Mounting a 420G to an m20b25

[JROM141]

Hi,

I've got a 1991 BMW 325i auto and have recently acquired a 420g out of an e46 m3 and want to mount it to my m20b25.

I've had no luck trying to find consistent information on what I need to adapt the 420g to my m20b25. I've had a well respected fabrication shop here in Australia tell me it will bolt straight to it and another one say that it won't work or will need heaps of work.

I understand I'll need a mounting kit to fit the 420g to the chassis but how can it fit to the existing m20b25?


Thank you!

[82eye]

Hi,

I understand I'll need a mounting kit to fit the 420g to the chassis but how can it fit to the existing m20b25?

will probably be tilted over ten degrees. you'll need to solve for trans mount and linkage.
couple 6spd threads:
can has 6speed manual transmission? (bimmerforums.com)
[HOW TO] e46m3 6-speed your e30 - R3VLimited Forums

the bimmerforums swap used the zf. the r3v swap is for the 420g.

[Panici]

It should physically bolt onto the engine but there are some points you'll need to consider:

• Flywheel/Clutch/Pressure Plate/Slave Cyl configuration.
• Driveshaft length and connection (giubo) to transmission output flange.
• Transmission mount to chassis.
• Shift linkages.

Why specifically do you want to run the 420G? I can understand E36 guys swapping over to get an Overdrive top gear, but the G260 already has an overdrive 5th.
If you're just looking for fuel economy, it's not worth the time and effort.

[hasa]

Perhaps much better gear ratios than G260, and 6th gear anyway

[JROM141]

It should physically bolt onto the engine but there are some points you'll need to consider:

• Flywheel/Clutch/Pressure Plate/Slave Cyl configuration.
• Driveshaft length and connection (giubo) to transmission output flange.
• Transmission mount to chassis.
• Shift linkages.

Why specifically do you want to run the 420G? I can understand E36 guys swapping over to get an Overdrive top gear, but the G260 already has an overdrive 5th.
If you're just looking for fuel economy, it's not worth the time and effort.

Only reason I'm going with the 420G is because I've gotten one with a parts car and thought that six gears would be a bit better than 5.
I've been told that the 420g is a better box than the G260 so I'm wondering if it's worth all the extra trouble since I've already got it on hand?

[JROM141]

will probably be tilted over ten degrees. you'll need to solve for trans mount and linkage.
couple 6spd threads:
can has 6speed manual transmission? (bimmerforums.com)
[HOW TO] e46m3 6-speed your e30 - R3VLimited Forums

the bimmerforums swap used the zf. the r3v swap is for the 420g.

Thanks for the links, I'll check them out.

[Panici]

I'm wondering if it's worth all the extra trouble since I've already got it on hand?

It's not worth it unless you've got money and time to burn.

[digger]

the 420g gears are spaced a bit closer especially 1st so you get less rpm drop between 1st to 2nd shift.
It would also allow less revs on cruise for the same 1st gear change point. Its not a massive difference ~200rpm with say a 3.38 final drive but you could go longer with 3.23 like the e36 m3 euro evo.

However, when you consider the e46m3 runs a 3.62 you have to wonder if 3.38/3.23 is too long for the old m20 in which case then 1st becomes pretty short with a low redline ~6000rpm vs 8000rpm. obviously a slight difference in wheel rolling radius to.

tbh i would consider for mine as i intend to buzz to 7500 but the angle seems like compromise the niceness of the shifting? ive never experienced 420g in an e30 to know

the diesel 320d fixes the angle but lots of custom stuff and potentially needing to retain the DM flywheel isnt nice im sure you could custom.

[TobyB]

The gear spacing on the od 6 speed is really nice...

...just sayin'....

t

[hasa]

The gear spacing on the od 6 speed is really nice...

...just sayin'....

t

This is what I'm saying too. Basically same ratios than in my full race 4-speed dogbox + creeping + overdrive gear. I can't figure out why it would not be worth of something and that's why I'll convert mine to GS6-37BZ (with diesel bellhousing to correct the angle).

[digger]

This is what I'm saying too. Basically same ratios than in my full race 4-speed dogbox + creeping + overdrive gear. I can't figure out why it would not be worth of something and that's why I'll convert mine to GS6-37BZ (with diesel bellhousing to correct the angle).

the 37BZ petrol ratios into the 37DZ diesel housing? have you seen any writeups?

[hasa]

the 37BZ petrol ratios into the 37DZ diesel housing? have you seen any writeups?

We have small language barrier here. I'm not 100% sure what you mean/ask. AFAIK the bellhousings (front part of the transmission to be correct), are interchangeable. However the input shaft is too deep for M20 flywheel/clutch combinations so it requires major work.

If it seems that I'm not aware of something important, please let me know...

[digger]

We have small language barrier here. I'm not 100% sure what you mean/ask. AFAIK the bellhousings (front part of the transmission to be correct), are interchangeable. However the input shaft is too deep for M20 flywheel/clutch combinations so it requires major work.

If it seems that I'm not aware of something important, please let me know...

The mounting issues for the 6sp diesel box are somewhat known, i mean are you talking about putting the petrol gearbox internal parts into the diesel housing to achieve more favourable ratios rather than just using the whole diesel box? do the internal parts swap straight in ?

does using the petrol internals in the diesel housing alleviate any of the known issues using the entire diesel box e.g. the input shaft and flywheel situation?

[hasa]

Front part should be the same (except the tilt angle). So taking DZ bellhousing and bolting it to BZ back side should solve the tilt issue. Input shaft distance is the same, so it is another issue to be fixed.

[digger]

Front part should be the same (except the tilt angle). So taking DZ bellhousing and bolting it to BZ back side should solve the tilt issue. Input shaft distance is the same, so it is another issue to be fixed.

https://www.brittanscustomparts.com.au/product-page/input-shaft-adapter-gs6-37dz-bz

this has been used before to extend the shaft. not the most elegant solution though.