Gearbox rebuild question

It would seem to me that a sealed bearing in a transmission would not be as well lubricated as an open bearing and the speed of the shafts would seem to me to dictate that an open bearing is a better system as far as long term wear. But I'm not a bearing expert so maybe that is not the case at all. Then again, as noted, tranny oil is going to get in there anyway.

One thing sort of related that I find interesting: here in Mexico all the long distance buses and most big trucks use oil-lubricated wheel bearings instead of grease-packed bearings. Apparently they have found that there is less wear/better life/less chance of a problem. It would seem to me that the additional parts associated with oiled bearings in this application would actually create more failure points but I guess it's well sorted out. Perhaps this is becoming common everywhere but when I'm in the US I don't notice big vehicles with oil-lubed bearings (there's an external, flexible oil feed pipe that connects to the center [hub] of the wheel bearing carrier so it's easily visible.)

MexicoMike said:

Really (reaming)? OK, I don't have the facilities to do that - at least to do it correctly - so I'll accept the current slight play as within spec!

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The first gear bush tends to wear and can contribute to jumping out of gear under hard acceleration. If you can wiggle the gear on the shaft at all, it needs to be replaced. "Reaming" can be as simple as running a small fine grit hone, the type sold for brake cylinders, in an electric drill. I've done that twice in the 35 years I've had my bike.

FIRST GEAR BUSH WEAR IS MY GREATEST FEAR FACTOR IN AMC GEARBOX !!!

I have crashed twice in same day, both times being EXTRA CAREFUL SLOW NO LOAD ON AT ALL COASTING DOWN HILL at 20 mph then at 12 mph, to have sudden loss of rear tire drag - crossed a gap in pavement that made bike just slide right out from under to leave me runing a few yards to end up standing in middle of my lane in the tightest blind spot road in USA watching Peel slide slowly into guard rail and traffic back up for me just in time. About an hour later easing into my drive way Gravel, bike flipped up from under to land on my pelvis on peak of 3 ft high berm, fracturing my L5-sacral vertebral joint and innominate bone to cripple me for 3wk till I grew a new joint back. But was very pleased that I'd saved the new paint job and corrected a genetic abnormality of fused L5/Si facet joint, now pops like a normal human pain free.

I'm especially freaked out now to hear worn 1st bush can come out of gear under load OMG! Next bushes set I'll get two for the 1st cog and switch out at first hint of slack wear. I'm still awaiting advise on how to best ream these bushes.

According to Mick Hemmings (in his Gearbox Servicing video)

Mick said: "...you shouldn't have to ream any of these bushes by the way, when they come genuinely-they are 'to size'....."


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The layshaft 1st gear bush is usually the one which requires changing.

"when they come genuinely"

Does that mean AN parts, (from Old Britts, for example) or is Mick referring to some bushings that he (mick) supplies?"

MexicoMike said:

"when they come genuinely"

Does that mean AN parts, (from Old Britts, for example) or is Mick referring to some bushings that he (mick) supplies?"

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I'm sure he means AN parts.

Layshaft 1st. gear bush should be a press-fit in the gear, unlike the layshaft 3rd, and mainshaft 2nd. which are a sliding fit in their gears.

The layshaft 1st gear bush is usually the one which requires changing.

I've ordered from RGM and Waldridge and Fair Spares, only one or two bushes fit right out the box. Your's is first mention I've read in a while that anything from AN was good right out the box. Sheeze. The Late Mr Huddson's video on gearbox does not cover this bush fit/ream issue, so was a surprise delay a couple times now, being fooled the first 2 times thinking just a bad batch. Now I know I don't know what to do next time real soon.

"Your's is first mention I've read in a while that anything from AN was good right out the box. "
You won't get any argument from me against that statement but I guess I'll give it a try and order the complete bushing set, bearing set and gasket/seal set from OB and see what happens as far as fitting the bushings... I'll be in the US the first week of Dec so I'll have them sent to where I'll be at that time. THen back to Mexico for about 2 weeks so I guess that's when I'll re-assemble the gearbox.

When I bought my bike it had a new kick start lever on a knackered shaft. The PO had then put bits of brass shim in to try & make it tight on the shaft (it didn't work!) I ended up fitting a new shaft & new lever. So the moral of the story is if you have worn spline issues consider replacing shaft & lever as a pair. even if there is only slight play when tight, it'll only get worse over time. I tend to treat the kickstart pinch polt the same as exhaust threads & check tighten regularly. Incidently I had the same issue as others with the pinch bolt binding.
Terry

My experiences are limited to the first gear layshaft bush. I suppose that if you buy the gear and bush together, it may come reamed to size? Awfully pricey though. When purchased as a replacement however, you will need to open it up a bit after pressing the new bushing into the gear. I recall it was a very small amount that had to be removed.

LAB wrote;

Mick said: "...you shouldn't have to ream any of these bushes by the way, when they come genuinely-they are 'to size'....."

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maylar wrote;

however, you will need to open it up a bit after pressing the new bushing into the gear.

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This was my experience. Once the bushes had been pressed into the gear centre, it seemed that the interference fit caused the bush internal diameter to slightly alter/compress? They would fit on the shaft OK before pressing into the gear, but not after

Maybe I'm just a very poor engineer, and didn't heat the gear up enough or something?

debby said:

My roller bearings had plastic cages, so the bearing sounds good. I'll defer to the experts on how to ID it for sure though.

A few other things to check while you're in: kickstart pawl, cracks in the kickstart shaft (at the bottom of that V notch), and end play on the kickstart shaft. You can shim it as required with rear iso shims (see the OldBritts tech article). Sounds like a previous owner has done a full rebuild, so you're probably good to go. The shaft crack is easy to miss though, so check carefully. My 850's was cracked and I almost missed it.

Debby

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It's worth shimming to get rid of excessive endfloat on the kickstarter, but you need some endfloat. With no endfloat the bike can pop out of first gear, because first gear won't fully engage. The first and second layshaft gears bump into each other before the spring loaded ball is fully in the detent in camplate.

-Eric

Roller layshaft bearing very good news , the mainshaft will accept a 3 rd bush for more support if only 2 in there , 1 st gear bush is a tight press fit in 1 st. gear , if after it slides onto shaft with minimal rocking all is good , if too tight it will need reaming with proper toolset to make gearbox a joy.

Reggie, its common for bushes and bearings to squeeze down on shaft or balls on interference fits and reason to order up C3 crank bearings and ream bush after fitting for best fit and long life. Exactly why I started post to get sense of how it should be done and what to be aware of. Now I think I do and it beyond my capacity to do it right so off to a tool/die shop next time.

Eric, thanks for another esoteric AMC detail to learn, 1st/2nd nipping each other w/o slight kick shaft end play.

One thing I learned over last decade, I'll be getting two 1st gear bushes per one set of the rest of them. And to put 3 bushes on the sleeve shaft plus oil groove them
but just short of the one on the DS end .