Another known fault with BA and BF Falcons is the driveshaft centre bearing.
I'm slowly ticking off the list of common problems, all tackled by your truly, next was this one (1. driveshaft bearing, done. 2. alternator, not yet but will be easy. 3 - Front ball joints, not yet must check. 4. Auto selector cable, done. 5. Indicator stalk wiring, done. 6. Diff bushes, not yet, that might be the end of the line if that happens...)
Symptom - as you take off from a stop, there is a clunking that feels like it is coming from the rear. Under heavy acceleration it is worse. Generally it smooths out as you speed up. It progressively gets worse over many months. What's going on here is that the drive shaft - from the transmission to the differential, is split into two pieces and there is a bearing it rotates in, attached to the floorpan.
It's easy to spot - lay down on the passenger side, no need to jack up the car, reach under and grab the drive shaft. You should not be able to wobble it.
How hard was this to replace? I did it, but unfortunately this is not just twist-a-spanner, in fact, I'm labelling it as "don't do this yourself" - read on for why.
First up, there are four models of centre bearing, you need the right one. You can narrow it down to two models by measuring the spacing between the mounting bolt centres. You can see mine here is 170mm. The second measurement is the shaft size, one is big and one is small. You can't actually measure it until you have split the shaft. So make sure you local parts supplier has both in stock, once you have it out go get the right one. The VIN number won't help you here. If I had to guess, if you measure ~35mm just before the bearing you probably have the 30mm version, if you measure ~40mm then you probably have the 35mm version.
Removing the drive shaft is straight forward. But mark EVERYTHING with a paint pen. So when you put it back together every single spline, nut and bolt is back exactly where it was. Nothing too hard here, don't be too afraid of where it bolts to the diff, it looks like it may explode open but it is actually a CV joint (weird!) and it stays together.
Splitting the shaft is hard. There is a 24mm nut holding the yoke on. I first tried putting the yoke in a vice and undoing it with a spanner. With a vey long breaker pipe. All I was doing was opening hte jaws of the spanner. Whoa, that thing is on tight! If you can crack it, excellent! I couldn't so I removed the universal joint. If you have never pulled apart a universal joint and put a new one in, stop now.
If you have the uni joint cup locating clips like I did here then you are in luck, you can remove the uni and split the shaft. Apparently some joints are not replaceable, watch for that.
Once it is split a rattle gun gets the nut off no problem at all. Replace centre bearing, rattle gun yoke back on with supplied new nut. Put the new uni joint in, grease it up, reassemble making sure your paint lines match up. Put back in car. Yah! you are done!
But wait, drive shafts are dynamically balanced. Even though you put it back together exactly as it was, you split the uni and put in a new one. Will it still be balanced?
Well it wasn't for me.
Sure, the clunking was gone - success - but now I have a vibration from around 30kph-60kph. Not terrible, but there. The drive shaft needs to be balanced. You can see the balance weights on the shaft. Some of them look small, and accurately placed. I thought briefly about putting some hose clamps on it (secure to the shaft, but with a weight on them where the adjuster is) - then I could tweak the balance through trial and error to smooth it out - but I never did, I've just lived with it. So far it has been 6 months, no sign of any problems, so not all bad.
If like me you aren't building a show car, you just want to keep an old Falcon on the road, this is fine. But, I was quoted AUD(2021)$500 to drop off the car and pick it up fixed and balanced, or $400 to drop off drive shaft and get back fixed and balanced. The $500 I reckon is worth it. I spent around $120 and the better part of a day - some of it visiting a mate with a vice and a rattle gun which was nice - but it was not a complete success.
So, in a very un-JAW stance; don't take this one on yourself. Sometimes, very rarely, it is better to pay someone else to do it. Good luck out there!