Author Topic: broken teeth of crankshaft (Read 6902 times)

Aurel · « **on:** June 29, 2015, 12:59:39 PM »

hi guys,
two weeks ago i make an oil change an find some metal pats in the oil.
So i disassembled the motor and saw that some teeth of the crankshaft are broken. Its a race engine with 836 ccm so i don't think that the power is the problem. Its a complete new rebuild engine. Did some have the same problem or did I make some think wrom??

754 · « **Reply #1 on:** June 29, 2015, 01:25:45 PM »

Did you change the primary chains?

Aurel · « **Reply #2 on:** June 29, 2015, 01:35:33 PM »

yes to a stronger one, all parts that you can change i changed.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #3 on:** June 29, 2015, 01:38:35 PM »

I haven't seen that before. You wouldn't expect a 836 to do that. Did you find any other pieces of metal that may have jammed between the teeth and the chain? perhaps something else broke or you left an extra 'part' inside?

Which chain did you go with?

Aurel · « **Reply #4 on:** June 29, 2015, 02:02:32 PM »

No i didn't find other parts that get between the sprocket and the chain.
I buy the chain in germany by bikeside a really good shop the chain is in perfect condition i think he sells only parts i don't know how manufacture it. Its no a OEM chain, this chain have solid rollers and looks different from the side.

bwaller · « **Reply #5 on:** June 29, 2015, 03:03:34 PM »

It was one or two teeth no problem but three in a row will be hard on your new chain. I've seen a crank in a race engine with broken teeth and it wasn't an issue....three in a row, man I don't think I'd use it.

kmb69 · « **Reply #6 on:** June 29, 2015, 03:30:17 PM »

Can you please post a picture of the chain?

Old Scrambler · « **Reply #7 on:** June 29, 2015, 03:59:03 PM »

The teeth on the other sprocket don't look very good either..................Did you assemble the chains with alternate side-plates to each other? Is you clutch shaft mounted in firm bearings?

scottly · « **Reply #8 on:** June 29, 2015, 06:34:30 PM »

Both of the teeth in the middle and the one on the left have a smooth, rounded look in the damaged area, in the shape of the chain rollers. Examine the chains, link by link.

754 · « **Reply #9 on:** June 29, 2015, 08:19:16 PM »

Any evidence of tensioner failure?? Thinking sudden slack may have caused chain to ride up
Try comparing length of chains to what you had in there before if the old ones are around..

johno · « **Reply #10 on:** June 30, 2015, 04:15:34 AM »

Hi Aurel,
The cam chain teeth look in good condition which would suggest the crank hasnt done a lot of miles, yet the primary teeth are extremly worn on the drive side which would suggest that it has had some extreme load on it wether with the old chains or the new chains ?? if you built it I am sure you would have noticed the teeth wear as it stands out on a normal crank check so if you havent done a lot of miles since the rebuild it may be due to the chains being too tight or too loose. To decide on which way that could be may depend on the type of tensioner ? the standard roller spring type may have been installed incorrectly as it has a locating dowel ? or the latest manual teflon type may have been adjusted too tight.
I must say I used the big mean mother sherman ja das auto type primary chains and I crapped my pants when I put them in as I thought they were too tight. I made special tool up to check the free play so I could check while I manually adjusted them.
The result was in my opinion they were right on the limit of tightness with no adjustment required ; just touched the chain and backed off half turn, man those chains were tight... after running them for about a minute or two ( as I blew the crap out of the motor and checked them on tear down) the tightness left them as I expected them to be when first installed with a fraction of movement.
Time be be sherlock holmes mate , have a good look and think

oh let us know too

johno

70CB750 · « **Reply #11 on:** June 30, 2015, 04:24:19 AM »

Not trying to sound stupid, but was the new chain same pitch?

PeWe · « **Reply #12 on:** June 30, 2015, 09:31:21 AM »

Is the crank modified too? Lightened version that might been damaged during machining, shipping.
Have you found all missing teeth in the oil or inside engine? If not, some of them missed at assembly...?

I remember another case with major chains damages, but crank seems to been OK despite broken chains, cam sprocket, cam sprocket ear off from cam...
http://forums.sohc4.net/index.php/topic,125377.msg1424921.html#msg1424921

Bikeside.de has some goodies to CB750. I bought my Mikuni TMR32's from them earlier this year. But I can not find reinforced primary chain there, only Honda OEM and another RK chain.

I bought my primary chains from Classic Cycle City on eBay.de. Much tougher and tighter, as well as the price that most likely is well worth the investment. (I forgot the rods...that are the weakest point today)
Scroll down on this page for information how reinforced primary chains from CCC look like.
http://www.ebay.de/itm/Verstaerkte-Primaerkette-Honda-CR-750-CB-750-Four-Heavy-Duty-Primary-Chains-/160659394773?nma=true&si=3yFGFS8LUrb8BnDpQSmQapouB3E%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557

kmb69 · « **Reply #13 on:** June 30, 2015, 10:12:06 AM »

Please, a picture of your chains so that we might be able to identify them.

Aurel · « **Reply #14 on:** June 30, 2015, 10:56:57 AM »

hi,

i will post a picture of the chain on Friday, its a crank from cyclex. On the back side there are also some broken teeth. So i order a need crank. I assembled the chains with offset links. I check the balance and the concentric run of the crank all was really good.I check the lenght of the chain and i can´t measured any differences between the two chains. the engine runs really good i didn´t hear any noise or so. The chain tensioner looks like new. It is an OEM tensioner am not sure if the spring is strong enough for a race engine up to 10500rpm.

PeWe:When you ask Klaus Dony from bikeside he will find the parts that you like. I have also the TMR 32 on my bike the work really well.

The chain looks similar the the chains that classic cycle city sells.

sorry what did you mean with:Is you clutch shaft mounted in firm bearings???

scottly · « **Reply #15 on:** June 30, 2015, 06:10:48 PM »

Are the damaged teeth on the back side 180 degrees apart from the ones pictured? Are you using the Cyclex Power-Arc ignition?

Medyo Bastos · « **Reply #16 on:** June 30, 2015, 07:43:44 PM »

sorry to hear, and see what you are going through! the seller is good in my book, i have no issue buying from classic city!

Aurel · « **Reply #17 on:** July 01, 2015, 02:34:33 AM »

No its no exact 180°, no i using a ARD magneto on my bike.

PeWe · « **Reply #18 on:** July 01, 2015, 06:08:02 AM »

Offset links... Primary chains are not endless? Both chains locked to each other, creating a duplex chain? I thought that the 2 primary sprockets on the clutch can flex independently of each other. My speculations might be totally wrong

Old Scrambler · « **Reply #19 on:** July 01, 2015, 10:13:17 AM »

by'firm' I mean the clutch shaft has no play........especially side-play.

Interesting.........do you think the cogs broke on acceleration or from down-shifting?

Did you send your own crank to Kenny?..........I have supplied more than one crank to Kenny. He always mic's them and gives them a good look-over. I don't know if they are x-ray'd for cracks.

kmb69 · « **Reply #20 on:** July 01, 2015, 10:17:07 AM »

PeWe, All of the primary chains I am aware of are endless - Honda, Cycle City, and Cycle-X. Duplex chain? You are correct - the sprockets on a stock primary do "float" independently. And it is not unusual to see uneven wear on a pair of chains. The "offset links" comment meant offsetting one chain by one tooth to the other so that you have the inside plates on one chain lined up with the outside plates on the other. Usually, the inside plates are thinner than the outside plates (or vice versa) and offsetting in theory "balances" the strength of the two chains.

Back in the day, RC Engineering would weld the two primary sprockets together on racing applications. These welded sprockets would have to be aligned perfectly to achieve the desired results.

kmb69 · « **Reply #21 on:** July 01, 2015, 10:31:24 AM »

Was this crank by any chance treated via cryogenics? Pretty sure the sprockets on the cranks are already case hardened. I have no idea what effect the cryo would have on previously case hardened material.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #22 on:** July 01, 2015, 10:56:00 AM »

Which brings up the condition of the rubber cushs inside the primary?

Aurel · « **Reply #23 on:** July 01, 2015, 11:30:36 AM »

the rubbers in the primary are new also all bearings in the complete engine I fink it would by a good idea to weld this 2 sprockets together but i am not sure if the sprocket on the primary side are good to weld i will take to sprocket out of my stock make a chamfer on the lathe and weld it and test it afterwords. i don't send a crank to cyclex its to expensive for me i am from Austria Europe so i have to pay +50 dollars. I am not sure but i don't thing that ken check the crank with magnaflux for cracks. When i have this problem again a will make a billet crankshaft out of 34CrNiMo6 Europe (4340 SAE USA).

754 · « **Reply #24 on:** July 01, 2015, 06:08:17 PM »

I am sure you checked the teeth before assembly?
..i keep getting this nagging thought that that is the kind of damage that would occur if you had clamped down on the teeth (even if you used a shim) on say a milling machine.... Perhaps that cracked them, then in running they broke off..............just thinking out loud..

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Author Topic: broken teeth of crankshaft (Read 6902 times)