A new attempt at making piston rings

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OscarII

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This post is not so much about rings, but it does show something very important. I am always amazed at how much a difference in gas tank height affects an engine. When I built this engine a few years ago, I had made a gas tank to fit underneath the cylinder, with a 3/32" diameter check valve in the discharge. This year when I went to start it, I couldn't get it to draw fuel up from the tank. Okay, when these engines set around on the shelf for years, check valves have a tendency to freeze up. That wasn't a big deal, I just grabbed a spare gas tank I had and mounted it on a block of wood. This allowed me to start the engine and see that it actually did run. After putting the cast iron rings in, the engine would run good for four or five minutes and then die, as if it were running out of fuel. When everything else is set at "optimum" and the engine dies for no good reason, I always suspect the gas tank height. In this picture you see a 1.6" tall aluminum spacer under the tank. That fixed it!! Now the engine will run until I turn it off with the switch in the electrical system.
As another old fart engineer I would like to add by 2 bobs worth to the discussion. . I have now idea of Brian's experiences except he produces some damn fine models and his posts have helped me in the past. To the problem of fuel feed and raising the tank it sound to me to be about basic fluid flow. This type of problem is found all through the world of pumps and pipes. An easy fix is to increase the fuel pipe ID and that of the check valve. I have had these problems in water supply to boiler pumps. Each small elevation of the fuel tank has the equivalent of enlarging the pipe ID. Every fitting or bend in the fuel line has the equivalent of lowering the pipe ID. Apologies to those who already know all this, Peter
 

Badhippie

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Brian
I missed your birthday sorry about that.
Happy Birthday Late. Lol my Dad always told me I would be late for my own funeral lol which by the way I don’t think that is a bad thing lol.
Thanks
Tom
 

Brian Rupnow

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The curious thing about being old, is that unless my arthritis is killing me, I feel like a young man. I would say that overall, I feel more like forty or fifty, but with 25 years more experience. I always used to think that when I got old, I would only be able to do old man things. it really isn't like that.---Brian
 

Charles Lamont

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...] To the problem of fuel feed and raising the tank it sound to me to be about basic fluid flow. [...] Each small elevation of the fuel tank has the equivalent of enlarging the pipe ID. Every fitting or bend in the fuel line has the equivalent of lowering the pipe ID. [...
Well, yes OK, but In the case of fuel flow to a model engine, the flow velocity is so tiny that bends, fittings, and pipe resistance are going to be insignificant. This is why you can get away with banjo fittings in fuel lines. Without a float chamber, and with any reasonable pipework, the flow restriction is overwhelmingly at the jet needle. Given a particular setting of the needle, what affects fuel flow is (a) the ability of the carburettor to create a vacuum, and (b) the head, i.e. the distance the fuel surface is above or below the jet. As the fuel level drops, the available vacuum becomes insufficient to lift fuel to the jet.
 

Gordon

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I have a question about ring grooves in the piston. Trimble says the width of the groove should be .0015 wider than the ring width. I have a note that says the groove depth should be .003 deeper than the ring thickness. I cannot seem to find that in the Trimble article even though I thought that is where I got that dimension. I seem to keep having a problem with the installed ring bottoming out and jamming when installed in the cylinder. I suspect that at least part of the problem is that the bottom of the groove does not have a perfectly square corner. Any idea where that dimension came from and is it correct? Would another few thousands make a big difference. It would seem like the width would be much more important.

Gordon
 

cds4byu

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Bob Shores suggests to make rings from the same bit of cast iron as the piston. It starts with the diameter .003 oversize, and ring grooves cut .003 "deeper than specified". I don't think that is describing the depth of the ring groove to be .003 less than the ID of the compressed ring, but rather recognizing that you are working on an oversize workpiece.

Trimble does not give a specification for the difference between the ID of the ring and the OD of the ring groove.

Walshaw suggests that the clearance behind the rings (I interpret this to be the radial clearance between the installed ring and the OD of the ring groove) should be .004 for compression rings and .010 for oil control rings.

Personally, I don't think a few thousandths extra is a huge problem. But I would be looking at (1) the corners of the ring groove (make sure your grooving tool has sharp corners) and (2) the internal corners of the ring (make sure there are chamfers on the ring).

Carl
 

petertha

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On page 7 (Apr/May-1989) Trimble says his objective groove clearance gap was 0.0005 - 0.001". This is the vertical distance viewing the piston from the side with the ring resting on one side of the ring gap.

Terry.M has provided a sketch for several of his successful builds. I believe he follows a similar recipe each time - Trimble ring math for the ring design & ring fixture and 0.001" vertical ring groove clearance. Post #453 has the sketch & Trimble equations all nicely laid out. 270 Offy
 
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Brian Rupnow

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Your numbers are right. 0.0015" side clearance and 0.003" greater groove depth than the rings radial thickness. If you don't have a specially ground tool to cut the grooves in the piston, the grooves will have a radius in the bottom. You have to do a really good "sand and chamfer" operation on the inside corners of the ring or it will hang up on those radii.
 

Gordon

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On page 7 (Apr/May-1989) Trimble says his objective groove clearance gap was 0.0005 - 0.001". This is the vertical distance viewing the piston from the side with the ring resting on one side of the ring gap.

Terry.M has provided a sketch for several of his successful builds. I believe he follows a similar recipe each time - Trimble ring math for the ring design & fixture and 0.001" vertical ring groove clearance. Post #453 has the sketch & Trimble equations all nicely laid out. 270 Offy
That is interesting. He is showing .0065 clearance behind the ring which is over twice what I have been using,
 

mayhugh1

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That is interesting. He is showing .0065 clearance behind the ring which is over twice what I have been using,
I do that to compensate for the non-zero radius of my grooving tool so the ring doesn't bottom out on a radius left behind in the groove. I can't see how the actual distance between the back of the ring and the facing wall of the piston is critical so long as there's space for compressed gas to flow between them and force the ring against the wall of the cylinder during combustion. - Terry
 

stevehuckss396

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Unless the tool is perfectly on center you are not cutting as deep as you think. I cut my grooves .005 to compensate.
 

Gordon

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It appears that I have been overly concerned over the groove depth. There is no such thing as a perfectly square corner and if the chamfer is not big enough there is interference.

I am presently working on an engine which was marginal running on CI rings and to proceed I put in an O ring and the engine ran well except that the only O ring I had was Butyl and after a short time of running it hardened up and was not sealing. I always intended to go back and get the CI rings right and that is what I am doing now.
 

LorenOtto

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The curious thing about being old, is that unless my arthritis is killing me, I feel like a young man. I would say that overall, I feel more like forty or fifty, but with 25 years more experience. I always used to think that when I got old, I would only be able to do old man things. it really isn't like that.---Brian
At 78 I can second that.
 

petertha

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I think piston ring grooves & retaining ring grooves are the few times I used my blade micrometer. It cant accurately measure corner radius at these widths of course, but they are handy for similar applications. I initially tried to take the groove measurement using conventional mic & little slivers of gage block in the groove but it was pretty fiddly. Mind you, I am also not very good at 3 wire method measuring threads and I have a soft spot for tools.

I will dig up my notes but I seem to recall the commercial (O.S.-56) piston/ring I used as a guide for my radial was pretty close to the gaps being discussed here. The inner corners of the ring had no appreciable chamfer so I think they are similarly relying on the groove depth to not cause interference problems.
 
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