Piston ring sizes

[Fleetwood Meadow]

Engine Rebuilding 101: 

 

-Can someone explain to me the difference between STD rings and .010, .020, etc.?

 

-Are the standard rings the same size as oversized rings, the oversized rings are just a wider diameter? Or are the oversized rings thicker in size? What I mean by thicker is if you lay the ring down and use a caliper to measure the size of the piece of metal, not the diameter, is it thicker/wider than a standard ring?

 

-Also, can you use an oversized ring on a standard piston?

 

-If someone bored an engine would they bore every cylinder or just the ones that were damaged? 

Edited April 4, 2019 by Meadowfleet
Added more questions (see edit history)

[TerryB]

The .010, 020, etc are overbore sizes for the pistons, they are that much larger diameter than a standard piston.  To accommodate a larger piston a larger ring is required.  So if the piston is .010 larger, the associated ring for it must be equally larger.  There are exceptions to the rules but usually all cylinders are overbored by the same amount.

[Bloo]

Typically every cylinder is bored because it incresases displacement, and possibly piston weight. Not always. It was not uncommon in the old days to fix one cylinder by overboring. A one-cylinder fix today usually involves sleeving the really bad cylinder to match the good ones.

 

.010, .020, etc are piston and bore oversizes. An .020 piston ring goes on an .020 larger piston in an .020 larger bore.

 

If a bore has wear it is possible to put in larger rings to compensate for wear, to some small extent. These are called "File-Fit" rings. They are made just a little too long and you file the ends to get the ring gap where you want it. They can be used in new engines when trying to get the ring gaps absolutely perfect (for racing, etc.). In old engines they can be used to make the wear situation a little less bad.

 

The limitation is that worn bores have taper, and if a ring gap closes when hot, the ring breaks and it destroys everything in sight.

 

You have to set the gap at the bottom of the bore (the least worn part), because the gap is tightest there. The part that matters most to the ring's function though is the top, and the top is more worn. No matter what you do the ring gap at the top of the bore will be too loose, it will just be a little less loose than if you had just put standard rings in. It helps, but is no substitute for a rebore and new pistons.

 

People used to also knurl pistons to compensate for piston skirt and bore wear, to keep them from slapping and making noise.

 

For the most part, nobody does this stuff anymore.

 

Edited April 5, 2019 by Bloo (see edit history)

[hchris]

7 hours ago, Meadowfleet said:

Engine Rebuilding 101: 

 

-Can someone explain to me the difference between STD rings and .010, .020, etc.?

 

-Are the standard rings the same size as oversized rings, the oversized rings are just a wider diameter? Or are the oversized rings thicker in size? What I mean by thicker is if you lay the ring down and use a caliper to measure the size of the piece of metal, not the diameter, is it thicker/wider than a standard ring?

 

-Also, can you use an oversized ring on a standard piston?

 

-If someone bored an engine would they bore every cylinder or just the ones that were damaged? 

 

 Bloo has given excellent insite, if  I might elaborate a little more though.

 

Consider a ring starts out as a solid piece of round metal, in simple terms if you turned off an end piece on a lathe you would have a solid slice of metal of a given diameter and thickness, if you then drilled /machined out the centre of this slice, you would finish up with a metal ring of given thickness and diameter, cut a slot in it and you now have a ring which you can carefully expand and slide over the piston top and fit into an appropriate width groove in the piston.

 

So pistons and rings are made to measure for given size cylinders, obviously wear and tear makes the precision sized cylinder oversize in due course, so there are now various options to overcome the problem of lost compression and excessive oil consumption due to this wear, a lot will depend on the severity of wear as to repair choices.

 

Oversize rings is one solution, usually coupled with remachining the cylinder bore and oversize pistons to suit. But just focusing on the rings, to answer your questions, consider a bigger diameter piece of round metal ( say 0.10 or 0.20 or 0.30 greater diameter) machining off a slice and going through the same process as before, you now have an oversize ring; generally they will be of the same thickness as before. 

 

So now you have an oversize ring and it can do the job of making up for cylinder wear, however it`s not that straightforward and Bloo has done a good job of explaining why.

[Fleetwood Meadow]

Thank you so much guys for the great answers. When I rebuild my ‘51 Dodge’s engine I want to make sure I’m getting the right sized rings. 

[Guest]

Make sure you measure every ring in it's bore and it's land in it's piston.  I have found myself and had seen in a dealership where I worked that not every ring is correct.  I measure every and file as required to get the correct end gap.  Personally have found six rings, in three engine overhauls, that were too tight in the land and five that needed to be filed to get the correct end gap.  All were well known name brands.  Measure everything.

[Ben Bruce aka First Born]

12 hours ago, Tinindian said:

Make sure you measure every ring in it's bore and it's land in it's piston.  I have found myself and had seen in a dealership where I worked that not every ring is correct.  I measure every and file as required to get the correct end gap.  Personally have found six rings, in three engine overhauls, that were too tight in the land and five that needed to be filed to get the correct end gap.  All were well known name brands.  Measure everything.

 

 And better a hair too much end gap than a half hair too little.

 

  Ben

[Rusty_OToole]

17 hours ago, Meadowfleet said:

Thank you so much guys for the great answers. When I rebuild my ‘51 Dodge’s engine I want to make sure I’m getting the right sized rings. 

You will need standard size rings for standard pistons, if your engine has been rebuilt with oversize pistons they will be stamped on top .010, .030 or whatever oversize they are.

 

If the cylinders are worn and tapered up to .010 there is an old trick, use .010 oversize piston rings and file to fit the lower part of the bore. Such a ring job will last for 20 or 30 thousand miles but of course, is not as good as a proper rebuild. It's something we used to do to keep a car on the road, but these days, when you may only drive a collector car 2 or 3 thousand miles a year, it makes sense if you want to save money.