Topic: CB750 mystery "fourth" piston ring?

[markb]

Here’s a pic of a set of early CB750 piston rings with the one piece oil ring. It has a fourth ring that I’ve never seen before. Anyone know what it’s for?

[754]

I would say it pushes out the oil ring.

[jgger]

+1 on what 754 said. I think that is an old school thing, I remember seeing it a long time ago in some car motors.

[Yamahawk]

It is a spring for oil control on the oil ring...it goes under that ring.
Charlie

[markb]

Is it required? I compared the oil ring that came with the spring to other one-piece oil rings that I have and they are identical. I have never seen or used that spring.

[HondaMan]

Is it required? I compared the oil ring that came with the spring to other one-piece oil rings that I have and they are identical. I have never seen or used that spring.

It's not required, but in certain situations it reduces the oil blowby at high engine speeds (if too-light oil is used).

[strynboen]

think it moves and klean up behind the oil ring..and give a minimal push aut..(.my small Honda cd-50) had it..use one piece oil ring..later 3 piece rings dont ..

[markb]

I don't think it pushes on the oil ring at all. It has no strength to expand especially compared to the expansion of the oil ring itself.

[754]

It's a shim, take it out you get clearance.

[Yamahawk]

If the piston ring manufacturer thought you didn't need it, they sure wouldn't include it in your piston ring set. Here is an excerpt from Federal Mogul piston ring page:

Federal-Mogul Powertrain two-piece oil-control rings are available in a variety of design variations. They include cast iron or nitrided profile steel bodies; standard, beveled or tapered ring lands; high-wear-resistance chrome, CKS or GDC coatings; and slotted or drilled drainage systems.
Two-piece oil-control rings are used in diesel and some automotive gasoline applications. The design of a two-piece oil-control ring consists of a cast iron or profiled steel ring body and a coil spring. The spring is a cylindrical coil spring made from heat-set resistant spring steel, and it acts uniformly around the entire ring circumference.
For most gasoline applications, Federal-Mogul utilizes three-piece oil-control rings. The design of three-piece oil-control rings consists of two thin steel rings (rails) and an expander-spacer, which holds the rails at the required relative axial distance while simultaneously pressing them against the cylinder wall.
For three-piece oil rings with an expander-spacer design, three fundamental styles have become established on the world market: VF System, MF System and SS50 System.
Oil-control rings are used in light-vehicle gasoline engines (two- or three-piece), light-vehicle diesel engines (two-piece), and truck diesel engines (two-piece). The main function of the rings in the bottom piston groove is to scrape oil from the cylinder wall, meaning oil-control rings play a major part in regulating an engine’s oil consumption.
Federal-Mogul Powertrain commonly produces five different types of two-piece oil-control ring bodies. Variations on ring land geometry and running face coating give many options for oil consumption and wear performance.
Three fundamental expander-spacer styles have been established for oil-control rings on the world market. When made from a nitridable steel, nitriding can provide effective protection against secondary wear.

Now, I know there are ring packages that do not include this oil control expansion ring, but in those that do, iy give proper spacing in the ring groove.
Charlie

[markb]

If the piston ring manufacturer thought you didn't need it, they sure wouldn't include it in your piston ring set.

Thanks for the reply, Charlie. What you say is completely logical. However, When I compare the oil rings from sets I have with and without the "fourth" ring, the oil rings are the same. Also what is described as a spring in the 2-piece oil ring in your excerpt is not what I have. I am familiar with the 3-piece oil ring in your pic but that is also not what I have. I believe they started using them on the CB750 K2s. Every K0 I've ever seen has had just the one piece oil ring. Here's another pic showing what I have. I think we are talking about something different.

That said, since I have them why wouldn't I just use them. But if they are necessary then why aren't they included in all ring sets?

[754]

Just measure the width and compare to the old ones.
 Then check the other two.
 Then try each groove with a ring and see how much wiggle... gentle here
You don't want drag.
 I had a nice set of forged pistons I was installing 2 days before Delores. I had slid the rings in at Christmas, seemed OK but top ring must have had less wiggle. I guess I didn't notice.
 Put the rings on first piston, checked with my fingers, rats... top one was not moving freely .
 Had to go thru 2 sets stockers, carefully clean grooves and tops , sort thru wrist pins, polish a few..
 But I got er done,  by then it was late in the game and I posted my progress.
 Nobody thought I would make it.. I did.
 One of the other surprises and ALWAY CHECK THIS , moreso if you send your cylinders for honing without a wooden box.
 One piston would always tighten up at bottom of sleeve in one spot .
 I cross measured using a T guage, then you don't need a micrometer.
 Had to work off up to 1.5 thou on a match book cover size are on the sleeves by hand, couldn't find the die grinder , it took quite a while.
 Had I not caught it and checked it, I am pretty sure it would have scuffed or even stuck the first day out.
Anyway as per usual check, check and recheck.

[Yamahawk]

I hear ya, but again, they must have a purpose. Those octagonal, or however many sided springs, are also included in my RD350 ring sets, at least some of them. I have seen the instructions, and know there is an assembly procedure to follow when they are given, including a top side to the ring it mates with. Can't remember what way they go now, as I haven't ringed the RD350 for a year or more, but I will be pulling them apart soon to port the cylinders and hog out the intake side for some YZ85 reed cages.
Are you sure that the one set of rings isn't slightly different than the other set with the oil control spring? Or, that they were for a different manufacturer of the piston? If they were for identical pistons, from the same manufacturer, I can't say why one set would have them and one wouldn't. I would contact the piston and/or ring manufacturer just to be sure. You probably don't want to assemble the cylinders and heads, put it back in the frame, and then figure out that you either did or didn't need them lol.... Anyway, hope you find a definitive answer from the company that makes them.
Charlie

[strynboen]

still think  it move araund and klean up the ring chanels in the piston.( behind the oil ring).so the oil ring not grow stock..or the oil canalels drilled in the piston vil fill vith sooth or blov aut  underkompresion..its a kind of "return valvet" so help,, the oil scrabet of the bores goes back in the sump..so the pressure not just blow the oil up in the kompresion champer

[754]

 Are they factory rings with early part number?
 Maybe the early pistons had deeper grooves..

[HondaMan]

In the aftermarket rings I have used, one of my favorites is the RIK brand. Honda used to supply these in the early 1970s, in Honda boxes wrapped over the RIK boxes. Today, when I find these rings, they often have this shim in them. The one "trick" to it is: these only fit the early pistons that use the 1-piece oil rings, as they are a slightly different ring land width and depth. I have always used the spreader, though, if it has one.

[markb]

Are they factory rings with early part number?
 Maybe the early pistons had deeper grooves..

They are MC brand rings.    Don't know when or where I got them. The only description on the box is Honda 750 STD. The other ones I have all have the Honda part number on them.

[HondaMan]

Are they factory rings with early part number?
 Maybe the early pistons had deeper grooves..

They are MC brand rings.    Don't know when or where I got them. The only description on the box is Honda 750 STD. The other ones I have all have the Honda part number on them.

Them's the ones! Those are the same as I have. I've got several sets of them in 0.5mm and 0.75mm size for upcoming builds. They all have the spacer rings.

[markb]

Are you planning on using the spacer rings?

[HondaMan]

Are you planning on using the spacer rings?

Every time.
They fit without issues on the early pistons that have the wider ring land at #3 position. They will work perfectly, too! I like these rings, and use them whenever I can. My own engine has them, now.

[markb]

Then like I said I will probably run 'em too but still OK to run without?

[HondaMan]

Then like I said I will probably run 'em too but still OK to run without?

Sure: it won't hurt anything. At best, it will increase the oil-scrape rate from teh cylinder walls, which helps the very-high RPM situation. On these engines with the 1-piece oil rings, we often see (saw) a slight oil haze from the pipes during long sojourns at redline speeds: this was caused by some oil not being able to escape thru the slots in the 1-piece rings. The 3-piece rings are far more porous (to the oil-release holes behind them, in the ring lands) and prevent this, but they also help reduce compression at those engine speeds as well, because they seal poorly when moving so fast. The spring-ring inside the RIK rings (and similar automotive rings) improves the lifetime of the 1-piece ring while also scraping the oil off better, but also increase the oil haze if the engine is run hard (racing), so they are not a great racing ring.

For my riding style these days, they are perfect: I seldom find redline anymore. Mostly I run below 7000 RPM. If I hit the interstate for a long ride, I gear up, either with tire sizes or even a 19T front sprocket: my tach reads about 5000 RPM at 85 MPH this way.

[scottly]

Then like I said I will probably run 'em too but still OK to run without?

If the rings came with expanders, they should be used. The ends of the expanders should be butted up against each other, not overlapping, and not aligned with the gap in the oil ring.

[HondaMan]

Are you planning on using the spacer rings?

Every time.
They fit without issues on the early pistons that have the wider ring land at #3 position. They will work perfectly, too! I like these rings, and use them whenever I can. My own engine has them, now.

I probably should clarify: my engine has the RIK version of these rings, which appear identical in everything but the stamp marks on the rings themselves. The boxes they came in look identical, except they are red for RIK: the rings inside are wrapped identically in the same kind of paper in both brands. The first time I got these was years ago and had bought 2 different sizes, one of each 'brand'. When they were out of the boxes, the only way to tell them apart was by their sizes (0.25 one set, 0.50 the other) and stamp marks, they were packed that identically inside. Appears like they came from the same factory?

[markb]

If the rings came with expanders, they should be used. The ends of the expanders should be butted up against each other, not overlapping, and not aligned with the gap in the oil ring.

I don't want to overdo this but are you talking about the expander that is used between the two oil rings in the 3-piece system or the ring that is used on the inside of the one-piece oil ring? I think we might be talking about different things.