Broken big end pin

[Lazy Dog]

Has anyone ever broken the big end pin, I did. Not sure how but after a loud snap, clank, click. tink, tink. The jug and piston were gouged badly 1/4" deep, Now i can move the magneto and the small gear independently.

 

[Al.Fisherman]

Has anyone ever broken the big end pin, I did. Not sure how but after a loud snap, clank, click. tink, tink. The jug and piston were gouged badly 1/4" deep, Now i can move the magneto and the small gear independently.

I can't see how a "pin" could break, I've seen the crankshaft break in two (V8 engines), but never a wrist pin. Since you say you can move the magneto and the small gear independently., I'm assuming that is what you mean, (crankshaft journal) where the the rod rides. The top of the rod rides in a bearing which is over the wrist pin, in the piston. Has to be a defect in manufacturing.

 

[Lazy Dog]

I'm assuming that is what you mean, (crankshaft journal) where the the rod rides.

Yep, sheared right in half. Motors been running strong for about a year and half. All the bearings seem fine, no grinding sounds and they spin freely, I can't see how it's even possible to break there, Everything else should have broken first. Don't know I'll post pics as soon as I get my gear tool back.

lol, must'a been the rubber around the frame.

 

[Al.Fisherman]

Needless to say Bad casting.

 

[Bikeguy Joe]

Ouchy..
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.

 

[Al.Fisherman]

Compared to how many crankshafts are made, breaking them isn't all that common. Google broken crankshaft (Images) and this is what you will find.
https://www.google.com/search?hl=en...urce=og&sa=N&tab=wi&ei=LjtqT7-kLKyAsgL9w-CfCQ

Back in the day of radial aircraft engines, it wasn't all that uncommon to brake a crankshaft an the propeller cuts loose, unfortunately at times they cut into the fuselage.

 

[Lazy Dog]

Pics, One sheared Rod pin. All the bearings/Seals are fine, The only damage is from the pin snaping.

 

[GearNut]

Nice set of paperweights you have there!
This is the first time I have ever heard of a China Girl snapping the crank pin.
What vendor did you get it through, if you don't mind me asking?

 

[Lazy Dog]

I wanna say Gasbike.net. It was a true grubee GT-5. It's been running great for over year and a half, at least 2000 miles. Rain, snow it is my car literally. Has to be a casting flaw like Al said. There just is not enough power created by these motors to break things like that. OH well... So about this crank balancing thing Who's got the best info on home balancing?

 

[biknut]

I can't see how a "pin" could break, I've seen the crankshaft break in two (V8 engines), but never a wrist pin. Since you say you can move the magneto and the small gear independently., I'm assuming that is what you mean, (crankshaft journal) where the the rod rides. The top of the rod rides in a bearing which is over the wrist pin, in the piston. Has to be a defect in manufacturing.

A lot of people refer to it as a crank pin.

 

[rustycase]

MSRFAN did a thread on balancing a crank... it's good info, and very do-able.
Good luck
rc

 

[biknut]

I wanna say Gasbike.net. It was a true grubee GT-5. It's been running great for over year and a half, at least 2000 miles. Rain, snow it is my car literally. Has to be a casting flaw like Al said. There just is not enough power created by these motors to break things like that. OH well... So about this crank balancing thing Who's got the best info on home balancing?

I balanced my crank. It's pertty easy. The problem is finding a crank without too much runout.

Go to about page 3

http://motorbicycling.com/showthread.php?t=34287