Date: Mon, 9 Dec 1996 08:02:09 -0800 (PST) From: Mike West Subject: Balancing Pistons etc. In this episode, like the last episode and very likely the one after that, it is raining in Mudville. In the last article I told you how to find the long rod, short rod, etc. but not why. And then we balanced them. This time it's "long piston, short piston" and I'll even explain what this is all about, tho it's covered in one of Bob Hoovers Sermons. We are talking about "compression ratios" and head spacing and cancelling out tolerances. Our man of the hour still has no tools, lives in a walk-up flat in Chicago down by the "L-tracks". The Noble Screw comes to our rescue this time. The "Gudgeon Pin Ploy" is not not so successful over the span of three pistons. We want the distance from the gudgeon pin to the top of the piston head. What I did with mine is set the heads on the table, run the gudgeon pin out a tad and use the depth gauge tail on my calipers. Do it carefully and a couple times and you have a pretty good reading. It must be square and just clear the top of the pin. Once again, it doesn't matter if my gauge is off a little as long as it's off the same every time. Accuracy only comes into play when I "mike" something here and have you build me a mating part which you then measure with your tool, somewhere else. It's like calibrating watches, as long as we both have the same time it does not matter what time it is. Our hero has no calipers, he found a piece of flat plate and has a 1/4-20 thread tapped in it. Total cost: a "smile and a thank you" He "procures" a screw of that persuasion and a nut to go with it. Probably the same place he had the hole tapped. It's long enough, 1-3/4" would do for me. Did I say it was a hex head? Let's look at a micrometer for a minute. Inside the fancy handle with the scroll-work on the barrel is a 1/4-40 screw. If it's metric it's a 6x0.5 screw. Whether you're outraged or amazed, our man has an accurate depth gauge. He runs the nut up the screw and then screws it into the plate. Set the plate on the top of the head and then runs it down till it just kisses the gudgeon pin. Slide it back and forth across the top. Lock the nut against the screw. The next one is a little shorter, so he writes "0" on the first piston and -4 on the second. Why minus 4? The amazing screw is a hex head. Has six points and six flats. A 1/4-20 has a pitch of .050", meaning it moves 50 thou. per revolution. The .050" divided by six flats is .0083" per flat, yes? And a point to a flat will be .004 etc. Soon you have all four gauged and the price of equipment was within budget. My own readings were 1.130 on two and a 1.132 and a 1.135" For the metric crowd, the pitch was 1.0 so each turn is 1.0mm. 1.0mm divided by 6 is .166 etc. Now we still have to balance them with each other. Piece of cake right? It really is just a matter of getting them all the same weight and they're aluminum, you can carve it with a knife. Our hero has no balance beam again, so he "rummages". He comes up with a wooden coat hanger with a wire x-bar that says "Savoy Hotel, Racine Wisconsin". He goes down to the building supers shop and borrows a line level that will fit over the wire piece and a screw-eye that the hook on the hanger fits into. Back in his room he screws the eye into the door jam overhead and hangs those matching shoe-laces on the ends and the line level on the wire cross-piece. Shifts the laces until this "beam" is exactly level. Maybe weights one end with a piece of tin foil to keep the laces spread. Is this guy picky or what? If you want real first cabin, you put a couple hooks of wire on the ends of the laces. Same wire, same lgth equal same weight. Within a few minutes he has them gauged by weight and knows the lightest one. Keeping the lightest one on end of the beam, he scrapes the little balance weights inside, around the piston skirt all the way around and keeps checking against the lightest one on the "beam". It won't take long to get them all the same weight. The "reason" for the long , short pistons and rods is so you can put the short piston with the long rod etc. and effectively "cancel out" any tolerance stack-ups on those parts. Your head spacing will now be as close as you can come without a lathe, and should be within 1 or 2 thousandths of each other. Bob Hoover covers it in some depth in a Sermon called "Compression Ratio". Anyway, the 1 or 2 mils I can live with. It beats a possible 10 mil stack difference if I randomly put the long head on the long rod. Then if I put the short with the short, I'd have another 7-10 mils or a total of say 20 between the two pistons. Thousandths, mils, and .001" are all the same unit if I'm confusing you. You can see where the .020" diff. is getting pretty ragged on CR. Then. . . if you want even finer divisions on your "depth gauge", get a Knurled Allen head screw. Mind boggling, ain't it? west, still pushin'