Piston problem

[Boombiladi]

Ok, so my fiancées blasters top end went last weekend after a long ride and we took the top end off and found a hole in the middle of the piston lol all else looks good minus some debris. My issue is I'm not sure of the bore and I'm pretty sure it's not stock. I found numbers on the top of the piston that say a38k0, but I can't cross reference numbers anywhere and can't find any info that tell me what size I'm at. With out taking my cylinder and piston somewhere and having them mic it can anyone help me out?

 

[Herpderplol]

You can measure it with a $12 caliper from multiple stores.

If that's too expensive get a trusty metric ruler out and see how wide the bore is. this will be very inaccurate though

 

[Awk08]

With out taking my cylinder and piston somewhere and having them mic it can anyone help me out?

unless that cylinder was recently bored/honed to fit a new piston, and detonation or an airleak fried it,
your chances of a new "same size" piston being within the specified piston to cylinder tolerances are slim to none.
more than likely that piston has been in there for quite some time, and the cylinder is worn or out of round.
a new piston of the same size is just going to flop around in there, possibly have poor compression, = waste that new piston $
at the least....it will need honed to restore the cross hatching for the new piston rings to wear in, if the bore is within specs.

do it right the first time and save $ in the long run !
taking to have it measured correctly at various places down the bore, and bore/honing it to fit the next size piston if needed, is the best option. you already need a pi$ton, hone, and new head/base gaskets, whats another $50/60 to have a fresh bore/hone matched to a new oversize piston = fresh, good compression, long lasting engine

 

[Blaaster]

we took the top end off and found a hole in the middle of the piston lol all else looks good minus some debris.

Unfortunately the replacing of a piston is not the only thing you have to worry about.

You are going to have to split the cases and remove all the debris from the bearings.

While you are in there you may as well fork out a little more cash and replace the bearings and oil seals.

 

[Boombiladi]

Ok. It sounds like I'm going to have to have it read at a shop. The background of the blow up is as follows, the cylinder looks fine. The rings were still fine where it went was it got a hole through the center of the top of the piston. I was hoping to not have to take the hole damn thing apart again.... I just rebuilt the whole bottom end. I started with a roller from a junk yard I was working at years ago, it sat for 5 years and I just got it running again. I think with a hone I'll be fine. I work construction so time isn't really available this time of year so just a trip to a shop will take away sleep time. But anyway thanks for all the help and advice guys

 

[Blaaster]

The hole in the middle of the piston was caused by a lean mix, which melted the metal.

That metal now resides in the crankcase, wrist pin bearing, rod bearing and crank bearings.

You will need to split the cases to rectify that matter.