Yoshimura pistons in a Z1

First some background:
Bike is a 1974 Z1,  around 1980 I sent the engine to Harry's Machine (a racebike machine shop here in central Mass) to have them add some more power.  They welded the crank, bored the cylinders and installed higher compression pistons, upgraded to Andrews 1X cams with adjustable cam sprockets and assembled the engine.  When I got it back the bike went like a scalded cat. Rode it for about 15 years like that and then life got in the way, you know the story. Never sold it and a few years ago put a little (!) work and parts (money) into it and started riding it again last summer.
Project for the off season is replacing the most likely dried out valve seals with fresh viton ones.  While the head is off, decision was made to freshen up the cylinders with a quick hone to renew the crosshatch and maybe new rings, since the head is off anyway. (Good thing because the cam guide in front is broken and flopping).  When I got the jugs off, the bores look good, no taper, no ridges.  Turns out Harry's installed some Yoshimura "Pop.Y" pistons, looks like maybe 10:1 or so?  Kind of exotic stuff but not surprising since they built race motors, and Rick Stetson who raced out of their shop built my motor.  The finished bores all measure 69mm (with my Harbor Freight calipers) but I was expecting to find 70mm.
Anyone familiar with the 1980 era Yosh bore kits? This would have been before Yoshimura switched to Suzuki race bikes. I'm curious if I can get replacement rings for that bore size.  If not, the existing rings look OK, but as long as I'm there...



edit: Now I know why they gave me a cool Yoshimura T-shirt when I picked up the motor.  Wish I still had it!

I don't have any direct information on Yoshimura piston kits, I have a few sets of there cams but no pistons. You could try messaging Yosh but I think the chance of getting a old style ring set is slim to none.

All of the newer rings are super thin so even if you were to get a 69mm ring kit they would probably be the wrong thickness.

I once called up MTC to ask about some pistons rings for a older kit they no longer made and the guy laughed so hard he almost fell off his chair.

I have had good luck lately finding replacement rings from ring manufactures. They all have catalogs you can look thru for bore/thickness then cross reference the engine/vehicle make. NPR, Mahle, Hastings, etc. Once I find what will fit then I go on Ebay to find NOS listings.

Most times I can get at least top and second rings, but hard to find exact oil scraper set (can reuse those ones) Car engine rings are a lot less expensive

 

Let me know Scotty if you want to soda blast anything.  

I just did a quick search for "Yoshimura 69mm pistons" and it now seems this is a popular overbore size for the Z2 engine.

They do offer "new" kits, but none list the ring set thickness.

I did find these out of the UK but again not sure on thickness.

imdpistons.com/products/yoshimura-kawasa...q=yoshi&_ss=e&_v=1.0

Also found this vintage info, seem to be a street kit for Z1. The race pistons were 69.4mm

www.yoshimura-rd.com/blogs/yoshimura-his...fjRS81-zvVIYBPze39DF

 

Thanks Injected, that vintage info is what I'm looking for.  Looks like I have the 69mm kit, as I did the math and came up with that same 987cc.  It was 1981, but I vaguely remember Rick Stetson showing me those Yoshimura catalog pages before he he built the motor. (things were sometimes foggy during those years, haha)
I had a feeling I'd be out of luck for new rings, but the old rings are in good shape, pistons and cylinder walls are clean with no ridges or taper. Shows the benefit of religiously changing oil and filter!  So I'm thinking a quick hone to revive crosshatch should work.

Thanks Ronnie, I'll be in touch about some soda blast of the jugs and maybe cyl head.

For fans of motorcycle history, it turns out the article referenced by Injected is part of a 25 short chapter history of Pops Yoshimura, his company and its racing history.  Interesting reading. Go to "Yoshimura-rd.com/yoshimura-history" to read more.--