M50 swap - M50 or M42 gearbox?

Post Sun Jul 20, 2008 1:43 am

I'm picking up a manual 525i tomorrow for my M50 conversion, but I also have an M42 gearbox kicking around. Is there any advantage in using the M42 box or am I better off sticking with the M50 box? Will I have to mess around with clutches and flywheels if I choose the M42 box?

Thanks

Post Sun Jul 20, 2008 1:49 am

Use the M42 box and prop.
Get yourself an M20 flywheel and use either the M50 or M20 clutch

Im going this route on OBs car next weekend.
Do a search for more detailed info,theres tons available on here

Post Sun Jul 20, 2008 9:48 am

M42 prop is too long, get M20 prop, fly, clutch, starter and cut 56.5mm out of both gear linkages. Gearing is still a bit short on the 3.91, so kep em peeled for a longer diff.

Post Sun Jul 20, 2008 10:06 am

what prop can you use with a m20 box??

Post Sun Jul 20, 2008 6:23 pm

I'll use the M42 box then, what I didn't mention is that it's for my E21, so the propshafts will be a bit of mystery until I get everything in the engine bay. I took the engine and box out of a E21 323i so a flywheel won't be an issue.

Why exactly should I use an M42 box compared to the M50? I've read the M50 thread, but that was a while ago and its a bit of a monster!

Thanks

Post Sun Jul 20, 2008 6:27 pm

Only benefit is you can keep your original diff, if the ratio is reasonable to begin with that is.

Is yours a 323i? if it is I would keep the M50 box as I believe they had a 3.45 ratio diff?

Post Sun Jul 20, 2008 6:36 pm

It's a 316 BUT I bought that 323i specifically for the rear axle. I was worried it was because the ends of the prop wouldn't match or something, but it's because of the ratios I take it.

Cheers

Post Sun Jul 20, 2008 9:52 pm

E36 ZF box on the M52 has a 1:1 5th gear ratio, the Getrag 240 on the M42 has a 0.85:1 5th...

I'm using the M42 so I could keep my final drive of 3.91 in my known good LSD and keep the same kind of RPM for motorway cruising etc... - HOWEVER. - Since pretty much doubling the power, 1st gear is useless and 5th gear on the motorway is over 3k RPM for cruising between 70/80mph. Not an issue in a 4 pot, but the 6 would happily pull motorway speeds below 2k RPM.

To be fair, I think the Getrag 240 box, coupled with the 3.45 diff will be pretty good. You really need a diff below 3.15 such as a 2.93 if you're going to use the ZF box.

If you use the Getrag 240, get some fancy pants Redline Lightweight Shockproof oil from Opie as a safeguard. Also look at the difference between the 323i (+30mm) release bearing versus the standard clutch release off everything else in the BMW manual range... Jordan in the US recommended this, although I've used the standard one and it works fine, just get a bit of chatter off the throw-out bearing with the clutch relaxed in neutral.

Can't think of anything else just now, but feel free to ask if we've missed something.

Post Sun Jul 20, 2008 9:54 pm

im using a m20 box what props best to use??

Post Tue Jul 22, 2008 9:44 pm

The M42 240 box is plenty tough enough for am M50 2.5. The E36 325i uses the same gearbox as the 316i, 318i and 320i. The E34 525i uses a bigger stronger box but you don't really need it.

Post Tue Jul 22, 2008 9:50 pm

jaffro wrote:im using a m20 box what props best to use??

Jaffro did you get me PM 'tother day mate?

Post Tue Jul 22, 2008 9:52 pm

yeh mate i did pm ya bk. it should be with you by now

Post Tue Jul 22, 2008 10:15 pm

Dont forget ,the E21 will need a speedo drive !

I'd use the M20 box, prop and linkage, all fit back in no issues at all. keeps the final drive the same so £££ saved and overall, a better car @ the end of it all.

I'm not a fan of the ZF boxes, bulky change and ratios dont suit the E30 IMHO.

M20 fly is a given though

Post Tue Jul 22, 2008 11:35 pm

Thanks for the replies, very informative. I know about the speedo drive and up until now I've just disregarded it. Originally I was going to be doing an M42 engine swap into my E21 316 but the engine crane collapsed and the head ended up cracked, so I've moved onto an M50 conversion. I was planning to run a bike speedo for the time being with the M42 engine, but I do like the idea of using the original speedo. I'm starting to think that modifying the linkage is worth it due to the continued inconvience of not having a proper speedo. I have quite a few E21s (6) and one of them has a dogleg box and I have thought about using that, but I do actually have the normal M20 5 speed E21 box when I removed it out of the 323i the other day.

I'm sure I've read that although the M20 boxes bolt up, the linkage has to be fabbed up due to the rotation of the gearbox in relation to the engine. I'm wary of chopping up my dogleg box linkage since these don't grow on trees!

The gearboxes I could use
- M50 525i
- M42
- M20 5 speed 323i e21
- M20 dogleg 323i e21

I'm starting to lean to the M20 boxes, but I'm very open to suggestions.

Thanks,

Rob