help with piston ring numbers

[Aussie]

My piston rings have 75 stamped on them as do the tops of the pistons. Correct me if I am wrong but the 75 on piston would to me indicate the size is 75mm and the 75 on the ring would be the same, or would it. As I have bought STD 75 piston rings from Japan ( part# 447-11610-00 ) and the end gap is like 2mm (HUGE) but my rings with 75 on them the gap is within tolerance, under .004..... But does the 75 on rings mean they are oversize rings. ? H E L P.
Here is a pic of new ring on top of my ring.
I have a bit of smoking from left cylinder that's why I thought I would replace rings.
Just not sure what to do now

 

[5twins]

Yes, I think the "75" means .75mm, or third oversize. Oversizes go up .25mm at a time. If your old rings are still in spec, you can just re-use them. You shouldn't need to hone the cylinders either. It's an old wives tale that you can't put rings back in once removed because they won't seal again. They'll seal just fine.

But if that "004" you mentioned for end gap is in inches, that's actually tight. I think the minimum gap spec is more like .008".

 

[Aussie]

But what about my smoking issue. Compression was 150+ in both cylinders. New valve seals. Not constant smoke. When idling then rev thats when it was smoking

 

[5twins]

Is this a recent purchase that may have been sitting unused for a while? If so, maybe the rings just need to bed in again? I guess you could hone the cylinders to help the rings seat in again. Use the bottle brush type hone, not the 3 stone type. The stone hones can enlarge your bore if you're not careful.

 

[Aussie]

What would you think on my pistons. Standard size or do you think they would be oversize. Going by the 75 stamped on top. Have lightly honed them. Didn't take a lot. Bought the engine last year as a new build. Built my bike started engine and it smoked like a crack whore. Stripped the engine and the guy had put the rings in upside down. Put them right way up and rebuilt. Smokes a little on left so made a tool and replaced the valve seals through valve adjustment covers. Seemed to get better I thought but i don't think it did.
Don't really understand the way oversizing works. Thought over-sizing the rings meant bigger pistons or can you oversize the rings in increments without swapping out piston

 

[5twins]

I'm thinking your pistons are 3rd oversize too. Measure them. Stock is just under 75mm, usually something like 74.9xx mm. Measure the width of the pistons, front to back, about a half inch up from the bottom edge on the skirts. Bores are just over 75mm stock, usually something like 75.004 up to maybe .010 or .012mm.

 

[XSLeo]

Ok, On the stock size piston it has a 3 digit number. This is 3 spots after the decimal point. As I 74.xxx. On any over size piston it will have the oversize stamped on them, as in 25, 50, 75, 100. This represents .25 mm, .50 mm, .75 mm and 1 mm. This was mentioned by 5twins. I added a bit more.
The factory went as far as 1 mm on the over sizes. This would be 4th over. After market you can go up to about 12 over. This will give you around 707 cc's.
The 75 on your piston and rings means you have 3rd over pistons and rings.
If valve stem seals improved, it may indicate the valve stems and valve stem guides are worn.
Hard to tell from my house.
If it don't smoke much at stat up and doesn't smoke warm, I wouldn't worry to much, just keep an I on the oil level.
The left side of my 750 kitted 75 does that.
Leo

 

[Aussie]

Blimey that would explain why the standard ones don't fit then. Bugger.
Well thanks for that guys, you are the nuts. Appreciate it.
Best look for 3rd oversize rings then or 4th.
This all became apparent as the guy who built the engine put all the rings in upside down.
Smoke was unbelievable. It is better since stripping and rectifying them but I want no smoke. But then I only put about 80-100 miles on it so I might just rebuild it and put more miles on it see if it clears up. I have a few gasket sets so just be labour time if it don't work and the standard rings were cheap enough and can sell them on to get the cash back. Thanks again and be prepared for more questions ha ha ha ha