
Interestingly, the rear axle was changed from spiral bevel to hypoid some 6,500 cars before the transmission was changed to four speed with the BN2. BUT they didn't reinstate first gear in the early gearbox at that time.
Quote, Special Interest Autos, Feb. 1982 interview with DMH:
"How come the first Austin-Healey used a three speed gearbox instead of a four speed?"
Quote, Donald Healey:
"It's no secret now. Austin used that gearbox in a car where they didn't take anything like the power out of that engine that we did. A car called the A90. It was a four speed box, but we couldn't get that bottom gear to stand acceleration starts with the power we were delivering. We used to tear them up. So Geoff (son Geoffery Healey) said, "Shall we lock it out & put an overdrive in it?"
It's well known that the torque applied to the transmission internal gears is proportional to the gear ratios. It follows that blocking out the original lower first gear would result in considerably less strain on the transmission.
The reasons may be forever lost in the mists of time. I think it's generally agreed that first gear in the original transmissions, if reinstated, was too slow to be of much use except in creeping parades, & the factory DID have to replace a lot of the three speed transmissions under warrenty. True the BN2 transmission isn't exactly a jewel, but it's strong enough to withstand the power of the later cars without trouble.

I once used a BN2 transmission & axle behind a Chevy V8 conversion. They handled it well.

PS - To illustrate the torque vs gear ratio difference:
The only difference between these two BW T10 transmissions is the gear ratios. Note the increased torque input rating of the lower multiplication ratios, 375 lb-ft, vs the 325 lb-ft of the higher multiplication ratios.

Another Quote:
"There were two different ratio sets used in the Warner Super T10.

The standard Wide Ratio 4-speed on the L48 and L82 had ratios of 2.64, 1.75, 1.34 and 1.00 to 1. It was rated at 325 lb-ft of input torque.

The optional on the L82 only Close Ratio 4-speed had ratios of 2.43, 1.61, 1.23 and 1.00 to 1, and had a torque rating of 375 lb-ft."


Country flag
Dave, that story certainly makes sense, I always wondered why they didn't leave first gear in place and put a lower numerical final drive.

My "3 speed" had the first gear unblocked, it was really to low to be practical, especially with the low revving nature of the big Healey 4s.

I still liked the 3 speed box, it suits the torquey, low revving nature of the motor. But I do blame my loss of an impromptu drage race with my brother's TR3 to the 3 speed box vs his 4 speed, although we never had a rematch after I rebuilt the motor. The cars have very similar power to weight ratios.



Do you have the BN2 gearbox already?, or is it a 100/6 early 3000. there should be a small brass disc rivited to the top section of the gearbox that say's "type BN2/BN4/BN7", take your pick. if it is a BN2 then things are easier. The next thing is are you going to do the work yourself or is someone else?. It is possible to do this yourself as people on this forum will "walk" you through it step by step.

The first thing is you will need is either a BN2, 100/6 or sideshift 3000 clutch "driven disc" (clutch plate) but don't bother with the 10 inch diameter one as the original 9 inch diameter one is strong enough. If you want to put a complete new clutch assembly I would go for the BJ8 diaphragm one as it is lighter and easier to get.

The clutch operating mechanism has the pedal mounted on a shaft that is supported by the main chassis rail and where the shaft comes through the main rail it has a confusion of rods and levers that are a headache to deal with. Keep the arm that attaches to the pedal shaft and the one that is mounted to the clutch bellhousing and make up a link that attaches the two of them together. this will make life so much easier for you.

The small crossmember that the rear of the original O/D mounted onto needs to be removed completely. With the gearbox in the car and supported with a jack so the new rear mounts are just touching the frame I drilled the 4 holes through the chassis rails and also picked up the same holes in a piece of 3/8 inch thick steel plate. This plate bolts underneath the chassis rails so that you don't crush the rails when the bolts are tightened. I then welded a vertical piece of 1/4 inch thick steel plate onto the 3/8 inch plate directly behind where the gearbox stabilizer bar projects back. Yes a hole has to be drilled in this plate so the bar can pass through. I also welded 2 triangular shaped gussets either side of the vertical plate facing back. With some flat rubber bushes on either side of the vertical plate this will give you some relief from any vibration. I actually made a new stabilizer bar as the one I had was worn half way through.

You will need the cover that goes over the gearbox that keep all the cruddy bits out when you drive along, i e nuts and bolts that fall off and pieces of low flying birds that try to kamikazi you along the way. This cover is also there to attach carpet to so your interior looks somewhat even. The BN1 has a 2 part cover and is of no use to you here. Any sideshift 6 cylinder cover will work but a "transition piece" that matches the front of the cover to the bulkhead of the BN1 will need to be manufactured. If you can get a BN2 one that would make life so much better as It would match up. The BN1 carpet will not fit but that's a minor detail.

This sounds a lot of work but is well worth the effort. By now you will have more questions so let's hear them.

If it's a 6 cylinder gearbox you have then let us know and we can tell you what to do as far as that.


Yes it would be just about impossible to find a BN2 O/D unit in OZ so why don't you get one off a 100/6 or 3000? they are the same thing.


The only reference to "B" series comes in regard to the engines of the MGA/MGB/ Wolseley 1500 type. These have a capacity of 1489cc to 1798cc so were never fitted to Healey's. The internals of the transmission in your Wolseley LOOK the same as the Healey ones but that's about all. The ratio's are different but everything else is the same size. The reason your Wolseley doesn't have 1st gear blocked out is your Wolseley weighs more than the Healey. Does your Wolseley have a column shift or floor shift? and is the starter on the right or left looking from the front of the car?.

Dave Russell

Yes it is interesting about the change points from the spral bevel axle to the hypoid axle and the 3 speed to 4 speed. One would expect them to be done at the same time but I suppose that was BMC and their ongoing political struggle. That is interesting (it is to me at least) about the torque ratings of those 2 ratios, the closer the ratio the higher the torque rating.

Altight that's enough for now.



Freshman Member
Hi All:

I'm a 24 year Healey owner, but a new member to this forum and am well into the process of rounding up the bits to fit a side shift 4 speed gearbox into my 1955 BN-1. I was very happy with the 3-Speed, but second gear broke and a broad search for the correct replacement (apparently they used two different P/N's and the one I need is extremely rare) yielded nothing.

I'm pretty committed now (although I have not yet cut out the BN-1 rear mount) having purchased a good OD gearbox (with a BN-2 button, no less) locally and a repro BN-2 bell housing from Denis Welch. Everything else I need is readily available except for (1) a BN-2 clutch fork and (2) a transmission tunnel - I'd really like to use an alloy tunnel rather than fiberglass if I can find one to work with. Any leads on either of these?

Thanks much.


Jedi Knight
Win -

I note you are in N. Cal.

You should give the boys at British Car Specialists a call... in Stockton. They have one of the largest and most complete stocks of Healey parts IN THE WORLD and a very nice guys to boot. Best part is stuff will come to you in a day or two.

I know they carry the BN2 bell housing in stock, and I am pretty sure they carry the clutch fork as well. The tunnel I'm not so sure, but if I'm not mistaken I think you can just do some very minor modification to a 100/6 tunnel and it fits.


Freshman Member
Country flag
Hello all. I see that this is an old post but I'm hoping that one of you who is familiar with the BN1 transmission mount can help me find one that's in good condition. I'm restoring a 54 BN1 and want to use the original 3 speed but the old mount that I have from a prior owner is torn up. Thanks.

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