Broken crank

[jon370r]

Has anyone ever seen a broken crank like this?

[jon370r]

another

[Dave83]

Yep,Easter Sunday of 07.I went to the track and got to playing around and noticed the bike didnt have any bottom end power.It ran fine wide open,but coming out of corners or hitting jumps,it just was weak.Hard to start also.Came home,pulled the stator cover hoping for a sheared flywheel key,nope not it.Pulled the head,looked good.Noticed that when I turned the flywheel,the piston was slow to react.kinda tripped me out.i put the head back on and kicked it over,started killed it.Pulled the heaad back off and started messing with the flywheel.i could turn the fly wheel about 1/2 round before the piston caught up.Tore the motor down and found exactly what you have pictured.It was broke uneven and the piece in the flywheel would hang the rest of the crank and spin it thru.the timing was way off,explaining the loss of power and hard starts.called Wiseco(fairly new build).The guy said they had never heard of a single cylinder crank snapping,only twins and never offered any other help.Thought something else had to have happened.If it hadnt kept running,I may of accepted that,but it would still run and never gave any indication of trouble prior.Did find a broke clutch plate and think it may have ran thru thr drive gears and possibly snapped the crank.IDK though

[C-LEIGH RACING]

Seen a Honda OEM crank broke like that few years back so its not just a Hot Rods thing.
OEM crank did have a weighted flywheel though.
If the cranks not that old, I would contact Hot Rods & see what they would do.
Neil

[86 Quad R]

i've only seen/heard of one other and if im not mistaken it was dave that posted it. cant be for sure tho.

it's most unusual to see that sort of thing happen. what top end and what flywheel are you running?

thats just an oem crank that been rebuilt?

[jon370r]

It was a new 4mm Hot Rods stroker crank with about 40-50 hrs on it. It was also a stock "85" flywheel and 82mm piston running 160lbs. compression.

[ESR350r]

I have broken 2 of ESR's 5mm stroker cranks in the exact same spot as you on both of mine. Was not running a heavier fly wheel or anything like that. they both broke with less than a season on them. ESR installed one and a local shop installed one. I put the last one in and have ran 2 full seasons with out a problem.

[Dave83]

Mine was a Hot Rods crank.Standard long rod.ESR 310 non-pv cylinder w/ spacer plate and stock flywheel.When you guys cranks broke,was there any indication before it went.Like I said,mine was just hard to start and had no bottom end power.Wasnt even noisy and no more vibration than normal.

[ESR350r]

both of mine would not fire and were hard to kick over.

[C-LEIGH RACING]

Something I see thats not on the OEM crank, that hole through the middle.
OEM is solid in that area.

Think about this for a minute.
When your putting an engine together, installing the crank, if all the parts are right, the crank will be a press fit into the flywheel side main bearing & a slip in fit on the clutch side.
Now that you have that in place, you go ahead & put the rest of the engine together & when you get to the part where the seal sleeve & two gears go on the clutch side & tighten the bolt down, that pulls the crank up tight into the clutch side bearing.
Now what you have with both side of the crank down into the bearings far as the ends can go, the crank is in a tention like state & unless you releave that its like the crank is running in a bind of sorts.
Normally what I do after thats all done, I put the nut on the end of the crank & then take a rubber mallot & whop the end releaving the tention.
Once you get the crank completely in with the seal sleeve, two gears & bolt locked down, you can feel a slite bind as you rotate the crank that even more than the seals rubbing, after the whop you can feel a difference, crank is free.
What I'm thinking, if that tentions not removed then the crank is running in a bind & could cause the end to snap off like that, just guessing though. You add the weight of the flywheel out on the end of the crank & it could very well just be whats happening.
I've never seen a clutch side break like that & you know for sure its getting abused every time you drop that clutch lever.
Just a thought.
Neil

[86 Quad R]

very interesting neil and i think that its possible that what you pointed out could be a major contributor to alot of the premature bottom end(crank) failures that take place after fresh builds, regardless of the crank manufacturer.

i use a similar technique as you mention but do it differently. :cool:

[Langbolt]

Here's another one....:(

I wonder if it could have been caused by the difference in crank bearings.....I just noticed that one has 8balls & the other has 7....I know the 8balls version is a CR250 bearing & the 7 is the TRX....but I would have thought the guy who built it would have seen the difference too ?

Could the harmonics out of balance have caused this ?

Or the fact it's a 350cc big bore ?

[Langbolt]

another pic

[Langbolt]

and another

[wolvman]

looks to me like it was cracked 1/3 of the way thru for awhile long enough to start rusting in the crack before it broke are those score marks like the bearing spun on crank shaft

[Motofool250r]

True that harmonics and or material defect cracked it then it broke later, I do Inspection for a living honestly these pieces are hard to get a good inspection on with their geometry and without high tech expensive methods for a 200$ crank.

[Motofool250r]

can you make a macro photo of the shaft and crank side where it was broken

[wolvman]

neil is right on with reliving the tension on the crank. most 4 stoke builds and deisel is recomended procedure when building bottom ends checking crankshaft end play .at least that's how i was taught to build a bottom end.