HELP! TL;DR did a 4 to 6 cylinder swap, kept the 4 cylinder diff and gearbox, swapped halves of the propshaft but it's too short
Donor car - January 1994 525i sedan (HD52)
M50B25TU
Receiver car - September 1995 518i Touring
(HG12) M43B18
Hey folks! Looking for a bit of advice...
Just swapped out my M43B18 (4 cylinder 1.8) for the M50B25TU (6 cylinder 2.5) everything is going fine, a few a niggles here and there but that's to be expected... But, I'm facing quite a big problem... I have kept the same Getrag gearbox and diff from the 1.8, and knew I'd need a shorter propshaft, so I swapped the halves of the props so that the flange for the diff (4 bolts) is on the prop and found the front flange is also different. This isn't an issue as I can get an adapter plate (78mm PCD to 96mm PCD) online.
The issue is that the prop is about 100mm too short, not counting the adapter... What can I do?
Do I need to swap the gearbox and diff for the ones from the 2.5? I read that I can still use the 1.8 gearbox and diff... Which I'd like to do (the gearbox and diff are spoken for).
Thanks for reading, and any advice is greatly appreciated!
Also, I'm sorry if this is in the wrong place.
~ Sweyn
Last edited by Sweynw; 03-24-2019 at 07:14 PM. Reason: Misspelling
Hello and welcome!
Few here will know much about the 4-pot E34, but I'll share what I can. If you're using the diff and gearbox from the 518, why not use the original driveshaft from the 518 too? If anything, the shaft might end up too long because the M50 is longer than the M43 and places the gearbox further back. But that'd only be the case if the trans crossmember were farther back too.
Two-piece propshafts are balanced as a whole; you can't mix-n-match without risking an imbalance. You can't even re-clock the splines on the same two halves; this is why marking the shaft prior to disassembly (to change the center bearing for example) is required. Sometimes one gets lucky and there are no balance issues, but it's a risk I would not take.
I don't know what ratio and gearset the 518 diff uses, nor what the transmission is. Chances are, it'll withstand the M50's decidedly less-than-fearsome torque, but you'd indeed have fewer headaches with a driveline to match the engine. However, I recall reading that the 518 subframe is different from the 525+. It might be shared with the 525td? As a first step, I suggest you research parts compatibility using RealOEM. Feel free to check back if anything is confusing. That'll determine your options for the next step.
Last edited by moroza; 03-24-2019 at 09:06 PM.
Sorry, Moroza*
That's handy, it deleted my post... Basically I tried the 525tds prop (I have one as a spares car) and it was too short but was the same flange for the diff case. I had also been advised to try a 520 prop (its 1660mm) but dont have access to one... yet. But I have now given my 1.8 prop to a machining shop to get it shortened to 1620mm as that is the distance from the diff to the gearbox guibo. And obviously get it balanced after. But cheers for your help Moroza and thanks for the welcome!
~ Sweyn
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