Notched Piston Rings

I’ll have a stab at this.

In a 4 stroke ICE the rings (among a few other responsibilities) prevent engine oil blow by. This is not an issue in 2 cycle ICE.

As I consider, this is kind of lame. I’ll think about it a little bit.

Well, wait a second… are you certain that a typical 2 cycle ICE has only 1 ring on a piston? This would seem to enable a fair amount of piston rocking within the cylinder.

I have rebuilt many 2 cycle motocross engines, and they all have a single notched ring. The notch prevents blowby very effectively, and the pistons are lubricated by the oil in the fuel mixture, but the engines require frequent rebuilding. They are built for a single purpose, racing, not longevity. I understand the tendency of the piston to rock with only one ring, and some competitive 4 stroke racing engines use a single ring. Engines are rebuilt after every race. Just a cost of competition.

VERY LARGE (3000+ hp) diesel engines, such as in large carrier ships have a cross crank to eliminate piston rocking. This ensures a straight line motion of the pistons. Not practical for smaller engines.

'cross crank'?

As in a cross-plane crank? How does that help reduce piston rocking?

NO. Perhaps I should have said a "cross pin" instead.

These gigantic diesels have a rail system that guides the piston straight down, preventing the skirt from rocking against the bore. Some have a long connecting rod that is supported by bearings near the center of the engine.

Think LARGE ,If you have never seen one, you will be amazed.

https://www.dieselarmy.com/tech/engine/dynamic-diesels-inside-the-worlds-largest-engine/

Picture a set of parallel rails.

Imagine a piston with roller bearings on the end of the connecting rod rolling on these guide rails.

The rails guide the piston in a straight line.

Imagine a second connecting rod that provides rotation for the output.

These engines are not designed for high speed, mostly less than 300 RPM. The piston also has many tubes leading to the top of the piston for lubrication.

Looks weird, but these are specialty engines that burn tramp oil, which has to be heated to be pumped. Almost tar-like. It is also the cheapest fuel available.

These engines do not have a "starter" as commonly perceived, they use compressed air to start.

Large freighters are still the most efficient way to move large tonnages on a cost-per ton/KM basis.

I will try to find an image of the cross-crank in a diesel engine.

https://en.wikipedia.org/wiki/Crosshead

Does if my reply to #3 help?

cross crank. Is this what I know as a "cross head?" This is a separate piston-like between the connecting rods and the piston; it takes the sideways forces and ensures strictly straight-line motion for the pistons. It would make the engine taller. My Ahrens-Fox piston fire pump has this cross head design.

Yeah I do. Steam Engine Clupet rings.

Clupet Piston Rings

I have rebuilt mainly motocross 2 strokes, and they do have a recesses indexing pin, but I have never seen one with a slot in the cylinder.

This includes Kawasaki, Honda, Yamaha, Suzuki KTM and others.

Even the small non-competitive 2 cycle engines have the notched ring.

I am sure there are engines that I have not been experienced with and I do not know your experience.

My experience is in small 2 strokes is now nearly obsolete.4 stroke engines have taken over the motocross races because they have become lighter and they provide a smoother more easily controlled torque range, whereas 2 strokes have high rpm power band.

My experience also.

I have not come across the single ring on 2 stroke pistons so far. The ones I have worked on have had 2 rings of some material like chrome moly and were only about 1mm in width, these as you say needed pulling down after every race meeting.

My 1970 Yamaha DT1 had a single cylinder 2 stroke cycle engine. It had 2 chrome piston rings. I never raced it, but only changed the rings once while I owed it, and put 12,000 miles on it. It must have been the worlds simplest engine. It had no valves, only ports.

The main reason why a 2 stroke has different ends to a 4 stroke piston ring is the the ring function and the differences which occur in the cylinder.
The 2stroke usually only has one ring and the lubricating oil is included with the fuel mix, a 4 stroke engine usually has 3 rings, two compression and one oil control ring.
The lubricating oil is held in a sump and does not mix with the fuel. Also the compression is higher in a 4 stroke and the engine has a longer life.
From a cost point, it is more expensive and time consuming to produce chamfered or stepped ends than a simple 90° cut. 4 stroke piston rings are generally fitted with 120° offsets.