Can piston rings wear before cylinder wall?

[greggilo]

I have two- two stroke outboard motors with low compression. 1979 85HP V4 and 1979 150HP V6. The V4 has 90 psi on all cylinders and the V6 has 60 psi on all cylinders. I am currently using the 85HP on a 17' deck boat that runs about 29 miles/hr WOT. I recently purchased the 150HP used but was disappointed with its performance. The 150HP is hard to start and doesn't run well. I have torn down the 150HP to find that it has been worked on before. The #5 cylinder has an 0.030 OS piston. The cylinders look find and have not yet had the bores measured. Everything else looks fine on this motor. I am sure the bore measurement will tell me more but is it a waste of time to just replace the piston rings?

 

[racerone]

New rings in worn ring grooves is a huge mistake.

 

[RBoyd1971]

Out of curiosity, did you put the rings in the bores and measure the end gaps. Wonder how much wear there is. I used to replace just the rings on two cycle racing motorcross bikes all the time with the same pistons with no problems. However, I will say that two cycle motors are hard on pistons. You can measure ring grove wear. You can also get one set of new rings to check things out, such as bore concentricity and wear as well as how tight the ring groves are. If you put a new ring in there and shine a light from the backside and can see light around the edge of the ring the bore is egg shaped. With all that said, having a qualified machinist do a facts check is the best. I didn't have a lot of money in my younger days so I did what I could to keep my rides going.

 

[ryan 98]

I have two- two stroke outboard motors with low compression. 1979 85HP V4 and 1979 150HP V6. The V4 has 90 psi on all cylinders and the V6 has 60 psi on all cylinders. I am currently using the 85HP on a 17' deck boat that runs about 29 miles/hr WOT. I recently purchased the 150HP used but was disappointed with its performance. The 150HP is hard to start and doesn't run well. I have torn down the 150HP to find that it has been worked on before. The #5 cylinder has an 0.030 OS piston. The cylinders look find and have not yet had the bores measured. Everything else looks fine on this motor. I am sure the bore measurement will tell me more but is it a waste of time to just replace the piston rings?

If the bores are good and close to round and its an issue of excessive wear and ring gap you can absolutely dingle ball hone it and file a set of rings to fit. Now it's the absolute wrong way to go about it and longevity will suffer to an unknown degree. If it's already low on compression from wear then I can guess the bores will be worn out of round and that doesn't help either. Piston to cylinder gap will probably be poor too but I've had 2 strokes that had wicked audible slap and still ran. Also right off the bat having one cylinder oversized throws balance out the window, but once again there's a lot of these omcs happily boating around with one cyl over. Boring it over and going all new is the only right way. But if you have the competency level of me, amd that ain't asking for much. Then you can make it at least run decently well with some cheap simple home rebuild bits. And of course machine shop ( dingle ball one)

 

[greggilo]

Out of curiosity, did you put the rings in the bores and measure the end gaps. Wonder how much wear there is. I used to replace just the rings on two cycle racing motorcross bikes all the time with the same pistons with no problems. However, I will say that two cycle motors are hard on pistons. You can measure ring grove wear. You can also get one set of new rings to check things out, such as bore concentricity and wear as well as how tight the ring groves are. If you put a new ring in there and shine a light from the backside and can see light around the edge of the ring the bore is egg shaped. With all that said, having a qualified machinist do a facts check is the best. I didn't have a lot of money in my younger days so I did what I could to keep my rides going.

Thank you for the response, I never knew the cylinders could wear out non-concentric. I will have a machinist do the job. Probably go with all new.

 

[greggilo]

If the bores are good and close to round and its an issue of excessive wear and ring gap you can absolutely dingle ball hone it and file a set of rings to fit. Now it's the absolute wrong way to go about it and longevity will suffer to an unknown degree. If it's already low on compression from wear then I can guess the bores will be worn out of round and that doesn't help either. Piston to cylinder gap will probably be poor too but I've had 2 strokes that had wicked audible slap and still ran. Also right off the bat having one cylinder oversized throws balance out the window, but once again there's a lot of these omcs happily boating around with one cyl over. Boring it over and going all new is the only right way. But if you have the competency level of me, amd that ain't asking for much. Then you can make it at least run decently well with some cheap simple home rebuild bits. And of course machine shop ( dingle ball one)

Thanks for the reply. I will not try to do it myself. I am taking it to a machinist. Just have to see what the 0.030 piston and cylinder condition is.

 

[greggilo]

New rings in worn ring grooves is a huge mistake.

Thanks for the reply. I will have everything checked out.