Top end rebuild but..

Using the old bearing shells?
This is the question i want to ask:
Can i change the piston rings in my (running) engine to restore original compression and leaving the old crankshaft and rods in place?
Will this create future problem?
Thank you

Changing the rings without touching the bottom end is fine, assuming there are no existing problems. You should change the valve stem seals while the top end is off and it's best to use OEM gaskets.

If you have compression problem it could maybe be good to check valves also?

Or have you conclude the rings to be the problem?

As Nessism stated the bigend crankshaft should be ok. Can you let us know the history of your bike i.e kilometers/ miles, long or short distance trips, servicing intervals. These things matter when it comes to engine rebuild recommendations. Also what is the reason for changing the rings?

Other things like camchain might be required etc

Thanks you all.
Bike is a gpz 900 r with 15k miles
No particular problems except a bit low the compression.
You pointed exaclty what i want to do. A top end. Job with new seals and rings. And valve lapping too.
My concern was only about the bottom end, since its old and im afraid that with new rings and restored compression can damage something.

15k miles is not much. Is the bike running now and have you adjusted the valves lately? Did you perform the compression check with a hot engine and throttle held wide open?
If the bike has sat a long time without running the rings may be a little sticky. I'd adjust the valve and ride the bike good and hard in italian tune up fashion and then check compression again before tearing down the engine.

I am trying to decide on just the top end on my '81 KZ750 motor, and it has 34k miles on it. Got it from the original owner, and was not a hard rider. Used it for daily transportation. I'm not sure what pitfalls to watch for on the bottom end (any special tools, hard to replace parts, etc.).

I don't understand why a KZ engine with such low mileage would need to be pulled apart. My KZ650 has 62,000 miles on it and the head has never been pulled much less any other serious engine work, so what makes one believe their engine with much lower mileage needs to be opened up? Ed

Just the fact i found oil in the 3rd spark plug.
So i want to raplace all rings and stem seals

The oil in 3 is most likely the stem seal or possibly head gasket leakage or maybe the wet plug is excess fuel overflow from a sticking float etc .
When replacing the rings you will also need the bores honing too otherwise the new rings won't seal but 15k on a 900r is barely run in and any compression loss could be down to valve clearances so those should be checked first before digging into the motor unnecessarily and if the "oiled plug" is not fouling and not affecting performance i'd leave it alone for now.

GPZ900RR...Is that the Ninja engine?