how to convert bn1 gearbox to 4 speed

[musme]

I hear some people saying that the BN1 and BN2 gearboxes are the same. It this true ? Is it posible to convert a Bn1 gearbox to 4 speed and what are the results.
Thanks,

 

[Randy Forbes]

The gearboxes (and sheetmetal transmission tunnel) are definitely different, but I'll leave explaining what has to be done to swap a later 4-speed into the BN1 chassis to someone that knows the earlier cars better than I do.

 

[Healeysince59]

The BN1 gearbox is a 4 speed with first gear blocked out and is different than the 4 speed in the BN2. To convert, you need a BN2 bellhousing, gearbox cover, front panel, and a BN2 gearbox. Original bellhousings and covers are hard to find but repros are available ($$$). I think there is also a minor change to the chassis at the rear mount required.

Marv J

 

[Legal Bill]

Just wondering... What if you removed the block so that you could access first gear and then install a higher (numerically lower) rear gear ratio?

 

[glemon]

You know I always thought that would have made, sense, but rumor has it the gearbox will not survive so well if driven spiritedly in first gear.

Although it seems to me that if the tranny was ok with first gear open in a heavier car (think Austin A90 Atlantic) you would think it would be OK in the lighter car as well.

I had a BN1 with the original 3 speed and the blanking plate taken out of first. It was really really low, and seemed to drag going in and out of gear, I messed around and used it a couple of times, but didn't feel much was gained as any jummp in acceleration was offset by the extra time needed to shift the not really quick BN1 box.

I too thought the higher rear end unblock first idea made sense, but I think even with the taller ratio don't know if the first would be that usefull.

 

[Randy Forbes]

We had a "daily driver" 100 in the shop I worked at in San Francisco (Austin-Healey West) back in the late 70s that I had removed the blanking plate during a rebuild.

As I vaguely recall, the car would "launch" a distance of about six (6) feet before you hit the lofty 4800 RPM red line, and needed to shift. Talk about your <span style="font-style: italic">Granny Gear!</span>

It was universally agreed to have been a waste of effort, and the blanking plate was put back in.

 

[Michael Oritt]

"As I vaguely recall, the car would "launch" a distance of about six (6) feet before you hit the lofty 4800 RPM red line, and needed to shift. Talk about your Granny Gear!"

Exactly. The BN1 transmission was taken out of a "Lorrie" and even if you were to install a Lempert differential you would only be able to get 10 or mph out of 1st gear before having to upshift. And just imagine how smooth that trip up through first and the shift into second would be. I imagine it would put great strains on the entire car.

 

[Michael Oritt]

Bill--

The only "higher numerically" differential available is the Lempert set which only provides an increase in ratio (or decrease in gear speeds) of about 10%. While I don't know what would be the maximum speed reachable in first gear, seeing as how this was a "Lorrie" transmission that was used in applications where they shifted out of "grannie gear" at about 10 mph just imagine how herky-jerky would be the climb from 0-11 mph where you would be turning redline speeds. I would imagine that the acceration and shift into second would be quite rough on the drive train and certainly no fun. Now if you came up with about a 2:1 gear set perhaps the results would be different....

 

[Bob Claffie]

My second AH was a 100/4 that I converted the stock gearbox from 3 sp to 4 sp. I have different thoughts from the majority. I used the car for autocross and never had a problem. There was never any thought to change the rear gear since with the OD it would go close to 100. As far as stress and strain nothing ever went wrong in the tranny or rear. Memory is vague but I seem to recall that there wasn't a plate in there to remove but a "boss" that had to be ground down to access the hidden gear, then again maybe not. Bob

 

[Genos2]

I converted MY '53 to a BN2 4/speed back in the '70's.It had a Corvette V-8,3 speed w/od. As I remember,mounts are different(drilled&tapped holes for BN2 rear trans mnt). Also had too cut part of frame in front of X-member(I saved&still have)Chevy to Healey clutch linage was easy,used stock driveline,speedo angle drive, ect. cheeers Genos2

 

[Randy Forbes]

My second AH was a 100/4 that I converted the stock gearbox from 3 sp to 4 sp. I have different thoughts from the majority. I used the car for autocross and never had a problem. There was never any thought to change the rear gear since with the OD it would go close to 100. As far as stress and strain nothing ever went wrong in the tranny or rear. <span style="font-weight: bold">Memory is vague but I seem to recall that there wasn't a plate in there to remove but a "boss" that had to be ground down to access the hidden gear, then again maybe not.</span> Bob

Nope, it's a plate that blocks the sliding member from being engaged.

I don't have a manual that shows 100 parts, or i'd post it. Maybe the Moss catalog...

... yep, item 90: https://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=28879#90

 

[mjobrien]

The BN1 and BN2 have different wiring for the O/D as well with the BN2 only using 1 relay.

If it's 4 gears your lookign for and were thinking of a swap a 5speed Toyota would be a good alternative. Keep the 3 speed so you can put it back to factory but having a nice reliable 5sp which bolts up is a great option.

https://www.austinhealeyclub.co.uk/tech_toyota.html


Michael

 

[BUNDYRUM]

Hello musme,

The BN1 and BN2 gearboxes are completely different gearboxes. There is nothing you can take from the BN1 and transfer to a BN2 and vice versa.

The original 1st gear as fitted to the A90 Atlantic has a ratio of 3.36:1 so is not going to do any BN1 any good.

The BN4 is basically a BN2 with some minor changes but nothing that cannot be overcome. If you are looking for a 4 speed Healey gearbox the easiest one to get is the BN4/BN7/BT7 ie 100/6-3000 pre centreshift.

I converted a BN1 to a BN4 gearbox back in about 1973.

Yes the rear gearbox mount is different but as I said before nothing that cannot be overcome.

Hello Michael Oritt,

The "Lempert" gear is not the only one available, surely the non O/D ratio is available too.

If the car is an early BN1 with the 4 stud rear axle it could be fitted with either the 3.667:1 ratio, a 2.92:1 ratio, or a 2.69:1 ratio as DWR have available. Yes I know about the price.

Best regards,

bundyrum.

 

[Michael Oritt]

<span style="font-weight: bold">"The "Lempert" gear is not the only one available, surely the non O/D ratio is available too."

If the car is an early BN1 with the 4 stud rear axle it could be fitted with either the 3.667:1 ratio, a 2.92:1 ratio, or a 2.69:1 ratio as DWR have available. Yes I know about the price."</span>

Bundy--
--------------------

Bundy

The non-OD rear is. I believe, the same as the Lempert but I was not aware that DW offered such tall ratios, though they ARE for the early diff. I wonder what applications they sell them to--high-speed runs? Thanks for the info.

 

[pan]

Re axle ratios: I believe the 100S sales brochure offered final drive ratios of: 3.6, 2.9 & 2.6. This model used the "spiral bevel", 4 stud axle like the BN1.
Even with the 3.6 in a BN1, "bottom" gear is still way too low to be any use on the road. I know, I have tried it.
The BN2 O/D relay wiring is different from the BN1 due to the deletion of the speed governor switch.
Randy is correct, the "block" on the low gear is in the form of a plate. See Figure 4 on page F/4 in the BN1 Service manual. Item 56 is the the "change speed gate". Only a small modification is required to remove the block. Most BN1s in Australia have been opened up in this way, but why this is so has always been a mystery to me.

 

[GregW]

Re axle ratios: I believe the 100S sales brochure offered final drive ratios of: 3.6, 2.9 & 2.6. This model used the "spiral bevel", 4 stud axle like the BN1.
Even with the 3.6 in a BN1, "bottom" gear is still way too low to be any use on the road. I know, I have tried it.

Hi Pan,
If you want a higher speed in first (for a given RPM), you'd want to go the other way and use the 2.6

 

[Healey 100]

My BN1 gearbox has the block removed. I find the granny gear useful for crawling in traffic jams, especially when going uphill. The regular first gear is a bit too high for that, you have to slide the clutch or clutch and de-clutch constantly. In the granny gear, the car can crawl along at idle smoothly at 1 or 2 mph.

I thought the reason the first gear was blocked out was because it overtorqued the rear axle on hard acceleration. So I don't do that.

I have driven lots of Healeys with the four speed side shift tranny that came with the BN2 and BN4/6. I would have to break my transmission and not be able to repair it before I would swap it for one of those four speed units. That four speed might be stronger, but it also has a very notchy shifter with a whiney non-syncro first gear. The three speed transmission is quiet, and the tall first gear is a pleasure most of the time -- you don't feel like you are rowing the car through stop and go traffic.

Bill

 

[BUNDYRUM]

Hello pan, nice to see you on here again.

I was one of those who changed his 3 speed BN1 box to a 4 speed BN1 box only for me it was very easy as I used the change speed plate out of am Austin Atlantic box (oh, I wonder where that came from).

If you can remember back all those years where I used to live in Melbourne and the condition of the driveway (it was a lot better when you used it). The extra low 1st gear was a boon as the car would virtually drive itself up the steep driveway in and out of the potholes at a snails pace thus giving extra years of service to item such as exhaust mufflers/engine oil pans and everything else that was likely to get knocked around.

Hello Healey 100,

The reason the 1st gear was blocked off was simply the car didn't need it as it would start off in the original 2nd gear without any bother.

The extra low !st gear is non syncro also so it would mahe the same whiney sounds you refer to.

Don't knock the side shift BN2/BN4/6/7 gearbox, it will give a lot of reliable service. I was also one of those that installed a BN4 gearbox into my BN1 and can honestly say I never looked back. That Healey did a lot of miles all over Australia and all I can say is I'd never go back to the original 3speed. I am certainly not giving the 3 speed a bad time it just wasn't what I wanted after experiencing the 4 speed.

Hello GregW,
Using the 2.6:1 axle ratio would give you a 2.02:1 final drive with the O/D. The problem with that is you would need mountains more torque than what the BN1 could muster and you would most likely lose out on top speed. You could use it without an O/D and it would still go better than the original.

Best regards,

bundyrum.