Total Newbie Questions About Changing Piston Rings - Miata Forum

Third gear  
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: Jun 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.