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The 100 Forum

Transmission and Overdrive BN 1 Vs. BN 2

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rockingchairmotors Avatar
rockingchairmotors Jeff Bayuk
Nazareth, PA, USA   USA
Getting ready to put together a '55 BN 1. I have the original gearbox but also have a rebuilt (suppose to be) BN 2 box with the OD unit. They are obviously different. Can someone tell me if this BN 2 box will work and what I will need to make it work. Thanks in advance.

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RAC68 Avatar
RAC68 Raymond Carbone
NJ, Jersey Shore, USA   USA
Jeff,

Although I am not up on exact differences between BN1 and 2 transmissions, I do know that the BN2 trans has been chosen by many BN1 owners to replace their original. As I understand, the original BN1 transmission had its 1st gear blocked as it was a real stump puller. This left only 3 selectable gears with overdrive available on the top 2. The BN2 transmission is a true 4 speed with overdrive in 3rd and 4th. If you are not concerned about concourse and the BN2 trans and overdrive is in good shape or can be reasonably rebuilt, I would expect most would suggest you go with the BN2 transmission.

Hope this helps but I know other have this combination and can present actual operation and implementation experiences.
Ray (64BJ8P1)

simon1966 Avatar
simon1966 Silver Member Simon Griffin
.Madison County, IL, USA   USA
1953 MG TD
1954 Austin-Healey 100 "BEV"
1977 MG MGB
This is a recent photo of a BN2 box and overdrive installed in a BN1. My car had this added back in the 60's. I gather it was a common period modification.


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PAN Avatar
PAN Alwyn Keepence
Ipswich, Queensland, Australia   AUS
G'day Jeff,
As can be seen from the photo supplied by simon1966 the main difference between the BN1 & BN2 overdrive is the mounting at the rear. The overdrive in the photo does not look as if the rear is mounted securely at all. I would suggest that a bracket be fabricated to provide a secure rear mounting.
The early BN1 had an enclosed solenoid lever as well but that would not prevent a swap.
Fitting a BN2 transmission would also require a BN2 type gearbox cover.
Personally, I find the ratios in the BN1 well suited to the torquey engine. The wrong-way-round shift pattern puts some people off, but on a 100 frequent shifting is not required, anyway!

wakani Katlan J
None, MD, USA   USA
Mystery solved. To replace the BN1 tranny with a BN2 tranny you will need a bell housing from the BN2. All else is "bolt on" The BN1 tranny was intergal to the bell housing.


Just so happens I have an excellent BN2 Bell Housing that could be delivered for a price. Includes the linkage, throwout bearing mount and if I remember a servicable throw out bearing. You will also need a new clutch plate. I think the pressure plate is the same.

rockingchairmotors Avatar
rockingchairmotors Jeff Bayuk
Nazareth, PA, USA   USA
I want to thank everyone for their response and there help. Katlan I would be interested in your parts. Can you email me rockingchairmotors@yahoo.com Thanks Jeff

Lambdaman David Egan
Wakeman, OH, USA   USA
Is the bell housing still available?
Regards
David

gramos Avatar
gramos Graham Foster
Kefalonia, Greece   GRC
Hi Ray ,
just saw your post and can add a couple of things to the advice given .

use a diapghram clutch cover and plate from the 3000. its a far better , stronger and lighter assembly ,
and the original aluminium gearbox cover /housing ?
the BN1 can be modified to suit with a little shaping and a small section added , and
yes , I have done these mods to my car !

good luck ,
Graham

Vintage n Classic Avatar
Vintage n Classic Silver Member Geoff Golding
Nelson, Tasman, New Zealand   NZL
The rear mount of the BN2 is in the form of a "V" and on a lot of BN 1 Cars the owners make the appropriate modification to the chassis to cater for this. Two rubber mounts are then fixed to the rear of the overdrive to fit into the "V" mounting.

Careful with 1st gear on the BN 2 gearbox can snap off the shaft. Use 1st gear to just get rolling and then change up to 2nd gear to accelerate. If the gear snaps you can get it welded up so its as new but its a specialist who completes this work.

A number of owners I know use 2nd gear to pull away on flat road - the engine is "torquie" enough to do this without undue load on the clutch

100M Avatar
100M mike lewis
harbor island, SC, USA   USA
Like Geoff said above, I almost never use 1st gear on my 100M. No need as plenty of torque. If I'm on a hill or something like that would use 1st.

gramos Avatar
gramos Graham Foster
Kefalonia, Greece   GRC
Me too ,I always start in 2nd , 1st gear if you want massive wheelspin grinning smiley

I converted to a BN2 box after breaking the original 3 speed twice .

simon1966 Avatar
simon1966 Silver Member Simon Griffin
.Madison County, IL, USA   USA
1953 MG TD
1954 Austin-Healey 100 "BEV"
1977 MG MGB
This thread popped back into consciousness via an email exchange this morning. For the purpose of completeness, here is my BN2 box 5 years on, having been restored and properly installed in my BN1 by the good folks at Fourintune. Alwyn is correct the rear mount is different, My frame was modified by the insertion of a new x member with the correct mounts, Photos show this modification as well as the box back in the car a year ago as the restoration neared completion,


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