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Discussion Starter #1
So Im doing a rebuild on the top end but was wondering what the heck would cause a piston to crack on the bottom like this?





bad pic sorry





Is the piston going too far down in the cylinder or something? what should i check on the rod if anything?

 

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Maybe the rods bent.Or it could have been rocking around in the cylinder.
 

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Discussion Starter #4
What wold cause it to "rock around in the cylinder"?

bearings?


Should I go ahead and replace the rod too while im in there?
 

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Well the cylinder can sometimes become severely worn from dust and dirt in the air intake and it will out of round the cylinder causing it to rock a round a bit.I'd just se if it's bent and if not if it needs shimmed.
 

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Have u checked 2 see if that is the right size piston? Cuz i saw what happened one time when the wrong size was put in and it looked exactly like that. Like 2 the T same cracks same look ill see if i can find the piston i got and take a pic but its scary how simaler that looks.
 

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Like I said rockin around in the cylinder.
 

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first thing to check is the crank rod..check for up and down motion..side to side also..that is either from a bent crank rod..or wrong piston was put in it..if the rod bearing is so worn out..it will cause a side to side problem..which in turn..puts to much stress on the piston's wrist pin area..soo..check your crank rod bearing play..start there..then check to make sure you got the right piston..from the looks of that piston..from what i've learned..it looks like the crank bearing is so worn..that on the down stroke..it smacked into the main crank..soo..check all the bearing's in the crank first.
 

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I'd say that it got outa round from debris in the cumbustion chamber.
 

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i think it's from the crank bearings being too worn..causing it to slam into the main crank..gonna be interesting to hear what the out come is..lol
 

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Yeah.It must have been neat on a bit to do this.lol
 

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well..yu'd crack up to if your doing about 1000 rpm..then decide to hit cast iron..lol
 

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Yeah I guess so.lol
 

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Discussion Starter #16
Thanks for the info guys.

Any tips on how to check the bearings and stuff?

Just grab the connection rod and see if it has play or what?

Ive never done a rebuild before so Im a little inexperienced here.
 

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Detination in the cylinder befor the piston reaches the top, a bad wrist pin bent connecting rod worn main journad or bearing, not enough oil pressure?
 

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Are you sure it hasn't been apart before? The inside of that piston looks pretty low mileage. Look at the crank real close too and see if you can see where it hit. It looks like it hit on both sides of that side of the skirt. I'd say contact with the crank counterweights. It had to be making some noise too.
 

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yup jrl305..that's what i think..it hit the crank's counter weights..bad crank rod bearings..allowing to much slack on the down stroke..posb a bent crank rod..it could also have been undersized at the factory on the crank where the rod connects to..causing it to slam into the counter weights from to high rpm..either way..before you put the top end back together..put the new piston on..rotate the engine by hand a few times..and check for piston/crank clearance..if it hit's the crank's counter weights..your going to need to tear the whole bottom of the engine down..too check your crank play..grab the rod..try to move it up and down on the crank..without it turning on the crank..see what kinda play you have..you shouldnt have very plug play at all..now..after you check the up and down play..move the crank rod side to side..same goes here..you cant have alot of play on the side to side motion..some..but not much.
 

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My guess would be on a bad rod bearing.
 
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