Hi Guys,
I have a Morris Minor with an A12 (B310 I think) motor and 56a 4 speed gearbox which is giving me some trouble.
The original gearbox I had in it gave me no problems for 5 years before falling apart. I got a replacement really cheap which I had reconditioned.
Since putting the new gearbox in I have had no end of problems with vibration and wearing out the bush at the back of the gearbox. I have had my driveshaft length modified too, so that there is what I thought was the correct amount of clearance at the back of the gearbox. Also new yoke/universal joint attached.

The problem is that once again I tried to take the car on a roadtrip and after an hour at 100km/h the back of the gearbox had gotten really hot and started vibrating more again.
What I'm wondering is what the yoke / gearbox join should look like when set up correctly? How much clearance should there be? If anyone has any photos it would be really helpful (I will put some photos of mine up soon). I have had trouble with this for so long now that I can't remember what it all looked like before the first gearbox died.

Any help or advice would be much appreciated.

this assumes a morris minor has a solid rear axle

the yoke (yolk) should go in half the length of the shaft on it, when the car is at ride height (or the car is held up by stands under the diff). this way it has room to slide in and out as the diff travels up and down

you probably have a balance issue with the new shaft, a good tail shaft guy can be hard to find. i have heard of many people having re-occurring issues using the same tail shaft guy, then taking it else where and getting a much better job

can you feel the vibration driving the car? if so it's either out of balance wheels, out of balance tail shaft, broken engine mount or gearbox/ engine jammed to or hard mounted to car

an obvious thing is if the steering wheel shakes when it vibrates, if so front wheels are out of balance if not then its one of the other things

Thanks for you reply. I have had the wheels balanced several times, so I don't think that's the problem.
I have only had one driveshaft place balance the shaft so it could well be that. I guess it's worth looking into.
I didn't notice the vibration while driving before, but now I do. It's not really in the steering bit it's really noticable in the gear lever so seems to be directly related to the drive train.

I have attached a couple of pics to show how it currently looks. I tried to include a tape measure for scale, but it didn't really work. The gap is around 10mm.

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  IMGP0002_s.JPG (110.38 KB)

That gap is way too big, nothing can spin concentric with that much gap.

The issue is that that peripheral flange on the adjoining face of the union is stopping the drive-shaft from going any further into the gearbox.

What is the tailshaft from?

The driveshaft is a custom made one with what I was told was a yoke off a 120y on the gearbox end.
Do these two parts not go together? If so then that would totally explain the problem.
I have a vague recollection that my original gearbox didn't have the copper coloured bit on the back of the gearbox (dust cover? or seal?) which would have let the yoke go in another ~20mm.
What is it meant to look like on an original 1200 or 120y?
I guess this is the problem with putting together a bunch of parts that weren't designed to go together.

Agreed it's way too big. if I was in your shoes, I'd get a new rear yoke altogether (if need be maybe someone could send you one from Aus if there's none available at wreckers yards over there) that has a 'front' on it that slips inside the gearbox 'sleeve' or cup (whatever you wish to call it) and a new tailshaft which is longer to suit it.

In very rough terms, you'd want about 0.5-1 inch extra room - i.e. if you push the tailshaft all the way into the gearbox (and it has to go all the way in, not just hit that sleeve on teh gearbox, in that case it can't 'ever' go in all the way do to the shape/particulars of the flange you have) - well with the tailshaft pushed all the way in, and the car at ride height, it should only be 0.5-1 inch forward from seating in the diff yoke, 1 inch absolute max

Another thing - due to the way universal joints work (as distinct from 'CV joints' whee the CV stands for constant velocity) - well believe it or not, since the uni joints are a cross and two way swivel, whilst the output shaft of the gearbox turns at a constant rpm, the tailshaft itself speeds up, slows down, speeds up, slows down and again, ALL IN ONE REVOLUTION - but because there's a second uni in front of the diff, it translates it back into constant velocity at the diff itself.

That's no big deal - it has been used and worked on cars practically from day one. HOWEVER - to enable it to 'translate' that constant rpm to the tailshaft (which is faster slower faster slower etc) and then translate it back to a constant diff pinion input rpm - well to do that, all it needs is two things. The first is that the tailshaft has to be made with the uni joints aligned perfectly, not even 1 degree out. Not hard to do, put in a jig and weld.

The second thing is a little trickier - esp in engine transplant setups. And that difference is simple - the gearbox output shaft angle and the diff pinion angle need to be the same at ride height. They don't need to be aligned (i.e. the tailshaft and yokes don't have to be in a straight line) but the two angles of gearbox yoke and diff pinion have to be the same (paralell) to one another.

If the angles aren't the same, it'll 'try' and make one end speed up slow down, speed up slow down, and that makes for a huge vibration, and no amount of balancing will help it.

You may find you either have to lower the gearbox rear mount (or raise it, if the floorpan space allows for it) or alternatively raise or lower the engine at the front engine mounts.

I do think that the biggest issue is how far out the gearbox yoke is from the gearbox, but I suspect you'll find that there's that angle issue playing a part also.

That might not be clear I'll do a diagram (admittedly a very dodgy one!)




The red lines are to show the angles are (or aren't as the case may be) parallel.

You can also alter the diff angle by either cutting re-welding the leaf spring perches, or using tapered wedges, though that would generally be a last resort, I guess. I mention it only because the spring perch clocking/alignment should be done when any diff 'transplant' is made to allow the angles to be right in the new vehicle (in a hypothetical situation any time a diff swap from a totally different car is done, so it might not apply here, I'm just adding it to cover all bases)

That is so dangerous, if it comes out somehow and decides to embed itself into the bitumen while you're doing 80 kph it might not be a pretty site.

You have a tailshaft that doesn't suit. Source the correct one I say.

Here are some pics I took this arvo but forgot to upload. That is about where I have them (the latter image shows it pressed all the way in, or most of the way at least). That is about all the clearance you need - which is still quite generous (but that depends on your vehicle as well of course).

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Great, thanks for the useful information and pics guys.
I will start with sourcing a new yoke and having a new, longer driveshaft made up.
After that I will check the angles, but I think they are ok.

oh yeah forgot to mention, I spotted this pic in the Tech Wiki under Propeller Shaft Swaps
http://datsun1200.com/modules/myalbum/photos/7474.jpg

It looks like the rustier one is the yoke that's meant to go with my gearbox (based on the info you guys have given me). The other yoke looks like mine.
Anyone know what the back of the gearbox is meant to look like that goes with that yoke?

Thanks everyone for their input.
It turns out the problem was the main shaft in the gearbox was bent!
I assumed this was fine since the gearbox had been reconditioned.
Here's a video of the problem in case anyone finds this in the future.
http://www.youtube.com/watch?v=EyqUHxPYFxk

Rich