Drat! Crack in Gearbox Between Main and Lay on Drive Side

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Hmm, the bores wallow out mostly retaining their centering and but not stifling races spinning, so maybe this weak web could be replaced with a steel piece that traps the races as well as removes the thin flex fatigue fracture prone stress riser. I'v got to figure this loose fit out before I can afford a TT tranny in magnesium with reverse. In defense of swoohes repelling compelling logic, each bearing replacement takes some of the meat and grip away from its bore, so how many times before bearings begine wallow in expanded shell for some cleavage to show? This implies to me another radioactive item with like decades long half life which means over a couple of decades some will crack and some will wait another couple decades but crack they will. Its now known the position of Earth to sun distance speeds up or slows down nuclear decay, so chains slack, hub cushion wear, sleeve bushes lower gear oil strave abraiding and throttle-clutch coordination intensity are like astrology influences on chances of seeing unappealing cleavage.

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

concours said:

I've put 5000 miles on this one, as is, not leaking yet

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Yes, and you can say the same about that lay shaft bearing not failing?

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Its just a matter of time spinning before eventually just lead for fishing sinkers or paper weights.

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Dances with Shrapnel said:

concours said:

I've put 5000 miles on this one, as is, not leaking yet

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Yes, and you can say the same about that lay shaft bearing not failing?

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All I can tell you with surety is THAT layshaft bearing had 11,900 miles on it. Not quite sure I get your question... :?

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

My attempt at sarcasm.

You were saying the gear box shell was "not leaking yet" .
I was seeing the dreaded Portuguese layshaft bearing and asked: "bearing not failing yet".

It's a philosophical view on knowing something won't fail versus just saying "has not failed yet".

It's all moot anyway.

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Dances with Shrapnel said:

My attempt at sarcasm.

You were saying the gear box shell was "not leaking yet" .
I was seeing the dreaded Portuguese layshaft bearing and asked: "bearing not failing yet".

It's a philosophical view on knowing something won't fail versus just saying "has not failed yet".

It's all moot anyway.

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I got it the first time and was jamming you back. THAT bearing and the folklore of pattern failures posted here is the whole reason I even dove into a perfectly operating transmission. It is my humble opinion that that crack was caused by me taking the sleave gear bearing out cold, and the resultant pressure from the shrink fit layshaft bearing. Had I heated the case and dropped both out at the same time, it would have lived. It's a non-issue to me now. I'll report back in a few years/miles :mrgreen:

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

hmm, what would happen if this thin cleavage [love that dual term] were to be drilled out and tapped for a set screw that also traps bearing races notched for this? Would it just wedge crack to outside or stop the crack spread at source as well as restrain races from spinning in bores?

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

hobot said:

hmm, what would happen if this thin cleavage [love that dual term] were to be drilled out and tapped for a set screw that also traps bearing races notched for this? Would it just wedge crack to outside or stop the crack spread at source as well as restrain races from spinning in bores?

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That is taint, not cleavage, AFAIK

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

On no sir concours, us hillbillies know if its got a crack in it - in ain't the t'aint : (

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

After reading most of the less rude comments on this thread, no one has appeared mentioned anything about the case crack that is being heated and cooled as the bike is used and the outer case of the bearing being allowed to run.
Generally speaking, a crack is not a good thing, depending on the extent, if it’s just a hairline crack, then at the owner’s discretion, reuse, provided he/she uses something like a Locklite bearing lock on the outer bearing surface.
Getting this case non destructive crack tested is another option that the owner might like to consider and then seeking expert help from an engineer, which should cost you nothing. :idea:

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

The other thing with the Commando gearbox is how the crappy chassis/swingarm design uses it as a stressed member..... If someone shut the engine off and coasted down a long winding mountain road at speed hanging off the bike road-race style, that alone might crack the transmission.

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

I do that fast coast down the Mt twisties in N with engine shut off to feel what's road texture vs engine sensation but nil drive train or chassis strain compared to the rush up to the top.

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Along with just about everything else that can possibly fail on the commando I found the cracked shell as well on my MK 111
Not just a crack between the bearings but the outline of the layshaft bearing visible from the outside
The bearing (a portugal special) was in pretty good condition which makes me think it failed prior (as the seat is scoured on the inside) and was put back in service
My investigation for what it's worth found that since the box never kept pace with the beefed up power plant (or the force needed to kick the mongrel over) the superblend bearing is the best option to overcome the problem
Anyone rebuilding should not use anything else and anyone riding one should consider installing one as they have some give compared to the rigid bearing

Thanks to the guys from the classic bike club I got another shell (minor crack on the inside also) and will experiment with the old one
There seems to be room to beef up the case but the drive chain may rub against it who knows but that will be later on and it seems a shame to throw it out without trying to resurrect it.

According to the guys in the club they all crack

Cheers and best wishes to the Sandy victims on the East coast of the USA

Jed

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Jed said:

My investigation for what it's worth found that since the box never kept pace with the beefed up power plant (or the force needed to kick the mongrel over) the superblend bearing is the best option to overcome the problem
Anyone rebuilding should not use anything else and anyone riding one should consider installing one as they have some give compared to the rigid bearing

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As we know, the standard ball bearing normally fails because the rigid metal cage breaks up, and, while the NJ203 is certainly one answer, an alternative is the FAG 6203TB ball bearing as recommended by Mick Hemmings in preference to the NJ203 roller bearing.


https://www.treatland.tv/FAG-6203TB-P63 ... -super.htm

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Thank you for that LAB

I was cautious about the bearing being stamped Portugal as so far all I have heard is to avoid that one. But this information changes that idea as this FAG bearing is obviously different to the gold looking one that causes so many drama

Jed

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Hi ,just a comment concerning why a crack would develope at this location. I guess I have been lucky over the last 35 years I have never had this situation ocurr and have rebuilt gear box no less than 3 times maybe 4 over this time. I would think a failure of bearing due to heat is a cause for this to happen. Because of improper fit the outer race could have been slipping/turning with the shaft. Also A poor installation job accompanied by a bearing that did not fit properly driven into place could cause this type of damage. If bearing was slightly oversized and then forced into place I think could cause this type of faliure. What I do believe is with normal operation and maintenance of gearbox this crack should not just develope. heat and improper bearing placement or removal would be my thoughts for this problem.
have a good one
Dox

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Dox I had come to think that the main reason for the crack was running w/o replacing bushes regularly, as that makes the shafts and clutch basket wobble in and out of synch, forcing bearings closer and further apart, so you answered my immediate question of how often you've been into it for preventive maintenance, so now about convinced that's the main reason, not bad fitting practice or bad fit of bearing races. If races don't go in cold yet slip in normal when shell heated to over boiling then the bore/race interference fit is correct ballpark, so not cracking for that mis-match reason. Belt drive primary may help prevent cracks if my logic is correct, as belts won't stay on with much basket wobble so tranny must be kept pretty tight inside compared to chain drive. As common as the cracks are reported yet as uncommon as show stopper destruction's of tranny reports are, tends to imply that the crack is a non issue unless applying racing loads to loose as goose insides too long. You could help investigate this by waiting to rebuild while running with increasing clutch/shaft wobbles. Which brings me to my next question...

What made you alerted redo tranny each time?

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

I heard that if the drive chain is too tight and the shocks/swing arm are traveled to their max, the swing arm/chain will stress this area and crack the case. I do not know if it is true and I have not test the geomety, but I can see the possibility.

I all keep the right amount of slack in the drive chain, leaning to the loose side. Never take in account for new chain stretch and do not keep it on the tight side for this reason.

Another thought is when people try to remove or replace the big left handed sprocket nut using a breaker bar laterally. That could do it too.

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

Certainly one [ 1 ] over tight primary chain event can jerk the snot out of any weakness in drive train - YET, d/t to this prior fear-experience of it- on Peel's first break in I ran primary chain and final chain on too loose side - but in Ignorance also ran a ***STIFF*** new 0-ring chain in cold weather for easy timid break in till creeping up on correct tension - but hit a bump that extended swing arm fully to put more slack in final chain - which didn't have enough tension in it to keep the STIFF links from kinking up - so a kinked pair of links did not fall down into the teeth valleys but instead rode on top of all the teeth which jerked final chain so tight I got kicked in ass through the seat as the shocked bottomed out trying to take up the slack, which soon removed all the hub teeth tips to just nubbins that chain just spun over w/o any drive forward motion. This horrific battering may of bent the shafts and gored out the bearing races but by golly there was no crack in the cleavage area but the new reamed bushes were still good. My current conclusion is if the bushes are good then something else breaks, like cog teeth or dog faces or race bores gored out, but cleavage stays intact.

Re: Drat! Crack in Gearbox Between Main and Lay on Drive Si

hobot said:

Dox I had come to think that the main reason for the crack was running w/o replacing bushes regularly, as that makes the shafts and clutch basket wobble in and out of synch, forcing bearings closer and further apart, so you answered my immediate question of how often you've been into it for preventive maintenance, so now about convinced that's the main reason, not bad fitting practice or bad fit of bearing races. If races don't go in cold yet slip in normal when shell heated to over boiling then the bore/race interference fit is correct ballpark, so not cracking for that mis-match reason. Belt drive primary may help prevent cracks if my logic is correct, as belts won't stay on with much basket wobble so tranny must be kept pretty tight inside compared to chain drive. As common as the cracks are reported yet as uncommon as show stopper destruction's of tranny reports are, tends to imply that the crack is a non issue unless applying racing loads to loose as goose insides too long. You could help investigate this by waiting to rebuild while running with increasing clutch/shaft wobbles. Which brings me to my next question...

What made you alerted redo tranny each time?

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works out I did gearbox roughly every 10 years or 30 -35 thousand miles since owned from 1978. The bike is a 69 about 3 years after purchase the gearbox layshaft bearing let go {exploded} which also resulted in damage to 1st layshaft gear so had to replace this too. After that I have opened up and checked all components about every 10 years. I always replace both bearings and any suspected slopy bushings at this time. When the layshaft bearing went the kick lever spun around and slammed into the back of my foot trust me it hurts so this prompted me to open gearbox and do some periodic preventive maintenance. aside from the initial lay shaft bearing failure I have not had any troubles with bearings, shafts, gears, or bushings in the transmisson gearbox. Bearings and bushings are low cost items as compared to gears and shafts.
Dox