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MG Midget and Sprite Technical - Checking gearbox
|I have bought a midget as a project. It was dismantled when I bought it so have never seen it running. I have no idea of the condition of the gearbox - it is a 1966 car with the ribbed case box. What checks can I do to figure out the condition of the gearbox? Or should I just bite the bullet and exchange it for a recon unit?|
I've connected an electric drill to the input shaft of a couple of A series gearboxes and used that to spin them while shifting the gears. Seemed to give a fair indication of the state of the box and synchros. You'll need a 1/2" drill IIRC to fit the input shaft, and make sure you have some oil in it.
|That's clever David. I would never have thought of that. I'll give it a try.|
|here is the set up I used to test mine for function and oil leaks (I had installed a front cover that was porous, and had leaked like a sieve over the last few years, so this time I wanted to make SURE the new one was oil-tight). |
This is running it with the full oil level (drive shaft is then necessary to keep the oil in, at the rear).
I took two rubber wheels I had from a past project and screwed them to a block of wood, for the drive shaft to run in.
The only "tricks' were these:
you have to use a 1/2" chuck to grip the input shaft (as David pointed out) - make sure to tape the input shaft real well to make sure your chuck's jaws won't damage it, which would cause bearing troubles later between it and the pilot bush in the flywheel.
the drive shaft must be at the very same level as the gearbox so that the U joint will turn true and everything will sit and spin smoothly like this. I used several thin shims under my wheel block to get it right
Then, you can just let it spin for as long as you want (though, I wouldn't leave it unattended, just in case!).
|Whilst I can see this as quite a fun pastime, what does it actually tell you about the state of the gearbox?|
Won't it generally behave differently and be more noisy when under load in the car?
|Guy Oneandahalf Sprites|
|That's what I would like to guard against Guy. I know these gearboxes of old, and how noisy and leaky they can be. I keep wondering if I should just bite the bullet and get a recon unit, but at £450-ish it seems a lot of dosh if the one I've got is OK. But how to tell?|
Why not just fit it in the car and go for a drive?
|Guy Oneandahalf Sprites|
|Mike ive just fitted a rib gearbox that i got for £6 i only needed the remote and the bellhousing had a small hole in it the size of a 20p so it was going to be scrap so i had the lot .I sorted the hole fitted the box thinking well its worth a try i have another box i payed £40 for so if its no good ill try that. .Any way the £6 box works a treat no jumping out of gear goes in all gears smooth .So if i was you i would fit the box you have look out for a other one as stand by. And if it does not work out you can always go for a recon.
|Firstly be aware that you don't have a Morris Minor or Austin A40 box. The ratios are quite different and the bearings inside are bushes and not balls, a lot weaker. Quick give-aways with a Minor / A40 box are: no reverse light switch (but this could also apply to pre Oct 66 Spridget boxes), different clutch fork (Minor) and occasionally the clutch rod bearing (in the gearbox side plate) housing will show signs of use.|
You can take the gearbox side plate off, but you still won't see a lot in there, it'll look great. A gearbox that is dirty (black deposits on the gears and in the oil) will indicate a well used box and a nice clean one will reflect in it's condition. If you can check the gap between the synchro ring and the gear as each gear is selected, this will also give an indication to it's condition. A worn synchro will sit very close to or even up against the gear, one with life in it will stand off the gear by .025"+.
Turning the gearbox by hand may give an indication of bearing conditions but you won't really feel the all important laygear bearings, these are the ones that WILL give you trouble if not looked at.
Finally, check for debris in the box, chunks of metal have to come from somewhere, usually the first and reverse gears.
|M T Boldry|
|As Mark says, the bearing are different, but they are needle rollers in the 1275 & late 1098 Spridget 'boxes.|
The reverse light switch isn't always a reliable indication, as the Spridget remote is sometimes fitted to Minor 'boxes.
You can have a look inside. The only thing you won't be able to see is first gear, as this is hidden by the selector fork. You can see the first gear end of the laygear, although you need to select second gear (IIRC) in order to see it and you will need a torch. If it isn't showing signs of wear or chipped teeth, chances are it has been rebuilt or hasn't had a hard life.
|Dave O'Neill 2|
|Can I thank everyone for their most helpful comments. As it is such a beautiful day here today I have been able to get the gearbox out into the light to have a good look at it. I think it could be already reconditioned, as the engine has been, because:|
The casing is very clean, much cleaner than is usually done at home with paraffin and rags.
It has a brand new clutch operating fork and carbon thrust.
Looking in through the drain plug with a micro LED I can see that the gears look perfect and everything is extremely clean and bright.
Using the LED to look up the tailshaft, the oil seal appears never to have rubbed.
So I shall use it just as it is, although that's a way off yet as the car is COMPLETELY dismantled. In my photo you can see how I found it - it has a new Heritage shell, which is why I thought it a good buy. Incidentally, the Moss catalogue gives the different ratios for the Spridget and Morris Minor boxes. I checked third gear on mine by turning the tailshaft 10 turns and counting the turns on the 1st motion shaft, and it was 13.6, so it is a Spridget box. Hooray.
This thread was discussed between 23/10/2010 and 24/10/2010
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.