The piston is an intricate little part, with both exterior and interior features cut into it.
That makes it difficult to rework should it be a little big since the work holding tool (vise or lathe chuck) has to grab a finished piece which runs the risk of marring it. The piston outside diameter is supposed to be 0.873" and the cylinder inside 0.875". A difference of .002" (two thousandths of an inch) is too loose for getting engine compression and the solution for that in pistons everywhere (your car, your lawnmower...) is to use piston rings - split metal rings that in this drawing fit in those two grooves on the left end of the left figure.
In order to test the fit of the piston, I decided to turn a handle onto it and not machine those internal features just yet. I thought I'd get the piston to fit and then do the interior work. That turned out to be a bad decision, for a reason I'll get to in a while.
The drawings come with the size needed and a link to a small business the designer knew would provide those. I bought two and prepared the piston according to the drawing.
This is the piston on the lathe. The tool cutting the groove isn't cutting, it's just posing there. On the left, in the jaws of the chuck, you can see the handle. The thin white layer is an old business card cut and wrapped around the handle so the paper protects it a little.
Getting the rings on a piston like this is nerve wracking. It involved thicker oil (like SAE 30 car engine oil) pressing the piston onto the ring while it was on my mill table (that is: big, heavy, iron) and working the ring onto the piston until it could slide along the outside of the piston, then sliding it further by hand until it snaps into the groove. The other ring is slipped on from the bottom end entirely by hand. (The handle, the smaller diameter in this picture, is the bottom end)
This is where I've been stuck for a while. I either make the piston a little smaller or the cylinder a little bigger. Remember I mentioned testing everything with the piston rings was a bad decision? That's because I don't think I can get those rings off the piston without ruining them and buying another pair ($15) so I can't machine the piston to any degree. That means I need to make the cylinder larger diameter, which means I need to measure the cylinder to see how much metal I need to take off. If I take off too much metal, the cylinder becomes scrap.
The problem is that until this point, I was afraid the cylinder was too big rather than too small.
So I'm putting myself back through measurement school. More precisely, I'm repeatedly telling myself to do measurement practice, but other things keep interrupting that. I'll be the first to say I don't know what I'm doing, which means I'll be learning lots on this engine.
Years ago when I worked in a shop I learned the hard way about the difference between radius and diameter. Fortunately I was working on a small project of my own and not A paying customers job. It cost me a small piece of aluminum stock and a couple of hours.
ReplyDeleteMeasure twice, cut once. You can't uncut...
It wasn't that. It's that I'm using telescoping gauges to measure the inside diameter and I must not have the technique quite down.
DeleteI squeeze the two plungers to minimum diameter, put the gauge at the depth I want to measure, unscrew the lock until they snap to full diameter, then re-lock them and rock the telescoping gauge to get it out of the cylinder.
Measuring a 7/8" ID to four decimal places that's 2" deep isn't quite the same as putting your caliper jaws on a block that fits in the jaws.
Try inserting the gauge, letting it open up, and then slide to the desired part of the bore. Rock it around to get a feel for when it feels tightest (smallest ID) and then gently lock it while the handle is centered axially. They take a lot of feel- and then the mic has to have some feel too when measuring the gauge. You correction tool is going to be based on how much stock needs to come off- a few tenths might mean a lap rather than a boring bar.
DeleteCaveat- I am not a machinist, I just read about them and their problems incessantly.......
When all else fails, go grab a beer and your guitar and take a break.
Yeah, this is a critical measurement. I'd almost rather do it with a lap and some abrasives. Except for the fear of getting abrasive on the lathe ways and damaging them.
DeleteI was playing with numbers on the calculator to see if I could visualize the measurement look off. It's a cosine error so tilting the gauge 10 degrees makes it look around 3 mils larger than it is. I think I'd see 10 degrees, though.
Think about putting the jaws of calipers around the end of a bar. You can ordinarily see if they're not in the same plane and not aligned well. Plus, you can watch the display and know that the you'll get the minimum size when the calipers are aligned right. That's the feedback I'm missing.
When all else fails, go grab a beer and your guitar and take a break The universal fix. Also do that when it's successfully done.
The only engines this small that I've worked on have been the little engines for my R/C models. All of them were two-stroke compression/glow plug ignition, and none of the pistons had rings.
ReplyDeleteHow loose are the rings on the piston? I'm thinking they should be easier to get on, but then my knowledge of itty-bitty engines is pretty limited.
They're loose enough to slide in their slots when pushed but don't slide around on their own. I don't know if that's right or not.
DeleteWhen I used to rebuild car engines (because I couldn't afford a mechanic) the specs for ring clearance axially was usually around 0.008" - 0.012", and were checked with a feeler gauge. They should slide easily to rotate around the piston.
DeleteThe gap is larger than you might expect because they will gradually load up with carbon and become stuck, at which point they don't work anymore.
They should spring out a bit from the groove. In full-size engines the gap is about.001" per inch of cylinder bore.
DeleteSooo... when are you gonna build a live-steam locomotive? He-he-he...
ReplyDeleteMakes sense to fit the piston into the cylinder before adding the rings. One of those 20-20 hindsight things.
Hope it works out for you.
No live steam. I don't have a big enough yard. Anything that would fit ends up being the size of the Lionel O gauge trains.
DeleteEver see a "49er" motor bike? 49cc engines? Those are street legal transportation here, and capable of doing useful work. That's the goal. It will take a few projects to get ready for it, I think.
You do know there are 'Live Steam' clubs that have huge tracks already to play on, right?
DeleteStill, making your own street bike sounds like fun. Since the powerplant can also be used for go-carts and other fun stuff.
Ever try soft jaws in your lathe chuck? I can see from the photo that the jaws in the chuck for the photo are one piece, but you may have another set for chucking rings from the inside.
ReplyDeleteSoft jaws are custom cut with almost exactly the diameter of the piece you want to hold, center up with extreme precision, and do not mar the part if you use a softer material than the work.
A full description is too much to do here, but the adaptations needed to make soft jaws work on one piece jaws might be beyond the scope or value to the project.
I have some that I made a few years ago, but for this, I was thinking of cutting a strip of aluminum out of a soda can and wrapping the piston or handle in that. Like the business card but tougher.
DeleteIt surprised me how well an internal lap worked when I sized a nominal 5/8" hole intended to be a cork-popping fit onto the nose of a shopsmith:
ReplyDeletehttps://www.gadgetbuilder.com/Lapping.html
Good link - thanks for that.
DeleteIt might be useful to find out how Cox mass produced engines as small as .020 cubic inch. There is quite a long and extensive history of this product, and new old stock is still being sold.
ReplyDeleteso... putting the handle in the lathe and using strips of sandpaper / crocus cloth won't worK?
ReplyDeleteFor what?
DeletePrecision ID measuring is always a fiddly affair. I've used pistol gages, ID mics, telescoping gages, all of them take touch and feel to get right. My only advice is to find a master in the same size range and practice measuring to get the technique down.
ReplyDeleteWhat is the cylinder material?
ReplyDeleteIt could be tricky to get it lined up on the lathe again to re-bore- a lap will follow the bore. Just not sure how much material a lap will remove, or if there is risk of abrasive getting embedded and causing a problem.
A double check could be made by turning a test "piston" to the correct size, then measuring the OD. Then subject it from the oversized real piston diameter to arrive at a correction number.
It's mild steel, 1018. It is tricky to line up on the lathe again, but I've been having success with using a dial indicator and marks on the lathe jaws and cylinder to put the same spot on the same jaw.
DeleteIf you and gap your rings first you will be able to tell if you have a straight bore. Since you have more time in the cylinder I would use that as your base diameter.
ReplyDeleteI cast the pistons for my smallblock so I made a 5C drawbar setup to pull the skirt to a shoulder so I could turn the diameter, head and ring grooves in one operation using a dummy wrist pin. If you make something similar, it would allow you to check the piston diameter to the cylinder without removing it from the fixture.
You really can’t beat a pin gage set for measuring internal diameters. They do get a bit expensive for the larger sizes like you’re dealing with.
ReplyDeleteSince I’m far better at measuring external diameters than internal diameters, I have been known to make a series of test cylinders with a variety of external diameters - 0.872, 0.873, 0.874, 0.875, etc - and then use those to measure the internal diameter. Basically I am making my own set of pin gages.
Definitely easier to turn an OD to size than an ID!
DeleteAs I recall, the piston (without rings) went into the cylinder fairly easily, but had a just perceptible side to side movement.
I went and looked at the piston and rings again, and I think they are too tight. It takes a lot to make them slide in their ring. I think that's first step is to take those out. It might mean buying a new set - or buying a cast iron bar and making two new rings.
In working with car engines there is no problem removing piston rings, they should not break. Take the rings off and put the rings one at a time in the bore and with a feeler gage measure the gap at the opening where the ring ends has a gap. The ring supplier can give you the necessary number. Secondly the ring should freely rotate in the piston slot. You should have a measurement for how wide the ring gap on the piston should be. I hope this helps, this sounds like a fun project....Tex.
ReplyDeleteThanks for the tips, Tex. I should be able to do those things in the next day or so.
DeleteGo get one of the smaller pin gage sets like the 0.061-0.250 minus set. Very reasonable prices on eBay. Then you can play with them sizing different IDs around the shop. You will begin to see how much site to side movement there is when there is .001 oversized versus .002 versus .003.
ReplyDeleteYou will soon wonder how you ever survived without these little miracles of precision.
I’m an idiot. They are called “gage pins”, not “pin gages”.
Delete$50 on Amazon and $53 on eBay. The 190 piece 0.061”-0.250” set is called M1. The M1 set comes in two versions, Plus and Minus. The Plus M1 set has each gage pin sized 0.0002” over its nominal size, while the Minus set has each gage pin sized 0.0002” undersize. Yes, we are talking only two tenths over or under.
All of my gage pin sets are the Minus flavor.
If you want a cheaper version to play with, get the 50 piece M0 set which is 0.011”-0.060”. You can find a set for $25-$30 on eBay or Amazon. You will be amazed at how incredibly small a 0.011” gage pin is once you hold one in your hands.
to help concentrecticity of the part in the lathe jaws, instead of a cut up bit of business card to prevent galling the part, try some 0.0025" phosphor bronze shim between the chuck jaws and the part being turned. gets better stability than the paper which will compress the fibers under load.
ReplyDeleteTo enlarge the cylinder bore, consider using an automobile brake cylinder hone. They come in two and three stone versions. The brake parts are normally cast iron, sometimes steel.
ReplyDeleteWhen using those telescoping bore gauges, it helps if you can slide a cylindrical item inside the bore to give you a perpendicular surface to rest the t-head against. a piece of pvc pipe may work for you. Has to have a properly cut/finished end, and can be a loose fit, just hold it to the wall. Make sure the same parts of the gauge head are touching both sides of the pipe to ensure perpendicularity.
Entirely possible that the rings don't have enough end gap to fit in the cylinder. You may have to file them to get the proper gap. Do you know what that spec is?
There are expanding pliers designed for piston rings. They work with the gap to spread them. Better than using your fingernails, normally.
Telescope gauges can be quite accurate if used correctly. Trying to hold everything perpendicular will not be very accurate. Drop the gauge into th bore (with the clamp loose), hold the handle slightly off axis from the bore, maybe 5°-10°. Slide the gauge slightly back and forth axially This allows the gauge to find center in the bore. Then carefully snug the clamp on the telescope gauge just enough to overcome the spring pressure of the gauge while still holding the gauge handle off axis. Then sweep the gauge handle past the bore centerline and in the same plane as the T part of the gauge. As the telescoping part passes thru the plane perpendicular to the bore it will find the minimum point and if properly snugged will stay there. After the gauge is removed,it can be carefully tightened a bit more so as not to be influenced by the micrometer. Do this 2-3 times when high accuracy is required. The result should be consistent if done right and you can measure within a couple of tenths (.0001)"
ReplyDelete^^What a great explanation of using a bore gauge!
ReplyDelete