"C" on top of O/S Pistons

I have a 1985 GPz550 Engine that somewhere down the line, was bored out 615cc. The pistons have the arrow on them (and yes, they are all pointing to the front of the motor like they are supposed to be), and there's also a letter "C" stamped on the top of each one. Now before someone here says, "Maybe it stands for center?" :laugh: I don't think so. Can anyone tell me if they've had aftermarket pistons like this? This engine was fittied with the kit back in the late 80's or early 90's I believe, because the Stage 3 Megacycle Cams were dated 04/86.

Here's the best picture I have of when I removed the head and verified the 61mm bore, it's in the red circle, near my grease-filled thumbnail:

It's possible they are using "A", "B", and "C" to designate first over, second over, third over sizes. The oversize pistons for mine just come stamped "+0.25" for the ones that are 0.25mm over size diameter.

That means it takes an XC style piston ring. When you order rings, you only have to know the bore size and the "C"

Jay

Thanks Jay.

Who makes these replacement rings? What is an XC Style Piston Ring as compared to a, lets say standard Wiseco 61mm 615cc piston ring, or a standard piston ring?? I bought a set from Wiseco for this engine, not knowing, and haven't installed them yet, but from your reply they might not work??? Looking to freshen up the cylinders and install new rings.

I also noticed there is a slight dome to these pistons as compared to the Wiseso Pistons in the kit I bought from you with the 61mm boreed cylinders. I don't know the history on them or this engine. Your help, and others, would be greatly appreciated. I will be removing these pistons for inspection.

Thanks!!!

Did the rings you got from Wiseco have a four digit number and XC on the box?

The rings didn't come in a box, if that matters, and the part numbers on the ring sets are 2402XA. After looking at Wiseco's Online Catalog, the XA is the only style they offer for the 61.00 MM Bore. I did find a description of the piston ring styles. Here's their site:

www.wiseco.com/Catalogs/Motorcycle.aspx




Here's just a part of the Piston Ring Section from Wiseco's Catalog, showing the XA and XC ring differences.

Note
Jay, I appreciate your information on this matter,(thanks for the educated guess bountyhunter) and being involved in a this site with you and your company's experience. I have the cylinder head off this "wicked" engine, and will be sending it your way for a go through soon. I liked the work that APE did on turning my stock 58mm cylinders into a 61mm(615), along with rebuilding the head a few years ago. There are other machine shops in Northern California, but I like APE's work. Thank you, and to any one else that has a business taking the time to assist in information, away from their daily work schedule.

So looking at the thickness of the XA versus XC Piston Rings from the chart above, I'd say those two type of rings aren't interchangeable. I'll need to remove the pistons, and measure the rings currently therin, and inspect them more once they are out.

Thanks for the kind words. The XC style has been around for a while. They are going to the thinner rings on the smaller stuff. Pull the barrels and measure the thickness of the top ring. If it is .039", there may be some 61mm XC rings out there somewhere.

Those rings are made by Riken or Nippon in Japan and Wiseco is the importer.

Jay

Thanks Jay. I'll do that when I get them out, and post the results here. I'll also ask Wiseco about them as well.

Well, I found out about the rings, and it's not good news. Wiseco doesn't carry them in the XC, so they refered me to Total Seal. I called Total Seal, and they said, "If Wiseco ain't gots them, ain't nobody gots them." Looks like it was smething that was done way back when the bike was new(5,000 miles ago)Got no choice but to buy a new 615 kit, and freshen up the bores on this "wicked" engine. I'll of course remove the cylinders, and inspect for any grooves before honing, and 615 kit buying. I'll still be sending the head to you Jay, for a rebuild towards the end of the month. If the cylinders require more than I can do on my own, you'll get them with the head.

Thanks for your help Jay.

Wait. I haven't removed the pistons, there's still a slim chance... probably not...:blink:

Best thing you can do is measure the piston rings once you remove the pistons. If you call with the thickness of the ring set, you can usually get what you need. I had the same issue when I freshened up my Z-1R that had a 1075 kit installed (20 years ago), and they were able to get me the set I needed.

Who was this "They", you speak of, that were able to et you a set?