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Hey all, still working on this project bike I picked up for 100 bucks this fall. 1981 kz440 a2 ltd. Bike sat for some years and definitely had some abuse in it's previous life, and a handful of people probably worked on it without a service manual... Engine is free after sitting for at least 12 years, and I have gotten the bike to start and run. Classic issues though - airbox was gone, starter clutch went out, dying as soon as you touch the gas because someone messed with the idle air screws, bowl plugs were literally rattling around in the bowls... You name it. Decided to compression test it and right side was low. Decided to tear down the top end and do rings and valves but now I'm wondering if these pistons are fried. Both had heavy deposits one definitely was oily. Flipped the pistons over and underside has some orange discoloration but is mostly dark not quite black which leads me to believe this thing was overheated repeatedly. 

That being said I'm looking for some advice - are these pistons usable or are they shot? I was hoping to be able to clean them up, put new rings in, hone the cylinders and be good but I'm having some doubts now. I have another bike I've been pulling parts from but it has also sat and the engine is in unknown conditon other that it being free turning. 

Here are the pistons after soaking in parts cleaner and some light hand brushing. Underside left piston has been wiped with a cloth right has not 

Pulled the parts bike motor apart and the pistons look like this. Not pitted, much less carbon, not black on the underside. Cylinders also didn't have any major scoring and appeared to still have some cross hatching. Thinking of swapping the tops on the two motors... Also upon closer inspection one piston on the first set has a clear divot from running into a valve probably on repeat for a while.... 

Anyone? I've done some searches and reading but I haven't found any threads where someone has actually completed this rebuild or answered this particular question. I understand throwing a caliper on the piston/cylinder to check specs but is there anything else I need to look out for? Replace? Thanks all

Any pics of your cylinder bore/hone condition would help. You should better do a compression test before your dissembled the engine parts. 

Ah yeah that probably would have been a great idea, oh well live and learn. The donor bike hasn't run in even longer and is a complete unknown and was further from being able to run so I didn't think of that as a possibility. Anywho, I'll get pics of the cylinders today. The ones I pulled had some scoring and those off the donor bike looked to be in much better shape to my learning eye. 

Got some shots of the cylinders. First 4 are the original bike with some scoring. Second two are off the donor bike. Thoughts? 

You pistons look fine.  You should be able to re-use them but....

You need to measure them to determine if they are still in spec.  The book will tell you where to measure and what the spec is.  You will need a micrometer or maybe you know a good machinist who can measure them for you.  Don't forget to measure the wrist pin diameter, and the diameter of the connection to the pistons.  All of the specs should be in the manual.  Try to put the same piston in the same cylinder when you put things back together.

The cylinder walls look OK but again, you should measure them to determine if they are in spec.  You can do this with a borehole gauge, or take them to your favourite machinist.  Since you are replacing your rings you should hone the cylinders before reassembly.

Be sure to clean all traces of the old base and head gaskets.  If you want to go first class you can have the cylinders aqua blasted to bring it back to looking like new.

 

Slowly been working at this and a few other things in the meantime...honed the cylinders yesterday and have been working to get the old base gasket off - that thing is STUCK on there! Been trying goo gone, brakeclean, pb blaster, wd-40, acetone...  Scraping (yes very carefully) with a straight razor and making slow progress. I ordered some roloc disks, but does anyone have any good suggestions for getting the other half of the base gasket off the base without getting a ton of crap into the crankcase? 

​​​​​​Got new tires and tubes mounted the other day which I did myself and was pretty proud of. Swapped the chain and rear sprocket off the parts bike as the existing ones were WAY out of spec (front sprocket was fine), and cleaned up the rear drum. Shoes and springs are of unknown age so replacements are on order. Front brakes are in good shape, cleaned up and flushed/bled, plenty of meat on the pads still. 

Scored a stock air element cap for a whopping $28 NOS. Hell. Yes. 

Looks like the rest of my parts and roloc disks are showing up today, a day early! Barring any other excellent suggestions for gasket removal looks like I'm going to fashion some sort of covers/plugs for the top of the crank case to keep gasket debris out. Thinking maybe a cardboard piece taped in with masking tape. Wish me luck!

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Looks like the rest of my parts and roloc disks are showing up today, a day early! Barring any other excellent suggestions for gasket removal looks like I'm going to fashion some sort of covers/plugs for the top of the crank case to keep gasket debris out. Thinking maybe a cardboard piece taped in with masking tape. Wish me luck!
 

Good luck! I'm watching for moral support, as I'm useless for mechanical support. I'm about to attempt a rebuild of my 305 and similarly, there is not much help out there except the shop manual which is pretty vague on certain steps for a non-mechanic like me.

I really really REALLY  hope you document the assembly thoroughly with detailed pictures and post them. It will help me on my 305, and might help me on the 440 I hope to get someday. 

Good luck with the 305! I took some photos along the way during disassembly, and will take some along the way and post them here. Looks like maybe tomorrow or this weekend I'll be getting into it.