A new attempt at making piston rings

[crueby]

Following this thread with interest. Somewhat related question (not to making them though) - are cast iron piston rings appropriate for use on steam engine models too? Low pressures/speeeds, would they still seal well on steam vs the higher impulse pressures on IC engines?
ChrisR

[Brian Rupnow]

Yes, they would seal on steam engines as well. I think that bronze would be used on steam engines because of the rust issues with cast iron rings.

[Jo]

Following this thread with interest. Somewhat related question (not to making them though) - are cast iron piston rings appropriate for use on steam engine models too?

Yes, you probably know that John Ramsbottom developed the split piston ring back in the late 1800s for steam engines and they immediately replaced hemp packing that was used to seal the pistons in existing engines (where possible) because of the improvement in power, efficiency and maintenance they provided. However in smaller sized models they can be a bit easy to break 

Anything over about 19mm diameter is ever so easy to make but for the smaller sizes its best to make some spares as they tend to break going over the piston   And it was standard to fit two rings in the same piston groove with their gaps on opposite sides of the piston in steam engines.

Jo

[crueby]

Thanks Jo/Brian!

[Zephyrin]

I suggest the following method, without heat treatment, which I use regularly for cast iron piston rings.
 First, you have to find the right cast iron, centrifugaly cast iron, without the crust.

1) Cut rings in a tube of iron with for external dia the dia of the bore plus the elasticity gap on the circumference and a small margin for turning.

I take 3.5 mm for this gap for a 25 mm dia and 2.5 mm for a 20 mm dia. piston.
This makes the outside diameter of the iron tube for a 20mm ring ((20 x 3.14)+2.5)/3.14 = 20.8 mm; or  21.3 with the margin for final turning.
The thickness of the ring is 1 mm in these diameters, with a clearance of 0.2 mm at the bottom of the groove; the inside dia of the tube for a 20mm piston ring is therefore ((17.6 x 3.14)+2.5)/3.14 = 18.4

Rings of the height of the piston groove (1 to 1.25 mm) are carefully cut with a thin, sharp tool, carefully deburring the exposed faces, so that only the side of the cut is left to be cleaned on a fine abrasive paper.

2) The 3.5 mm (or 2.5 mm) cut is removed from the circumference with a jeweller saw; this cut can be angled, if you wish; this is a delicate job to do as it will give the final inner dia.
By closing the ring completely with a thin brass wire around the ring and twisted, (a more convenient set-up to close the ring could be made !), the inside dia should then be the final internal dia of the ring, and this closed ring should fit on a mandrel turned at this diameter.
(The ring groove on the piston of course is at this diameter, minus a 0.2 mm clearance.)

3) This mandrel has a screwed washer in front of it to clamp one or a few rings and keep them closed on removing the brass wire; the exact external dia of the ring required is then finished and polished on the lathe.
On dismantling the ring from the mandrel, the ring is ready to fit the piston after removing sharp edges with a Norton stone, which is sufficient to give a final gap on the closed ring.
and it's over.

The elasticity of the rings obtained in this way is good; the compression after running in is top notch. 

[Brian Rupnow]

Thank you Zephyrin.---Time for an update on what is happening with this. I don't have any problem with machining the rings to size on o.d., i.d. nor width. The width is easier than you would think, because cast iron machines very easy and is very easy to cut down in width on a piece of emery paper on a cast iron surface plate. Solution there is to make sure and cut the rings a few thou "over-width". Cutting the ring grooves in the cast iron piston is easy, but you have to really pay attention to the width of the groove you cut. This is one of those weird times when "what you see" isn't necessarily "What you get".  I don't know why, but a 0.062" parting off tool gives me a 0.055" wide slot on a straight in plunge cut. You need to have the ring in your hand and keep trying it for fit in the groove you are cutting. Recommended groove width is 0.001" wider than the ring, with square sides, no taper. The groove should be .003" to 0.004" deeper than the radial width of the ring. When pushed down into place in the groove, the groove has to be deep enough for the ring to set below the outer surface of the piston groove.------------I don't have any problem breaking the ring on one side by putting the ring partially into the vice and pressing sideways on the ring with my fingers.------I don't have any problem cutting the ring with an 0.020" slitting saw like Philip Duclos recommends. Heat treating the ring and doing the same heat treat to all of the rings with a "gag" piece holding the ring open doesn't seem to be a problem, except that you can't really use an oxy aceytlene torch.--Too hot. Too fast!!!! Flame from a propane torch seems to do a much better job.----I don't have a problem setting the heat treated ring into the cylinder squarely  and checking the gap, and no problem filing the gap with an ignition points file until it reaches 0.004" wide.-------------Fitting the ring over the piston and down into the groove isn't as much of a problem as you would think---just use lots of #30 lubricating oil and a bit of thumb pressure and patience.--------It's not a problem to hold the cylinder up to a bright light and check that no light is coming past the sides of the ring (Which means that that ring is bad and won't seal.)-----The problem comes when after all of these things are looked after properly, the rings are not sealing in compression. My belief, and that of many people reading this post, is that the outer diameter of the ring is not perfectly circular, and as a consequence is not tightly contacting the wall of the cylinder.  Chadock (A ring guru) agrees with me/us. So---What to do? The only thing left to do is to assemble all of the rings on a fixture, align all of the rings with a slide on fixture that aligns the outside diameter, and then clamp them, remove the slide on alignment tool, and then take a cutting pass on the outside diameter of the rings to ensure that they are perfectly round. The outer diameter of the ring does not have to be completely concentric with the inner diameter. This cutting pass can be made by either a very sharp tool or by a toolpost grinder, or by lapping.----and should be less than 0.001" deep.
-----The problem after doing all of this is that the rings aren't se

[Roger B]

Ok here’s how I make my liners, pistons and rings. This description is for a nominal 25mm bore and the pictures are from two different builds.

The liner is bored to around 0.1mm under nominal size. I only have a dial calliper to measure with so I can’t be more precise. Take care at this point that the clamping pressure from a chuck etc. doesn’t force the bore out of round    Next the bore is lapped using an Acro barrel lap and 40 micron diamond paste at 250 rpm. When I feel even contact all along the bore and the machining marks have gone it is cleaned in the ultrasonic bath with an industrial detergent.

I then make a close fitting plug gauge to fit the bore. (You can do this the other way round, make the plug gauge first and lap the bore until the plug gauge fits which is probably better for a multi cylinder engine).

The piston can then be finished to size, what size? For a cast iron piston in a cast iron liner probably 0.05mm (2 thou) under the size of the plug gauge, for an aluminium piston 0.1mm (4 thou) undersize but this can be alloy dependant. It is better to have too little clearance as if the piston starts to pick up when running you can always polish a little off the diameter. The ring grooves are then cut with a 1.5mm blade type parting tool (because that’s what I have) The depth is not critical so long as it is more than the thickness of the ring but not so deep as to part the top off the piston (done that  ). The surface finish on the lower face where the ring seals against the piston is important.

On to the rings. These are bored to give a thickness of around 1.5mm, I don’t think that it is that critical, and the inside of the bore is polished to reduce the chance of breaking when fitting (either abrasive paper wrapped round a wooden dowel or a round abrasive stone (never a finger!!!!    )). The outside is then finished to the size of the plug gauge as the rings will grow when heat treated. The rings are then parted off to a nominal 1.5mm with the same parting tool as before, deburring as much as possible on the way. I split the rings with a pair of piano (music) wire rated side cutters.

The mandrel of the heat treatment fixture is turned half a ring shorter than the number of rings I want to treat. I could do some clever sums to determine the diameter of the mandrel but I just turn it down until the ring blank fits with a gap of around 3mm (1/8”). The fixture is then clamped up and heated to a moderate red heat with a Camping Gaz plumbers type blow lamp and then allowed to cool.

Next up I make the sizing ring for the skimming fixture. This is bored so that the plug gauge and a 2 thou feeler gauge are a close fit. It needs to be wide enough to cover the planned number of rings and the land on the fixture. The skimming fixture is an offcut of mild steel bar bigger than the finished size of the rings and long enough for the planned amount of rings. I wouldn’t recommend trying more than four at once.  It is first turned down to a close fit in the sizing ring and then the front part is turned approx. 0.1 mm (4 thou) smaller. This is checked with the sizing ring and a 4 thou feeler gauge. It is then drilled and tapped for the clamping screw ( 6mm or ¼” is what I have used). The first 6mm or ¼” is then parted off and drilled clearance for the clamping screw.

[Roger B]

Part 2

The rings are then carefully deburred and the gap is set to 6 thou in the sizing ring. The required number of rings are loaded into the sizing ring and the ring is slid onto the fixture. The clamping plate is then screwed in place and the sizing ring is slid off. If you have a DRO you can probably set the cut directly but I tend to touch the tool on the sizing land and then go in 0.2mm and skim the rings. I then polish the rings with abrasive cloth and check that they fit in the liner. Finally I polish the faces of the rings, checking that they fit in the grooves with some clearance, what is some? At this size I have around 0.1mm (4 thou) clearance. Again I don’t think it is that critical. The important point is the finish of the sealing surface between the ring and the piston.

The final step is to put it all together and start the engine. I spent months trying to get my 3cc vertical engine running, it did run with an O ring but it wasn’t until I developed the procedure above it would run with a cast iron ring. The subsequent engines have all started first time, set the timing 10-20° BTDC, crank it with a drill and open the needle valve until it starts. Running will probably be poor until the carb and timing are fine-tuned but it runs.

I hope this helps but as ever please ask for clarification. The piston ring is not stand alone it is part of a system.

[Zephyrin]

Hi Brian,
on the setup described on my previous post, the rings are held fully closed on a mandrel for the final turning, which obviously is truly circular, the crucial point for rings as you mention it.
And if the ring is too large you have the possibility to remount it on the mandrel to remove a fraction of an hair, if the mandrel is still in the lathe chuck !
nb : the mandrel I describe is a crude version of the turning setup shown on the Roger's tutorial...
you may notice on the rings on the picture I show that inner and outer dia are not concentric : a very slight eccentricity of the mandrel (+0.1 mm under the cut), allows to reduce the thickness at this place and to distribute the compression of the ring on the cylinder wall more evenly on the whole perimeter, I m not sure of the effectiveness of this refinement, but I simply saw that the rings of the old Micron 10cm3 engines were like that !

[Brian Rupnow]

Thank you, Roger and Zephyrin.----Brian

[Craig DeShong]

My no heat method of making piston rings has been covered in my numerous past projects.  My first rings were made for my live steam locomotive.  Cast iron rings in a cast iron cylinder.  The engine has been running for over 20 years without problems.  I used my method on my current Otto Langen project, and as you can see in the video I just posted in that project thread, they seal quite well.

[Brian Rupnow]

There is no joy in Mudville today. I put a new piston and two rings I had purchased from Debolt  in the newest vertical i.c. engine-2021 and had no compression. I squirted some oil down the sparkplug hole and had lots of compression. The new rings are not sealing. I drove the engine for 2 hours with an electric motor, hoping that the rings would "bed in" and seal, but it's not happening.----So---I have one trick left. That is to build a fixture as per Mr. Chaddock, and turn the o.d. of the rings to be perfectly cylindrical. That is exactly why I bought the toolpost grinder. I doubt my ability to take a 0.0005" depth of cut with a lathe tool, but with the grinder it may be possible.

[Brian Rupnow]

Here we have Mr. Chaddocks fixture for turning the outer diameter of piston rings to make them truly round. The sleeve in my hand has an internal bore of 1.00". The rings are sprung into the sleeve, the sleeve with rings in it slides over the part held in the chuck, then the washer on the very end is tightened down with the bolt, which squeezes all of the rings together. The o.d. of the 1/4" thick washer is slightly smaller than 1", so after the bolt is snugged up tight, the sleeve is slid off over the washer, leaving the rings ready to be turned or ground to a perfect diameter. I am going to use my toolpost grinder to do that.

[Brian Rupnow]

I'm not absolutely sure of what I'm doing here, but that has never stopped me before.  I wanted to have a good "visual" on the rings to know when I had reached a point of perfect roundness, so I coated everything with blue layout dye before I went to bed last night. This morning I mounted my toolpost grinder and taking .002" depth of cut, I made repeated passes over the rings until all the blue was gone. At this point, knowing my rings are perfectly round, I will dismount them from the grinding jig and try them into a cylinder.

[Laurentic]

Brian - may I respectfully suggest covering the lathe bed before using your grinder, all that grit that will come out the grinding process will not be good for it!!

Chris