10 Questions You Should to Know about Piston Wear Ring

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Join Date: Oct Location: Baltimore, MD Posts: 784 Total Newbie Questions About Changing Piston Rings Since I have to put a new short block in my car, I want to get the engine in tip top shape while engine is out of the car and accessable. I've bought a new used short block from a low mileage 96 or 97 Miata. The bores are supposed to be really clean with no scores or the like. I want to change out the piston rings so I can get nice even compression since this engine will be FI.

A friend of mine is going to lend his expertise and his tools to help me with the honing process. That part is taken care of. Here's my newbie questions...

1. I called Rosenthal Mazda to get a quote for a ring kit. The first thing that was asked was whether I wanted stock, +25 over, or +50 over. Since I plan on only doing a light honing and keeping the stock pistons, is there any reason to order any size other than stock?

2. Is there any way to test the compression before putting the engine in the car? Do I need to or can I assume that new rings will most likely give me good numbers? I would rather fix a problem while the engine is still on the stand rather than in the car.

3. Is there any other advice you can give a newbie such as myself?

I do have the old block to fiddle around with while I wait for the new short block and piston rings. I plan on taking everything apart, honing the bad block, and putting everything back together just for practice.

Thanks in advance. Sixth Gear  
Join Date: Jul Location: Mi, USA Posts: 8,832 Cylinder honing must be done correctly, and to the correct finish, for the type of rings being used. Along with this, the bores must be straight, round, and correctly sized to the rings and pistons. If not, wear and/or sealing problems will result. Adding a turbo or supercharger makes this even more critical. Unless you know your friend's hone is appropriate for this job, and that he? knows what needs to be done and how to do it, you would be well served to have this part of the job done by a qualified, experienced, professional. Because you are adding a supercharger you may want to use higher quality rings. Depending on boost, pistons may need up-grading as well.

1. Don't order any rings until the bores in your block have been inspected and measured.

2. You could do a compression or leak test, but until you have successfully broken in the rings the results of the tests are of limited value.

3. I don't mean this to sound rude, and I'm not trying to discourage you, so please don't take it that way. That said, you obviously know very little about what needs to be done, checked, and measured; and how to do it. While people in your position can and do recondition engines, a successful end result will happen only if a lot of things you are unaware of happen to be ok as is. I suggest you read everything you can get your hands on, and get all the help you can get from the most skilled and knowledgeable motorheads you know.

Good luck......Al English First gear  
Join Date: Sep Location: Hampton, VA Posts: 73 Cylinder honing is really not that difficult. The main concerns are cylinder wear and if oversized pistons are required. If the hone marks remain from the original assembly, you probably (almost certainly) don't need oversized pistons and accompanying rings. If there is a cylinder ridge, caused by wear then the engine might likely need .025" (+25) dia oversized pistons and rings. They make measurement tools specifically for measuring the engine cylinder diameter.

Rings will not show good compression until they have been 'seated'. Sometimes, with chrome plated rings, this can take awhile because they don't wear much. Steel rings seat quicker but don't wear as well.

The most important part of installing piston rings is to put the ring in the cylinder and measure the end gap. The end of the piston can be used to keep the ring square in the cylinder for this measurement. Without proper ring gap the rings will never seat, or they will break when the engine gets hot.

The best advice I can give is to find (borrow, beg, or buy) measurement tools and learn how to use them. Don't trust 'Bubbas machine shop' to give you an engine you can just assemble. That said, if the clyinders still show hone marks, I doubt you can do much to improve the piston rings in that used engine. Just my opinion of course.

Good Luck!

Stan Third gear  
Join Date: Oct Location: Baltimore, MD Posts: 784 I don't have to beg for the tool, thank gawd. The head of my employer's truck repair facility is loaning me all of the tools I would possibly need. Bearing mics, bore mics, honing tools, ring compressor. The only tool I will have to buy is the ring expander and that's because one of the machnics misplaced it.

So if I can still see hone marks in the cylinders of the used block, I should be good to go without honing and resringing? I don't know what mileage is on the used block other than the guys at Planet Miata are tearing down a lower mileage 96 or 97 engine for me because they weren't happy with the condition of the short blocks they had already in stock. If they are taking their time getting me a good short block and being picky about it, I can't complain.

What caused this type of wear? It's on a 1 1/2 economy engine. End gap was .018 and grove .003-.004. Center of ring face shiny only contact. Engine had lots of blow by and low compression, may why it won't run. Looks like the previous owner honed the heck out of it. New rings see what happens?

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looks like the rings where not flat on the outer edge....however before you put in new rings check the bore dimensions with a inside micrometer or other good thing like a bore gauge.
Measure the piston and see what you have...it may need a rebore and sleeve but need to check so you don't waste your time and a set of rings.
Those ones you show don't look too old.

Looks like the rings weren't sized for the cylinder, may have taken oversized and filed the gap to fit.

Yup. What's the PtW clearance OP. If it's been 'honed to crap' makes me wonder what with? Measure for out of round and taper of the bore. You've got to use a rigid hone when removing any defects. Those three stone spring hone are only ok(ish) for breaking glaze on a good cylinder. Otherwise they just make a bigger messes up cylinder.... Also the rings might be barrel faced which is why they're not worn all the way across. They should be worn all the way around though. This can happen if honed too fine. The cylinder glazes before the rings seat. I'd Hone no finer than 200 grit if it was me... The only history I know is the previous owner said it ran on battery but never on the Webster magneto. He had it rebuilt but never could get it to run. Timing and wouldn't trip but have worked that out (I think) So I measured the ring groove wear it's .003-.004. Rings seem fine in grove other than the end gap .018 measured at 3' from the back. I'll see if I can find someone to help measure the bore

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To be brutally honest, that hone job looks like crap. No cross hatch and pretty fine. The 0.018' end gap is fine. 0.004' per 1' bore diameter is absolute minimum so if it's a 4' bore, 0.016' gap is minimum. It's a ring seat issue causing blow by. The 0.003-4' land clearance is fine also. I've seen smaller rings far looser. I think you have the wrong diameter rings.

Slide the piston in the bore with no rings and see how large a feeler gauge you can fit between the skirt and cylinder wall. Will give a good idea of piston to wall without micrometers. I would use the proper ring size for the bore even if the eng gap is large. Ring fit in the bore and seating has far greater effect on compression than end gap. Thank you for all the good information. I should have mentioned it's a 3 1/4' piston with 1/4' ring .013 gap is what I should shoot for. With the suggestion of measuring piston and bore at the bottom, I get .008 with the feeler gauge. I'll remove the head later and measure at the top. Then maybe I can address the hone job. I didn't do And look at the rest of the bore. Good info all help is Appreciated Sounds like the piston to wall clearance is fine. I would suggest taking the cylinder to a shop with a proper sunnen home or see if you know anyone with a portable hone. Must be a guided, rigid hone sometimes called a 'plateau hone'. This will true the bore round and straight. If you use a portable you cross hatch manually. Take some light passes to get the 'feel' of the stroke rate required to make a nice cross hatch. 180-200 grit stones. Then measure the bore and order rings for that nominal bore size. Don't use over sized rings to get a file fit for the end gap. If the gap is over the 0.004/'bore, don't worry about it, it's not a race engine and the rings fitting the bore is more important than compression losses through the gap. Good luck with it! Maybe new rings in an oval bore. I've been at this over 50 years have not replaced rings in but a handfull. I'm currently working on a 6 horse "M" , had to drive the piston out with the prescribed 16# tool. All 5 rings were married to the piston, but about a half a day & probably a couple thousand taps with my little plastic hammer, they all came loose. Have been off the piston, cleaned & reinstalled. I have no doubt it will have good compression. The first thing people want to do is replace rings, & in my humble opinion, that's a mistake & usually unnecessary. [only worth .02 & you probably have change due]

Maybe new rings in an oval bore. I've been at this over 50 years have not replaced rings in but a handfull. I'm currently working on a 6 horse "M" , had to drive the piston out with the prescribed 16# tool. All 5 rings were married to the piston, but about a half a day & probably a couple thousand taps with my little plastic hammer, they all came loose. Have been off the piston, cleaned & reinstalled. I have no doubt it will have good compression. The first thing people want to do is replace rings, & in my humble opinion, that's a mistake & usually unnecessary. [only worth .02 & you probably have change due]

If they're not worn thin I'd leave em too. Just put a cross hatch in the bore to help them re-seat and go. OP is forced to do something as those definetly don't sit in his bore nice... The rings are not worn thin, just the odd uneven wear on the ring face. Who really knows how many other people have had there hands on this engine. It just one off may little things keeping it from running. I will do a nice light cross hatch in the bore and see what it looks like For me I've never seen wear like this. Thank you for all the good information and learning Just to clarify, I think those rings are the wrong size for your bore... Skip could also be correct that the bore is oval. Some proper measurements with an inside micrometer will tell.

I think Skip means (and correct me if I'm wrong Skip) that the rings probably didn't need to be replaced by the previous owner... if the new rings you have, that are worn funny aren't the right size for the bore, they're no good to you. I guess if you had a lathe and tool post grinder you could make a fixture to hold them and grind them round at your bore size... my guess is you need the proper sized rings in this instance though.