420g will not physically go into 5th gear

[tantumaude]

I have an interesting little challenge ahead of me. 1988 750iL with a manual swap--somebody took a 420g and cut+welded the auto V12 trans bellhousing onto the front.
I can't get the transmission to go into 5th gear, running or not.

I took the gearbox out of the car, still no joy. Feels like there's a blockoff or something. Usually these gearboxes have issues with 3rd, never seen 5th issues. spinning either input or outputs while trying does nothing either, nor does touching other gears first.

All other gears worked fine when the car was running, and I can easily shift into them still.

I know about the arrested ball/sleeve SIB, but I don't know if that would help.

Any insight appreciated!

[bmwdirtracer]

If you can't get it in fifth with the box out of the car, I'd have to guess you've got a bent shift fork. Try using a tool to move the shifter shaft independent of the linkage and the shift rod.

[tantumaude]

If you can't get it in fifth with the box out of the car, I'd have to guess you've got a bent shift fork. Try using a tool to move the shifter shaft independent of the linkage and the shift rod.

Thanks for that; please pardon my ignorance, but can you share some advice as to how I would do that? I've never opened these before.

[bmwdirtracer]

Do not open the gearbox. The shifter shaft is the rod with the ball knuckle "gearshift rod joint" which the shifter linkage attaches to, in order to manipulate the internals of the gearbox.

All I'm saying is that you should take every EXTERNAL part out of the equation, including that "gearshift rod joint (part 7 in the link), because external parts become sloppy - that joint is highly prone to slop.

If the damage is internal to the gearbox, you're kind of stuck, because parts are not available, and assembling these things isn't within the realm of a shop or DIY anyway; only a specialist takes apart a 420G, and I don't know any.


https://www.realoem.com/bmw/enUS/sho...diagId=25_0009


I seem to recall that the Euro 750 had a 420 G six speed available. I'm sure they're rare, but maybe you could purchase a salvage used one from Schmiedmann in Denmark.
https://www.schmiedmann.com/cms/en/used-bmw-parts

[tantumaude]

Noted, thank you. I've found that there are two repair kits for the detents, I'll look into those as well.
The v12s only ever came with 560Gs in 850Ci guise insofar as I can tell--no 750s were manual from the factory sadly.

[bmwdirtracer]

Damn...you're right...no manual 750's! I'm afraid I'd always thought that Jason Stratham's (Transporter) E38 was a 750 -- I just looked it up, and it was a 735.

A friend built an E38 M740, with the S62 and G420 - a very lovely beast. I take it that the 750 didn't have a bellhousing that would bolt up to the G420 and so had to be cut and welded?

Okay, then....onto finding a gearbox rebuilder. Seems to me that if you found such a rarity, you could take him a used G420 (from, say, a 540i E39), and your gearbox, and he might well be able to fix the issue.

I Googled "manual gearbox rebuilders ontario canada" and THREE came up.

I hope you'll be able to fix the issue - sounds like a very cool swap. Please let us know.

- - - Updated - - -

EDIT:

Wait - look here - Metric Mechanic rebuilds G420's! This is an excellent company, and I recommend you check them out.

https://www.youtube.com/watch?v=7Sj1Kfzcjvg

[tantumaude]

You're correct, the first 10cm of the auto 750 bellhousing were welded onto the 420G (whose own first 10cm were removed).
I've contacted a couple of local places, but nobody seems interested in taking on the rebuild at all. You read my mind about bringing them another 420, but it seems the input shaft bearing is shimmed into place for each gearbox so there isn't much chance of getting that right either.

I'll do some hunting over the winter.