A new attempt at making piston rings

[nj111]

Hello Brian,
There has been much discussion on this subject for decades and some years ago I found myself researching the subject in some depth.  Here a few things I feel are important in making good rings.
The thickness (height) of those rings you have there looks quite large.  Various builders experimented here and found better results in reducing this.
Myself , I had excellent sealing right away for 1" bore Westbury Kiwi with 0.022" ring thickness, as opposed to the 0.040" specified in E.T.Westbury's drawings.  I made these using Trimble's method in SIC 7/8/9 (except for heat treat temperature).
Bruce Satra (SIC No25) writes that he experimented with rings down to 0.015" thickness with much, much better compression.
With regard to heat treatment, Trimble stated an incorrect temperature (above the critical 720'C).   In his publication "An Essay on Piston Rings" (free online) T.D.Walshaw suggested 590'C is the maximum as otherwise "undesirable changes in the material will take place" (Young's modulus reduced). In the same document Walshaw stated that Trimble's heat treatment fixture "does not and cannot create exactly the right shape"  and therefore post heat treat trueing of just 0.001" was essential. -  I see you already have a solution for this above.
Assume you have been able to source good quality, close grained, centrifugally spun Grey Iron and not S.G. Iron?     
With the cylinder bore, I found it impossible to see what was going on here using a bore gauge with the standard 0.001" dial indicator. With this swapped out for an indicator reading in 0.0001" the difference is staggering and you can see exactly what your hone / lap is doing in terms of removing ovality and taper. It amazed me that even a toolroom lathe was not producing a truly circular bore. Until I switched indicators I had assumed all was spot on!
I'm certain if you persevere you'll find the solution to making excellent rings and I look forward to your updates here!

[Brian Rupnow]

nj111--I thought that a ring .062" wide as purchased was too wide. Everything I have read about it (And I have read a lot) suggests making the ring  1/30 to 1/25 of the bore size. With a 1" diameter bore, that comes out to a ring 0.033" wide to 0.040" wide. My reading material also suggests that a ring which is square in cross-section is proportionally correct. I purchased these rings from Debolt and then machined a piston to match the rings, with a ring gap 0.063" wide x 0.055" deep. (The rings are 0.051" thick). I paid $50 for two rings, so I haven't been rushing to throw them away, but I think I've just about ran out of things to do to make them work. I'm more or less "lost in space" now. I could make another cylinder with an exact 1.000" bore and see if that worked with the existing piston and rings. I could make new rings and a new piston to suit  my cylinder "as is". I know I could make a new oversize piston and lap it into the existing cylinder and not use any rings, but the whole point of this exercise is to make my own cast iron rings with my own equipment.---Brian

[nj111]

As a comparison I'm just looking at a set of new Honda 50 rings, (39mm bore). The height of these is 62% of the radial thickness. (41 thou in height and 62 thou radially)   - A similar ratio to what I have made with good success.

[crueby]

How does the gap at the ends of each ring work into it? What keeps the pressure from just flowing through there? I've seen some rings made with a tapered overlap, some just square ends. I know next to nothing of metal piston rings, just asking so I can understand better.

[Brian Rupnow]

Rings with plain ends should have a gap of 0.004" between the ends on a 1" bore. As the ring heats up from running the engine, it will get longer. If the ends don't have that gap to expand into, they butt heads with each other and then bulge out and score the cylinder wall.---Brian

[crueby]

Thanks Brian!

[Brian Rupnow]

Crueby---As to compression flowing thru the gap in the ring---that 0.004" gap is there only when the engine is cold. As the engine heats up to operating temperature that gap will close to about 0.0015" to 0.002". Events in a four cycle i.c. engine happen very quickly. An i.c. engine idles at about 1000 rpm. That works out to roughly 17 rotations per second. The engine only comes up on compression stroke every other rotation, so it "fires" 8 1/2 times per second at idle. The compression simply doesn't have enough time to leak thru the gap in the rings (at least not enough to affect the running of the engine). That is one of the reasons that a cold engine is harder to start than a warmed up engine.

[Brian Rupnow]

Today I will try a new trick. Conventional wisdom says that a cast iron piston should be 0.002" less than the cylinder bore. Conventional wisdom also says that the piston rings will take care of sealing that .002" diameter difference between the piston and cylinder. I am going to work from the assumption that my new piston, grooved for my purchased rings is perfect. I am going to assume that my purchased rings are perfect. I will make a new cast iron cylinder with a bored hole of 0.996" then use the new 0.998" diameter piston (with the rings removed) to lap the hole in the cylinder to as perfect an air-tight fit as I can get. Then I will put the rings back on the cylinder and fly with that. That is what I did on my vertical hit and miss engine, and it has wicked compression.

[Brian Rupnow]

On two previous engines I have used purchased cast iron rings. They both performed very well. I am at a loss why this engine has no compression when dry, and by dry I mean no pool of oil setting on top of the piston. I have just looked in detail at both previous engines where the purchased rings worked fine, and in both cases I see that I lapped the cylinder with the piston which was to be used in the engine, then added the rings to the piston afterwards. On this current engine where I am having so much difficulty, the piston was deliberately machined to 0.002" less than the cylinder bore. On my engine "Thumper" I made a note to myself saying that even though I had used the piston to lap the final cylinder bore, it wasn't a tight enough fit to run without the rings. There is a lesson for me there. Even though I have read in numerous places that the piston should be 0.002" smaller than the cylinder bore, that doesn't work for me. I just paid $32 for another piece of cast iron 2 1/2" diameter x 4" long to make a new cylinder. I will bore it to almost final size, then use my new piston to lap the bore to final size.

[Roger B]

Why not lap it with an Acrolap?

[Brian Rupnow]

I could lap it with an Acrolap--I'm just not sure of an acrolap giving me the exact match for the piston diameter. I am a master of machining a little bit , and then a little bit, and then "Oh $hit--I machined too much off"

[Art K]

Brian,
The beauty of the lap  is that you can bore it a couple .001 under hone it to size then turn the piston to fit.
Art

[Vixen]

Brian,

Make sure the chuck is NOT squeezing the new cylinder out of round while you  hone or lap it to the finished size. The finished cylinder must be perfectly round with no taper, otherwise the iron rings will never seal.

Good luck

Mike

[Brian Rupnow]

So here we have it---Old cylinder in background, new cylinder, and piston acting as a final lap. Bore was brought to "almost" with Acrolap and 600 grit paste, then final bit was done with 600 grit paste and using the piston as a lap. For first stage of lapping, acrolap was held in lathe chuck and cylinder was held in my hand. For second stage of lapping cylinder was held in lathe chuck, and T handle of piston was held in my hand. This can be very, very "grabby"--You have to know when to let go of the handle. Twice it grabbed so bad that I had to let go and shut the lathe off and take piston and cylinder out to my arbor press and use a short length of wooden dowel to press the stuck piston out of the cylinder. Definitely not for the faint of heart!!!

[crueby]

Hi Brian,  if the piston is lapped to such a close fit, is the ring still needed? Will the piston be a good enough seal as is?