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Hello guys! long time without posting anything technical related.

I hope this post would help a lot of people and break some myths about the M42 swap on a car with mechanical speedo.

This is not the cheapest way, not the easy way, but in my opinion its the best and correct way to drive the speedo on any E21 or 2002 M42 Swap, and you have a Gearbox with mechanical speedo and the correct mountings for the car

Yes i know you can retrofit an electronic speedo, but after reading a lot about it, i think its a bit of "frankestein" on that mod, althoug iám an electronic technician and i dont like it.


You need 2 things to do this conversion, first of all ,of course an M42 Getrag 240, and the rare E21 M10 Getrag 240 from any 4 cilinder car.



Yes i know its hard to find but not imposible, i found 4 gearboxes like this here in Uruguay so for you i think its not going to be hard.

In this conversion you have 2 ways, the Easy way and the Hard way. wich way you are going to choose depends on the condition of the M10 Getrag 240 you get.

The internals are the same, the only 3 things different between the 2 gearboxes are:

1- The type of lubricant they use, the M10 uses gearbox oil, and the M42 uses ATF. (The difference is in the material and composition of the sinchros and gear clutches) . Remember if you follow the "easy way" you have to use Gearbox oil, and if you follow the "hard way" you have to use ATF.

2- The output shaft of the Gearbox its a bit longer, to hold the speedo gear ,and mechanism.





3- The rear and front bellhousing, are diferent but they share the same bolt pattern, and the internal casting is identical.





Easy and Hard way explain:

The Easy way : Put the front bellhousing of the M42 Getrag in the M10 Getrag 240

Just that, swap the front bellhousing and you have a ready to go, mechanical speedo gearbox to put on your M42 Conversion.

This is not hard, and you can do it, if you have minimal technical skills.


The Hard Way:, in my case i have a Worn out M10 Getrag so i have to swap the rear housing, the internals of the M42 gearbox, the output shaft of the gearbox and disassemble entirely the 2 gearboxes.

Importan Note: You only need to disassemble the entire gearbox if your M10 Gearbox, is in bad condition or worn out. If you have a good condition M10 Getrag 240, you Just swap the front Bellhousing, with out all the "titanic" disassembly.

Disasembly and etc:

So first of all, a bit of cleaning of the gearboxes, in this picture you can see the similarity between them, and this picture gives you a clue that internaly the gearboxes are exactly the same:



After disasembly, the gearboxes are identical, and almost all parts can be switched between them:



Main gearset comparison, as you can see they are identical!



And finally the Front M42 bellhousing and the rear M10 housing, they fit perfectly:















-

Next i´m going to post some pictures of the disasembly of the gearboxes, i hope this photos serves as a guide and reference for any work in this type of gearbox.

Unfortunately, i have to disasemble entirely the gearboxes, is not an easy task and i recomend you to do this job only if you have good mechanic skills, or give the job to a competent mechanic.

The reason i have to do the "hard way", totaly worn out gears on the BMW M10 gearbox:



First of all, take out the front shaft seal, and the front shaft secure ring:





To do this job you need an inverted set of pliers like this:





You must take out all the screws that hold the 2 housings togheter and the screws that hold from outside some parts of the gearbox:







Also very important the lateral plug, that holds all the mechanism in place, if you don´t take this out yo can´t separate the 2 housings:





With the help of 2 hammers, and after removed all the screws, plugs and holding rings, you can split the 2 pieces as shown in the pictures:











This is how it looks with out the front housing, all the gears and levers are hold in place by the rear housing:





So thats it, as you can see i think this, its the better way to run the speedo, withoout the "frankestein" style mods.

I´m not going to put photos of the "Hardway", because if you can do it, you are a mechanic and you don´t need the photos, but if you need the photos for technical reference, i can post them.

I hope that this guide, break some myths and doubts about this conversion, and help all the guys that are planning to swap a M42!!

That's awesome, and would work on m50/m52 due to the same bell housing pattern as m42. Is there any risk of things falling out as you separate the housings?

wiz0r wrote:That's awesome, and would work on m50/m52 due to the same bell housing pattern as m42. Is there any risk of things falling out as you separate the housings?

You have to be careful with the drive shaft not falling apart. the rest of the gearbox its attached to the rear housing.

I'm glad someone's finally done this, knew it was possible. Good work.

Jacko wrote:I'm glad someone's finally done this, knew it was possible. Good work.

Thanks Jacko!

I made some changes on the first post:

Easy and Hard way explain:

The Easy way : Put the front bellhousing of the M42 Getrag in the M10 Getrag 240

Just that, swap the front bellhousing and you have a ready to go, mechanical speedo gearbox to put on your M42 Conversion.

This is not hard, and you can do you it, if you have minimal technical skills.


The Hard Way:, in my case i have a Worn out M10 Getrag so i have to swap the rear housing, the internals of the M42 gearbox, the output shaft of the gearbox and disassemble entirely the 2 gearboxes.

Importan Note: You only need to disassemble the entire gearbox if your M10 Gearbox, is in bad condition or worn out. If you have a good condition M10 Getrag 240, you Just swap the front Bellhousing, with out all the "titanic" disassembly.

Impressive, and great work, excellent write up and puctures too. Thanks man! I'm going to move this to the Top Tech Topics!

legend - i have some spare boxes to practise on!

Jeroen wrote:Impressive, and great work, excellent write up and puctures too. Thanks man! I'm going to move this to the Top Tech Topics!

Thanks Jeroen!!!

I learned everything i know of E21 from this forum, so this is my little retribution to the community!

Well most welcome!!!

You mention a 4 speed M10 box, does it work with a 5 speed?

Thinking M10 5 speed box with front end swapped will connect to M52, work with 3.91 diffs on 316 for engine swapsies, or even manic trackday beast!

drjim wrote:You mention a 4 speed M10 box, does it work with a 5 speed?

Drjim: all the Getrag 240 Gearboxes are 5 speed (M10, M42, M40 etc)

The 4 speed M10 box its a getrag 242, and this swap only work with the 240`s

I read it again, you said 4 cylinder car not 4 speed!

Interesting option for engine swaps.

This is a great piece of work, got me thinking about the M42 swap all over again!