After the excellent comment from Ownerus, I put myself back to school on measuring IDs with the telescoping gauge. Instead of the digital calipers I was using, I went over to my micrometer.
Before I start, the drawings say to machine the cylinder to 0.875" Inside Diameter and the piston to 0.873. The rings are to predictably take up the extra .002"
I measured both ends of the cylinder three times. For the combustion chamber end of the piston, the first two times I measured .8827 and .8829, a difference of "2 tenths" - .0002". On my third measurement, I got .8837. Even averaged that's 0.8831. That gives me some confidence the combustion chamber is close to that 0.8830. On the opposite end of the cylinder I measured 0.8827, 0.8812 and 0.8817; more spread and average of 0.8819". This says the cylinder is a little narrower at the end the piston mounts from, and I suspect it would be better for any taper to be a little narrower at the combustion chamber end.
The problem was that when I measured across the piston rings, I got 0.896. Which explains why I couldn't get that piston into a hole .014 smaller than the piston.
After the comments on that post, I had decided to remove the piston rings and deepen the slots because the rings stood much too high above the 0.879 piston. The rings didn't feel right and I thought the reason was that I made the grooves too shallow. Plus, the rings felt too tight in the grooves, hardly movable at all. Here are the results from trying to remove the rings today.
You can see the portions of the shattered piston rings next to the piston.
For an engine like this that will be run a few times at relatively gentle settings, a polymer (Viton) piston ring is probably a viable choice. That means making a new piston to fit the too-big cylinder, and that's probably the best thing to do. Remember, the drawing had them .002 apart in size. By that, I should make a new piston 0.880 or 0.881 in diameter.
I've got to say I'm a bit tired of working on these two parts, and want to get on with the engine, but it's worth trying to get this right now. There's a handful of these critical parts in the engine - the valves in their valve cages are the biggest ones - and the rest is pretty routine. I hope.
Cast iron rings?
ReplyDeleteYou got it.
DeleteYeah, they don't have much elasticity in them. I've only run across them a few times, but seem to remember always having to buy a few extras because they were somewhat fragile.
DeleteSounds like you're making progress in the measurements. The only "little" things I ever machined were for itty-bitty millimeter wave things.
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ReplyDeleteI suppose they don't make piston ring compressors that go down that small? Before I bought one, I just used an 89¢ gear clamp. Standard procedure is to compress the rings with the clamp, then tap it into the cylinder. Usually the top edge of the cylinder has a slight taper to facilitate this.
ReplyDeleteRings normally stand much higher above the surface when relaxed. Where they are is unimportant. A dial caliper makes a sufficient depth gauge to make sure the ring land is deeper than the radial ring thickness.
IMO- What the drawings say, and what may work fine, might be two different things. In the real world, on manual machines, a .002" clearance between two parts is a tight tolerance. We are talking a bit under the thickness of a human hair.
ReplyDeleteMy guess is that your .003 to .004 clearance will work just fine if the rings fit. If you are worried about piston slap lightly knurl the skirt.
I have been playing with tenths a bit, when grinding an accurate spindle taper - but for this sort of thing rounding them to the nearest .001 seems to make sense.
Disclaimer- I am not a machinist nor do I play one except in my hobby shop, and as long as the arrow on the crank points the right way I can do good work so there lill'l mikey, you arrogant piece of crap.
To Malatrope Nothing I could find would compress the rings. I used a hose clamp, and a black tie wrap (zip tie), both with and without a piece of aluminum cut out of a soda can that completely covered the rings. Nothing could get the rings compressed. The reason was that my slots are so tight that the rings don't go to the bottom of their grooves. I measured the grooves' depth and they seem good. If both grooves were just a little wider, I've read .003 or .004, that would probably do it.
ReplyDeleteThis is also for Raven. To get another piston to this point isn't hard so much as I wonder if it's worth it - as you say. I'm talking about making a piston maybe .001 or .002 bigger, and starting from a 1-1/4" stock, going down to 0.880 it will be slow going.
My gut feel is to order another couple of these cast iron rings. It isn't the cheapest alternative but might well be the most conductive to maintaining sanity.
Do you suppose steel rings would work? I know nothing about engines, but turning some steel rings out of tube or bar, then parting them off and cutting a gap could be done.
ReplyDeleteI don't know. I suspect that in a low compression, fairly low temperature engine like this everything is less critical than in engines demanding more performance; more power per cubic inch.
DeleteSiG, how low a temperature range? Because I'm thinking heat might be a method to get your rings set properly. Make the rings out of a material which when heated reduces the diameter of the rings. It would have to be a high enough temperature so the rings don't lose their 'set' during normal operating temp.
DeleteI don't have numbers. I've never even seen one of these in real life, so no real idea of what they're like. I assumed they got to maybe 150 or 200F, but based on very little.
DeleteIt might not even be as low as I think because it doesn't have any sort of cooling system except some oil in the fuel and the air around it. I'm just thinking that cars run around 220 for long periods while this will run for a minute or two.
Likely the heat will be well above that. Piston faces run about 575F in a normal engine (not a racing engine).
DeleteRaven's comment about knurling is a very good one. My father used to purposely knurl the thrust faces of pistons in order to help spread oil around in racing engines.
ReplyDeleteI also like your concept of using viton O-rings. They may not last very long, but it's not as though you are going to use this engine to power your lawn mower.
I'd make your ring lands a little wider, as you suggested. They may have ended up with a little bit of a taper, also. Did you check the axial clearance by putting a ring in "upside down" (outer ring surface first) and using a feeler gauge? Please don't be offended if these are obvious things you thought of instantly.
I'd make your ring lands a little wider, as you suggested. They may have ended up with a little bit of a taper, also. Did you check the axial clearance by putting a ring in "upside down" (outer ring surface first) and using a feeler gauge? Please don't be offended if these are obvious things you thought of instantly.
DeleteI think I've said it before, but it's safe to assume I know nothing. The amount of time I've spent working on engines, as opposed to radios, is a tiny proportion. I'm coming from almost zero useful background, and the last time I had an engine apart was about 1970.
That said, last night someone gave me the tip to try putting the ring into the piston's groove upside down to see if it fits. The grooves are tapered, tighter at the bottom than the top. I can hold the piston and piece of ring in front of the monitor and see light between the ring and the bottom.
I'm seriously considering trying to make some rings from a bar of steel like Raven suggests. Maybe 12L14 Ledloy? Not sure what kind of steel is appropriate.
Rings are supposed to be high-carbon spring steel, I think (I'm not too familiar with steel, but I don't think you need my advice on titanium). I've never heard of anybody trying to make them. But as you make your decisions, remember that you aren't building it to last 100,000 miles. That opens up a universe of possibilities.
DeleteI love to work with aerospace aluminum alloys (7xxx series). They are easy to machine and very little less strong than steel. You might consider trying them out for your next projects – you might never use steel again. McMaster-Carr has a wide selection of stock.
When you make your second piston, check the ring land taper before you take it off the lathe. If you have a problem, you can go in and take a little more off the bottom sides...using a really weeny teeny tiny tool bit, of course! You need a jewelers lathe and a stereo microscope for this!
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ReplyDelete(I am "unknown" above. I am not a fan of this commenting system, grrr. At least before coffee.)
ReplyDeleteWhat is the cylinder material? I was wondering if bronze would work for rings.
ReplyDeleteIt's 1018 Steel.
DeleteI spent the better part of an hour trying to remove the slightly smaller diameter at the mouth of the cylinder. According to the dial on the lathe I should have taken off .008 or .009 (moving the cutter .0005 per pass), but the diameter doesn't even measure .001 larger. I'm guessing I work hardened it and now the cylinder just bends out of the way of the cutter.
What sort of tool? Many carbide inserts don't like tiny cuts- a carefully hand honed HSS might work better, or even a lap for such a small amount- but if the indicated cut is .008 you should be seeing something- grimace- like a huge overcut as soon as it grabs....
ReplyDeleteThat is a small bore- does the tool have enough relief on the edge so the lower part of the tool is not hitting the bore?
Taking off .001 to true a tapered bore is going to be difficult with a boring tool, I think. Both from a centering issue, both radially and axially, and from a cutting issue, because when that cutter breaks out of the cut midway down the bore it is going to leave a ridge- unless you cut the entire bore length to a larger diameter. .001 is one third the thickness of a human hair- you could just hope the rings take it up.
It's a carbide insert tool and it looks to have OK geometry. I put it inside the bore and pulled it toward the wall until it just touched. It took some tiny shavings off the bore (only ran it on the first inch). I moved the bit .0005 deeper, ran it again and remeasured. There were no shavings and the bore measured the same. This repeated a few times. There was very little evidence it cut anything.
DeleteI think I still have some 1/4" HSS blanks that I could make a cutter from.
If you have a carbide insert for aluminum , that might work- they usually have a ground edge rather than a "honed" edge. The honed edge is slightly dull, in close up, to resist heat and wear. Tool catalogs, like Travers, have sections dealing with this info.
ReplyDeleteTo sharpen carbide I've used both the "DMT Dia-Sharp Diamond Mini-Hone" diamond files, and a "straight style diamond grinding wheel" from ebay. Each about $25. I filled in the 1" diameter hole in the wheel with two-part polyurethane casting compound, then bored the smaller hole I wanted.
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