Do I need new pistons?

Hey guys, I have been doing a lite tear down of my '81 KZ750 and wanted to ask for your opinion of my pistons. The bike supposedly has 46,000 miles on it right now. 3 out of the 4 pistons have wear marks on just one side of them, the other side looks great. The inside of the head has some small wear marks on it as well. The actual piston rings look to be smooth. I was dumb and didn't do a compression test before pulling the head off, so I can't say weather I've got good compression or not. So my question is, is this wear normal, or do I need new rings and have the block bored out a bit, or new pistons too? I'm trying to not blow my budget, but I also don't want to damage the bike further by ignoring a problem. Sorry some of the pictures aren't that great. Thanks for any advice!

I would say it time for new ones or a good used set, but if clean them up replace the rings with the next size up and bore the cylinders. They may work, but i would replace them.

All I know is when I had my 750 I checked sources for new rings and pistons. They were NOT cheap. Plus, I think Z1 had issues finding oversized ones too. By the time you go through all that, you could maybe find a good used set on ebay or at a junk yard.

One possibility would be to spring for the Wiseco big bore kit, which would take you to an 810 cc, but then you would have to tinker with the carbs, and the kit itself isn't cheap either...

I have brought pistons before off ebay used it was 45 dallors for the set but i didn't need the over size rings.. plus i got lucky last year and picked up a whole engine with 12k on it for 50 so i have 6 extra pistons laying in a box.

well, you could always do that then. I've heard people say for best results you should keep the same pistons in the same sleeves when using used ones...but if they are are in good shape and all pass all the specs in the manual, you could try and just ball hone the sleeves and get a regular set of rings.

What do the pistons and jugs look like on the used motor? Maybe just hone them and re-ring, install on your installed one and do that?

I know that when I blew a piston on my 550, I bought a new piston (it was $50 through Z1) and just honed that cylinder and put new rings in it. Cut new valve seats too. Money was really tight and I didn't want to spend anything on the repair. It runs ok, but I'm not hot rodd'ing around either. Problem is my compression on that cylinder is lower than the others...

Thanks for the tips guys! I guess first I'll see if I can get new rings somewhere. I've never done anything like this before so it will take me a while to figure out what I need. I did see a head with sleeves and pistons on the "bay" for $200 I think, which is doable, but I'm not sure what condition the pistons are in. I of course would prefer the "810" kit, but that is quite spendy. I'll talk to my local Kawi shop and see if they can hone the head out for me.

i am by no means a terry vance or anyone else, but the cylinders in your pics look pretty bad to me you mite want to let someone look at them before sending any money on rings iam not sure a hone job is going to do the trick you mite need a bore job

When I tore down my 750 twin with 75k miles on it, I did not see any cylinder wall scoring like that. I don't think that is normal wear.

bountyhunter wrote: When I tore down my 750 twin with 75k miles on it, I did not see any cylinder wall scoring like that. I don't think that is normal wear.

+1 - cheapest route will be a good/used block, piston, and ring set from fleabay/classifieds.

I would guess all that scoring is on the thrust (exhaust) side of the piston and liner.
Looks like it's been run low of,or in very contaminated oil and picked or has overheated and been close to partial seziure.
You could give it a light hone and re-ring but the piston/bore clearances will be shot and most likely won't last long till its burning oil and down on compression.
Oversize pistons (either new or good used)with a re-bore and new rings is the only way to go if you don't want to take it apart again in 500 miles time.

Well, it sounds like I'm going to have to source a better used head and pistons. Anybody want to get rid of theirs? I'm going to the dealer tomorrow to show the parts to them and see if they have any recommendations for a local machine shop. Sounds like this is going to be a pricey fix, darn it! I'm a bit nervous that the bike is going to destroy whatever new parts I put in there, but hopefully if I keep the oil topped off it will be fine. I'm also partially tempted to just put it back together the way it is and see how it runs, but I'd probably just be pulling it apart again like zed1015 said.

Not to be out of line but the best way to fix the
junk you have (and it is junk ) buy the Wisco 810 kit.
Simple bore job and put it back together