AMC gear box failure modes dangers

I'm still tingling from Texas Mile and want to give it a go someday but pensive I'll bust drive train especially the gear box, so need idea of what is worse to expect. I'm am mainly afraid of rear wheel lock up of course but let go can be murder in mild turning conditions.

I've had most the classic failures and one caused me to crash twice in same day both going as slow and safe as possible. Let me list the ways I know so far.

1. Thin bush on 1st gear can wear so it pops out of 1st when no power added to keep load on cogs, just sorta coasting w/o a worry of crashing SPLATT>
Going down hill in 1st 15 mph in 10 mph turns to not get close to cars ahead, yet enjoying leaning far over, rear crossed palm size 1" deep missing section of pavement, to unload 1st letting bike just fly out from under sides ways to leave me runing a few yards to end up standing in middle of hwy watching Peel continue to slide down hill into guard rail un hurt her self. Fooled me good as I thought is was the loss of traction that got me so easy and surprised.
An hour later coasting slight down hill easy turn into my driveway on THE Gravel going 12 mph, as I looked to make sure in total safe zone, I let off power for just engine drag but instead it poped out 1st sped up so I had to turn a tad sharper SPATT dumped bike on me on top of berm crest to fracture pelvis and LS, I was born with it fused, but not no more.

2. Lay shaft ball bearing failed, pulling kick lever to the ground but clutch pull released the lock up, no crash.

3. Bent main shaft form prior owner or me getting triplex chain way too tight when hot. No crash but big bill to replace the damage.

4. Four or five Third gear teeth let go on drag race with Duci Monster but didn't know it till some very fast miles later it stopped shifting and made bad noise in gear box, no crash but limped home in 4th.

5 Ain't broke a main shaft, so don't know what to expect, please fill in my blanks.

If you have the Manley outrigger and good bearings I don't think you'll have too much problem with top speed runs. It's the sudden power that may cause the most problems. Of course it will be you behind the handlebars at 200mph so I'm not worried... about me. :mrgreen:

I had a gearbox explode flat out in high on the banking at Daytona, locked up momentarily but came loose before I got too bent out of shape to recover. I didn't figure out what broke first but nearly everthing was broken except the mainshaft. Jim

OH Crap! I don't want to die or destroy bike just having fun but feel like moth near a flame.

Hobot,

As I posted in another thread, the only significant failure I've had with an AMC gearbox while riding was a seized sleeve gear bushing. Luckily it happened coming off the track. I was over to the right with my hand up in the turn before pit-in when I felt the bike surging as if the rear brake was binding. I remember at the time wondering how I'd overheated the rear brake as I hardly use it on the track. By the time I got to my paddock spot the bike would no longer move, even in neutral. The box that I was running was a home-built one using a Quaife outer housing, original Manx shafts and gears (which means old and used) and various AMC parts from my Commando bin. We did have to machine and weld a boss in the Commando inner cover for a layshaft bearing. The outer cover was a Manx cover sourced from Mick Hemmings. My box was not stock, but the failure didn't seem to have anything to do with what we did in building it. The failure appeared to me to be oil starvation to the sleeve gear bushing. It looked dry and galled. I don't understand how that could happen given that the box was appropriately filled with Mobil 1 gear oil, but that's my diagnosis. Some of the track guru's stated that Mobil 1 gear oil is too thick for an AMC gearbox. I'm not so sure, but the bushing did fail.

My racing buddy over the last several years raced several Manx's with various Norton variant gearboxes. I've seen him completely blow up at least two (both very expensive boxes). Both times the gears were destroyed to the point that nothing bound up; at least not for a sustained period. Just a jerk at speed as the box temporarily locked, then freed up when the gears sheared. When he took the cover off the boxes, parts just fell out. I think the only thing he had intact were the shafts.

We never conclusively figured out what caused either of our failures (at least to my knowledge).

Another anecdotal datapoint anyway.

OH my Steve, thanks for reminding me about the sleeve bushes but design not able to retain oil nor be exposed to oil once spinning. Even w/o your alert I'm too familiar with the ceramic produced by the friction of brass on steel. Peel is going for torque power, so most the high speed work would be in 4th so no spinning in sleeve bushes. On this note which has be a bit baffled but intrigued no end is the stories of two locals spending a bunch on Commando's in their day and then taking on all comers by clutch drop standing starts in top gear and just hanging on WOT.

Still disconcerting even Manx single power could over power or destroy AMC type tranny. More upsetting about internals jamming and shearing to save lock up. I might want to add a sump in gearbox the pieces can get out the way in.

Swooeroo mentions 200 mph as that is the magic number to get past in land speeds. The bikes I saw doing it had near 250 hp via turbo and NOS power adders plus full race fairings with extra holes and seams and gaps taped over with suspension strapped down as low as possible. Can AMC guts in Qualfe box stand the power loads to go say 175? I doubt the crankshaft could stand the torque to go 200 mph w/o torpedo body. If it just broke and slowed down, fine, but if locks up to crash, what a way to go eh. But rather not.

hobot said:

Swooeroo mentions 200 mph as that is the magic number to get past in land speeds. The bikes I saw doing it had near 250 hp via turbo and NOS power adders plus full race fairings with extra holes and seams and gaps taped over with suspension strapped down as low as possible. Can AMC guts in Qualfe box stand the power loads to go say 175? I doubt the crankshaft could stand the torque to go 200 mph w/o torpedo body. If it just broke and slowed down, fine, but if locks up to crash, what a way to go eh. But rather not.

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By the time you hit 100 the crankshaft is flopping around, from there it's a race between the crankshaft taking the easy route out the bottom of the cases or the gear teeth vacating their locations. Third crankshaft bearing and Sportster gearbox are your best choices.

And to think that Burt Munro attained over 190 mph on a modified 1920 Indian Scout...

By the time you hit 100 the crankshaft is flopping around, from there it's a race between the crankshaft taking the easy route out the bottom of the cases or the gear teeth vacating their locations. Third crankshaft bearing and Sportster gearbox are your best choices.

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In my short Norton saga's I've learned its rpm not torque that flails crankshafts. Boosted engines don't have to spin their nuts off to make hi power. I don't want to wonder to far from Norton-doom on parts or design so do expect to run into innate limits but seek to push them far as 'practical'. I fully expect that to mean higher power load planting for turns and similar acceleration and speed as can be reached by other hi end bikes in many tracks. No real Norton can take it, but maybe one modeled off Commando basics can surprise em no end.

I've interviewed a handful of Commando drag racers, all said around 100 hp power level sprinting - AMC spills guts on track so Harley tanny or others used.

I'm not cavalier on speeding, I flash on images of pilots bodies in 3 big pieces after crashing over 150 mph in public. Also seen heads/helmets inbeded in dump truck grill. Yet in right time place many get away with maxing out.