Honda GX270 Crankshaft broken in two

J

joler1302

New member
So, yesterday, we managed to break a crankshaft into 2 pieces at right at the journal. This happened when the throttle was let off at the end of the straight.

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Thoughts?

Crank journal was just renovated, brand new con rod, new crank bearings. Any thoughts on why?

My theory is, that it was a combination of metal fatigue and maybe detonation. I don't know how old the crank was, so there could have been a weak spot in the metal.
 
Broke exactly where the ones that break usually do. No way to predict it. A little more radius on the journal when initially made, may have helped. How about a little better picture of that plate attached to the counterweight on the flywheel end? Did you make that? Purpose? Polish the next crank up a lot more, Get it real smooth all over, shot peen it and maybe a little cryo treatment. I doubt it would have helped on this beak however. Good luck.
 
Was running the Stock 2:1 Oil clutch so not too much load on the crank. The plate on the crank is a centrifugal oiling system. Can post more pics if you want!
 
Wow. First 270 crank I've seen broken. I had to weld the sides of the journal up to make it more narrow so I can fit my long rod and also ground the crank about .040" smaller too. Good thing it's solid and don't have a hole though the journal. I hope mine holds up. My is been balanced and smooth as it can be. Maybe that makes a difference. Maybe your next thing is to have the crank balanced?
 
A guy we race with has an engine that pushes 18hp at 8000rpm. He has 4-5 broken cranks. All In the same place as this one. We are looking into balancing already! Did you balance the crank by itself or with flywheel and did you count in the conrod and piston? We race stock bore and stock stroke btw.
 
But this engine had stock gx270 carb, skimmed, ported, 55lbs springs, stock valves, aftetmarket camshaft(too big to be run by a stock carb. Next purchase will be nr 280 rpm grind)
 
We balanced my crank with Bob weights and I think it was 65% over throw factor I think. No we don't balance the flywheel with it.
 
Both. Added and took some off on some places. We didn't even have to use heavy metal. Just welded some on the places that needed it and grind it down till it's right.

Here's another thing. Minidragbike balance his cranks by it self. No bob weights or any over throw factors. He said it works.
 
Things like this happen from detonation. All it takes is a sharp edge left on the edge of the combustion chamber when a head is milled and you've got a pre-ignition point.
 
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