Fresh rings?

I am considering my options on some engine work this winter. My 900 has low compression but generally runs OK. I do have some leaking valve seals. Compression is 100 psi across all cylinders and comes up to 160 with oil added through the plug hole. I haven't taken it apart so I don't know the condition of the pistons and cylinders but they can't be real bad with such even compression. The increased compression with oil leads me to think the low compression is due to the rings and not the valves. If everything looks smooth in there, what do you all think about just honing the cylinders and replacing the rings instead of going to the first oversize pistons and rings. I did look back in some old posts and sort of seems like mixed opinion. Back in the day, we used to just re-ring our two stroke dirt bikes all the time.

I suggest that all you have discovered is that your pistons are not sealing. What do you expect to "SEE" when you look at the bores? They will most likely be smooth looking. Does this mean you can just toss a new set of rings in and expect to get good compression? I think not...

You had best buy a digital caliper and set of bore gauges. LOOK AT A SERVICE MANUAL. It tells you how to measure the piston to bore clearance. If either the pistons or bores have worn (or perhaps both) out of spec, the distance between the piston and bore will increase to the point where new rings won't seal. It is at this point where you put new pistons in (with new rings) and bore the cylinder or find a replacement cylinder with matching pistons that is within spec.

You will want to determine if the bores are round as well... they tend to oval a bit and the pistons will rock and eventually the low compression can turn into blow-by.

if I rember correctly even a 160 with oil is not so high

OK George, I did it the right way. Got hold of a set of micrometers, telescoping hole gauges, and a nice dial caliper. I pulled the head and cylinders fully expecting to find worn out cylinders or rings. I spent the afternoon checking every dimension in the book for the cams, piston, wrist pins, rings, and cylinders. Everything is well within specifications and looks very nice. I haven't taken the valves apart but know that the number one exhaust is leaking oil around the seal. There was some carbon buildup on three of the four piston tops and inside all the head chambers. I also haven't checked the cam bushings with plastigage. Visually, the bushings do show some wear. The rubber/plastic cam chain tensioner parts are all hard as rock. Seems I remember that those parts are somewhat pliable in their new condition. I wonder if there is a kit with all the tensioner parts?

What really has me baffled, is that when I tested my compression, it was low. It came up when I put a tablespoon of oil in the cylinders. From what I see and measure now, I ought to have great compression. Maybe my compression tester wasn't correct. I'm sort of back to my original question except now I am wondering if I ought to do anything with the pistons, rings, and cylinders. I could run a hone through to deglaze the cylinders. Any suggestions?

I do think a valve job is probably in order. The engine has 39000 miles and it's apart. Definitely needs valve seals.

OK George, I did it the right way. Got hold of a set of micrometers, telescoping hole gauges, and a nice dial caliper. I pulled the head and cylinders fully expecting to find worn out cylinders or rings. I spent the afternoon checking every dimension in the book for the cams, piston, wrist pins, rings, and cylinders. Everything is well within specifications and looks very nice. I haven't taken the valves apart but know that the number one exhaust is leaking oil around the seal. There was some carbon buildup on three of the four piston tops and inside all the head chambers. I also haven't checked the cam bushings with plastigage. Visually, the bushings do show some wear. The rubber/plastic cam chain tensioner parts are all hard as rock. Seems I remember that those parts are somewhat pliable in their new condition. I wonder if there is a kit with all the tensioner parts?

What really has me baffled, is that when I tested my compression, it was low. It came up when I put a tablespoon of oil in the cylinders. From what I see and measure now, I ought to have great compression. Maybe my compression tester wasn't correct. I'm sort of back to my original question except now I am wondering if I ought to do anything with the pistons, rings, and cylinders. I could run a hone through to deglaze the cylinders. Any suggestions?

I do think a valve job is probably in order. The engine has 39000 miles and it's apart. Definitely needs valve seals.

76LTD wrote:

. . . a valve job is probably in order. The engine has 39000 miles and it's apart. Definitely needs valve seals.

And maybe valve guides as well.

I own a 1973 Z1900 & 1977 KZ650. A few years ago I had the same problem with my 650. First of all, if you already have it apart considering the miles, have the head done. My engine looked the same when I got it apart. I honed the cylinders, then ordered rings 10 over. By placing the new ring a third into the bottom of the cylinder. Square it up by tapping lightly with the piston. The new ring should overlap. I use an awl,
carefully mark at the end of the overlap. Remove the ring and with a fine grit grinding wheel carefully grind the excess up to the mark on the ring. be patient,
grind,check,grind check etc. do this for each ring. Check the end gap with a feeler gauge until the factory gap is acheived. Remember to mark which rings go with what piston and that piston in what hole, 1,2,3,4 cylinder. I would also suggest new wrist pins and clips.
Assemble. My 650 was a little hard to start with the starter but, it kicked fine. One week of of break in miles later, the rings set and it started fine.
A good plus was the bike hauled a#@.


I have been working on these old Japanese bikes for thirty years. I have Many classics. If you know your bike. If you have a little experience, you can do this. My 650 still runs and still hauls a#@.

Good Luck
Dave

I own a 1973 Z1900 & 1977 KZ650. A few years ago I had the same problem with my 650. First of all, if you already have it apart considering the miles, have the head done. My engine looked the same when I got it apart. I honed the cylinders, then ordered rings 10 over. By placing the new ring a third into the bottom of the cylinder. Square it up by tapping lightly with the piston. The new ring should overlap. I use an awl, carefully mark at the end of the overlap. Remove the ring and with a fine grit grinding wheel carefully grind the excess up to the mark on the ring. be patient,
grind,check,grind check etc. do this for each ring. Check the end gap with a feeler gauge until the factory gap is acheived. Remember to mark which rings go with what piston and that piston in what hole, 1,2,3,4 cylinder. I would also suggest new wrist pins and clips.
Assemble. My 650 was a little hard to start with the starter but, it kicked fine. One week of of break in miles later, the rings set and it started fine.
A good plus was the bike hauled a#@.


I have been working on these old Japanese bikes for thirty years. I have Many classics. If you know your bike. If you have a little experience, you can do this. My 650 still runs and still hauls a#@.

1973 Z1900
1977 KZ650
1977 KZ100
1982 Honda CB750SC NightHawk
1980 Yamaha XS850
1980 Honda CX500
1980 KZ750 LTD
1981 Yamaha XJ750

Post edited by: Z1Rider1, at: 2007/12/02 13:57

ZIrider1,

Are you saying you put 10 over rings in an engine that had not been rebored? It seems that would result in the ring tension against the cylinder walls being more than the engineers contemplated. What kind of longevity is on that engine now? Any problems?
/

I would really like to know how well this works as well...I have a set of 1mm over rings hanging out (I am hoping to find the matching pistons one day on ebay...)


But this sounds interesting...

the ring tension is not going to be a issue, I have bulit a ton of race car motors using .005 over the bore size rings and fit them to each cyl with the proper end gap, you can buy a cheap ring gap file tool that keeps the rings square to the cut that is the most important thing .
you have to watch the oil ring to make sure the rails dont overlap.

make sure you dont remove too much material when you hone the cyl and a good cross hatch pattern is used