Hole in piston

So I recently bought a 1978 kz650sr only to have it break down. After checking compression: 135, 80, 80, 0. I was worried. found a nice big hole in the #4 piston, caused by detonation.

back story: I bought the bike 3 hours from where I live, was driving it home, in the middle of nowhere of course, when I started to lose power, I kept it as full throttle just so that I could get it to a gas station, it stalled right when I got there. Went to start it right after, it shot out white smoke everywhere, turned it off. Let it cool down and drove it 20 minutes to a small town where I had a friend, it ran fine.

Previous owner: Said he bought it with the #3 piston frozen, took it to a shop were they checked it and bored the cylinders, he put everything back together. He also mention that at one point the petcock failed and gas flooded the crankcase, he said he replace the filter and oil and it ran fine, that was 4 months ago and about 400 miles ago.

Sorry for this being so long, I guess my ultimate question is what should I do? Have it rebuild or buy an engine on ebay and start from there? What year engines can I use?

depends on the collateral damage, prolly head damage as well, i'd rebuild it if it were mine, great learning experience, odds are you have a blockage that didnt allow oil flow, ( thats what happened to a friend of mine) and screw what POs say, most of em are liars anyway, if u rebuild it urself the YOU will know its done right, dont let it scare you,

Well ive never done engine work on a motorcycle, so I guess I can learn. Just scared of the gearbox and the crankcase, the top end I can disassemble and assemble fine.

since I have to buy a new piston, yall think I should just get a kz750 jug and pistons and go from there, I mean might as well have a little fun out of this rebuild .

Nah, get the 750 jugs and a set of Wiseco 810cc 10.25:1 pistons.

750 conversion...hmmm....
Whatever you decide keep in mind that you need to figure out why this happened so you don't go down the same road again. Is it something on the top end that you need to look at? I'll let others with more experience comment on the possible causes and checks but just wanted to put that cautionary note in your mind.
As for the rebuild, it is just a big erector set. If you are mechanically inclined it's a fun project. Make sure you have a manual and take lots of pics during the disassembly.

As soon as I get some time, I will dismantle everything and take some pictures and post them.

So I got some time to take the engine apart and see the damage



There is the big nasty hole



This is everywhere including wedged in between the valve and the valve seat

Should I just flush out the bottom end? Im trying to avoid having to take the crankshaft out and the gears out but I have no problem splitting the case and opening the bottom and flushing it out with kerosene or whatever you use.

This is how it sits as of now

That is a classic example of a detonation hole. Does the bike have pod air filters? Ed

Yes it does, the previous owner rejetted the carbs for them though. I think it started detonating because the faulty petcock let fuel overflow into the engine and dilute the oil, what do you think?

I don't think the petcock had anything at all to do with the detonation that clearly burned the hole in your bike's piston. I suspect the pods created a lean condition that the previous owner had no clue how to deal with, and that the lean condition caused overheating that triggered detonation. This may have been accompanied by an ignition timing screw-up in which the timing is too far advanced. I admit am a skeptic, but I would put zero faith in anything the previous owner said. You can replace the piston or install a new block and pistons, replace the head, etc., but before you start that puppy up again you should take a look at the jetting to make sure it won't cause the bike to run lean and take a look at the ignition timing to make sure it is not advanced beyond normal specs. I'm sure others with hands-on experience with blown engines will add to this conversation. Ed