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Gearbox layshaft bush wear
#1
I've been cleaning up and reassembling the 4 speed box I have so I can mate it to the engine. As part of the cleaning I degreased it all in water based degreaser and then dried the parts with a hot air gun. That showed me the reason why the dog teeth are all loose, it's because the synchro cones have slipped down the splines. When I heated them to dry them they moved further. So I heated them up and used the vice to press the cones home again which seems to have done the trick.

The inter-shaft needle roller bearing is very loose but I know I can replace this with a bush from spares. I will replace all the springs and balls.

But what do you do about the layshaft bush? This seems quite worn.

[Image: main.php?g2_view=core.DownloadItem&g2_it...1355ab7218]

This A7Club (http://austin7club.org/How%20To%2014.htm) link is very useful but from this it sounds like fixing this bush is a specialist job since it needs line reaming?

Simon
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#2
Simon, I'm not aware that any of our suppliers stock replacement bushes.

My local small machine shop turned up a pair for me from phosphor bronze. The bore will shrink very slightly once pressed into the gear cluster, but the fit on the shaft was still only fractionally tight - maybe I was just lucky. I 'adjusted' clearance by running a honing tool down the bore in an electric drill, lubricated with light oil. This was in 1995 and it's still going strong.

I'd suggest assessing the condition of the shaft before going to these lengths, it may stand a light grind to address uneven wear; then bushes can be sized to suit.

When all is assembled and tightened up you should have about 5 - 10 thou of end float (n.b. it will be about 5 thou more at running temperature). Shim as necessary.

I guess option 2 is find another gearbox - they are still fairly cheap and plentiful - but it may well have the same issue. The wear is principally due to the axial force generated by the single helical gears.
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#3
The bushes seem to wear little on the shaft. With care can be removed and if not too thin spaced with a washer. Others have described a drilled plate with flats manipulated behind the bush to enable it to be pulled out.

The loose syncro is a common and more difficult problem. Would expect curious symptoms but dont seem to hear of. I suppose Loctite is now the cure but difficult to clean the splines first. If removed tricky to refit; must go back in same palce.
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#4
The difficulty of option 2, finding a better gearbox, locally is why I am persevering with this one for now. I need something to get the car finished and at least the engine running. At least then I can look at getting the car registered.

Our local spares guru kindly rang me for a chat and the solution they've used before is to face off the bush to make it square then with in in the box measure the endfloat (that's for the figures Chris, I would have asked that next) then remove the bush and put a suitable shim underneath it. Bob, that sounds like what you suggest.

The synchro cones didn't seem loose at all, just shifted. After heating it took quite a bit of force to push them back down. I did wonder about putting Loctite on them but I didn't want to remove them fully.

Simon
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#5
If the cones are heated and spline ends centre punched might help. If a car is hill parked in gear, with weight on the gears, then pulled out of gear, quite a force on the retaining cones. It seems to be the only real flaw in the boxes. Under load, with all the fretting action, there must be quite a tendency to loosen the dogs.

Some time ago someone enquired about remedying worn synchro cones and it was suggested that perhaps these could be spaced. If anyone has done they would have experience of refitting cones.


And a puzzle. When in Austin and myriad similar boxes, say top is engaged what stops the female cone meandering along and contacting the bronze 3rd gear cone? Or is that what wears them? Does it explain why RNs seem to go so fast?
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#6
I agree you could get back on the road just fine by facing the flange and putting a shim / washer behind it, or similar - but now it's all in bits I wonder is it really much more work / expense to turn up a new pair of bushes?

I did make enquiries once about fitting new synchro cones but they were fabulously expensive at the time. I think Bob's right, it's not good for them to be able to move. But other than Loctite (or simply living with it) I'm not sure what alternatives you realistically have.

You don't mention what the car is Simon but I've recently replaced my synchro box with a 4 speed crash box. They have some minor 'disadvantages' (gear change a tad slower, but not as awkward as I expected; intermediate gears are lower by about 4%, which some would regard as a backward step; plus an amazing 'supercharger' howl in straight-cut 2nd gear which you will either love or hate. The top turret is a different shape / size so take care it will fit in). However, weak they are not. I bought 2 complete boxes in a local auction for £15.

If looking for a replacement box autojumbles are probably the best bet - on eBay they are no doubt 'rare collector's items'. (n.b. Where are you based?)
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#7
I'm in Wellington, NZ. The car is a special so it doesn't have to be original, just period Austin 7.

I managed to flatten out the bushes on the lathe but not how you'd think. Was too big to fit in my tiny chuck so instead I mounted the shaft in the lathe and cut a disc of wet and dry to slide over the shaft onto the boss at the end and taped it in place. With that through the cluster and chucked up in the lathe I could spin it while holding the gears still and it sanded down the bush back to flat. Worked very well.

I put it back in the box with no gasket and bolted it in place and measured the end float at 30 thou. I did look at trying to get the bush out but could see now easy way to do it (heat and trying to tap it out with a bit of wooden dowel didn't work). I think I would have to make a puller somehow, maybe a piece of 10mm square steel turned on the lathe so the ends are the same radius as the inside of the cluster but bigger than the inner diameter of the bush. I could thread an 8mm hole in it then use a bolt and a piece of pipe as a spacer against the gear to pull the bush out. I can then fit a suitable shim under the bush head and push it back home.

I should really get in touch with Andrew Bird about a set of close ratios before I go too much further.

Simon
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#8
I can't honestly remember how I got the bushes out - but I hadn't any plans to re-use them so it was probably a drift and a big hammer!
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#9
Having rebuilt many gear boxes I use this method to remove the layshaft bush you can then skim the bush flat and place shims behind the bush to take up any wear.
     The syncro rings can be peened with blunt centre punch to help retain them (see photo)

Terry.


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#10
Hi,
The bush removal tool looks good but all the ones I've done have come out easily with a 12mm bar. It does chew up the bush end a bit but that dresses up and the bush is long enough for that not to be significant. Measure the end float without the bush, then minus the residual flange thickness and fit a shim behind the top hat on refitting. Final adjustment on the gaskets behind the layshaft flanges with various grades of brown paper, or nothing other than sealant. Swopping syncro dogs from another cluster has worked for me, they don't seem matched or difficult as Bob suggests; a smear of Loctite then pened in place?

Dave
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