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Thread: How to turn a piston?
03-06-2006, 07:08 PM #1
What is the correct method for turning a piston? Is there any way to do it with the rod still attached? They are not full floaters and don't want to risk breaking a piston removind the wrist pins.
I was thinking maybe I could make a small plate that the rod bolts would go through. I would put a 3rd hole in it for my tail stock center then grab the crown with my 3 jaw. This might work for the skirt area but when I try to do the ring pack area I was thinking maybe I would press a pin through the 3rd hole mentioned above, grab that with my 3 jaw and use a steady rest to go around the circumferance? This does not sound too practical. I suppose I could perhaps JB Weld an aluminum block to the top of the crown and maybe center drill that.
Sorry to ramble and I may not be making any sense. Any constructive ideas are appreciated. Any "Don't do its" please don't respond.
03-06-2006, 07:27 PM #2
a piston is elliptical not round...you'll have to factor that into the equation
03-06-2006, 07:57 PM #3
Have a look at the inside diameter of the skirt, sometimes there is a small champher there or a turned portion for locating the piston. I have never done this, but think you would need to locate on a plug of some sort, not using inside chuck jaws (distortion).
I bought an "unfinished" piston years ago for my Matchless - it was located like this. But you will have to think of some way of pulling the piston back on this location - not so easy with a rod in place.
The piston I am talking about was actually finished on a special grinder (I think), it has several different diameters (top ring land is smaller etc), plus cam ground skirt.
BTW, the original piston I took out was "wire wound" an AMC thing, a groove filled with wire, wound and anchored under tension for expansion control.
03-06-2006, 08:48 PM #4
Like the Cat said. Pistons are not round. This could make things a little challenging. [img]smile.gif[/img] You will also notice that it is likely tapered small at the top. Aluminum pistons are made so that when they are at operating temp they are the right shape. They tend to grow more at the top where there is more heat.
I know your not looking to just buy new ones. If your real set on machining these I would think about using a boring head in a mill. I would think that keeping the assembly stationary would be more healthy.
03-07-2006, 12:50 AM #5
I think it depends on what the piston is for..... which I don't think you mentioned
newish engines very likely have all sorts of funny optimized shapes....
Older ones aren't sophisticated... if its for an old oil field engine, etc, its round.....
One dumb question.... Pistons are normally the right size when assembled..... and they don't grow that much.... usually they wear down, and if they don't the cylinder does.
So why does it need turned?
I'm not kvetching at you about doing it, just asking, as it may affect the correct answer.
03-07-2006, 01:27 AM #6
03-07-2006, 02:02 AM #7Older ones aren't sophisticated... if its for an old oil field engine, etc, its round.....
Pistons for IC engines are also ground not turned and often have micro groove finishes to retain oil. Unless you are looking to generate a piston for 1907 Gnome rotary aircraft engine just bite the bullet and buy one.
03-07-2006, 03:13 AM #8
I distinctly recall the AE pistons for mid 90s GM 3800s were single point turned and not ground.
I think at miniumum you'll need a 4-jaw setup or a long soft-jaw setup that's machined to size....just trusting concentricity to a 3 jaw is a big mistake in my book.
Normally the wrist pin is a shrink fit of sorts on the rod's small end...but IIRC it doesn't take more than about 300F to make the removal quite easy.
Another "what are you trying to do?" not that I want to throw darts at your reasoning, but need to know more about what area needs to be worked on.
Be very careful of modifying the top ring land...it is really one of the two most sensitive area of the whole thing in my thinking. (the other being any valve pocket undercuts in the face which require major cooling efforts because of the sharp edges that invite localized heat buildup)
03-07-2006, 07:57 AM #9
I worked at Curtiss-Wright back in the eighties. One of the jobs they ran was pistons for an obsolete military airplane from the WWII era. The pistons were finish turned on a Mazak lathe, so apparently were round.
03-07-2006, 08:56 AM #10
When regrooving old piston rod assemblies, we used to just centerpunch a dimple on the bottom of the rod cap, just enough for a good sharp tailcenter to find it. Of course, we would use a 4 jaw to center it up nice, and cut a spacer out of a piece of tubing to fit between the piston and the chuck face. That way, you could apply some pressure to the tailstock to keep it engaged in the dimple.
03-07-2006, 09:02 AM #11
03-07-2006, 10:31 AM #12
I've read an article about making pistons on a CNC lathe that was making the motions to compensate back and forth with the tool and produce an elliptical piston. Wish I could remember when and what magazine. All I mean is that it's done with single point methods as well as grinding.
03-07-2006, 11:41 AM #13
If you want to cut the squish band or something like that on an existing piston using the lathe I've used two methods.
You can use clamp blocks that catch a ring groove and take light cuts. I used a piece of square stock, bored a hole in it that is a close fit on the piston, then bolted plates to the top that were machined to catch the ring groove.
Or make a faux connecting rod. Use a piece of round or square stock, bore a hole in it, run a circular rod through it that has been machined to be a good fit on the piston pin. On the back end of the rod put some threads so you can pull the piston down firmly against the block, then grab the block in the chuck. This method (it doesn't hurt to have the block machined to catch any machined registers in the bottom of the skirt, or lightly recessed so the piston fits down into it snugly) will let you machine the skirt as well as the crown.
03-07-2006, 12:30 PM #14
03-07-2006, 12:38 PM #15
Cool. Some excellent replies. Thank you. I will read and try to digest this information. They are .030 over factory pistons for a Ford 2.3 turbo motor. I got them at a JY for a 2.5 turbo stroker experiment I am working on, not knowing that they were oversize. I paid $90 for them and waited past the 30 day warranty before measuring them. I did notice that they are larger in the skirt area than the ring pack area. I'll measure them some more and see if I can ascertain whether or not they are elliptical.
03-07-2006, 02:28 PM #16you will see that even 90 years ago the pistions were tapered and eliptical, often times with slots and steel struts inside of alloy pistons to keep the piston round when hot.
But most likely a piston for a simple hit and miss (not what he has, now that we see what he wants) would nearly surely be round as round can be. Those guys were not sophisticated, the 1920's are pretty new and modern by comparison.
Tapered pistons are easy enough to do.... and surely were done way back when, by some folks.
03-07-2006, 02:55 PM #17
I also question the straight turning of an automotive piston. An old friend had an automotive machine shop with engine rebuilding machine tools dating to the 1920's and 1930's. One of the things they had was a "Tobin Arp" (if I remember the maker's name right) cam grinder specifically for grinding oversized pistons to final size. At the time, I asked the automotive machinist there what the "cam grinding" was all about. He explained that the cam grinding process created a varying diameter ont he body of the pistons so that they "came into round" when the piston and cylinder walls stabilized at running temperatures. The thinking was that the piston body had a cross-section of varying thickness, being considerably heavier where the wrist pin went thru and lighter at 90 degrees to the wrist pin areas. They used to get oversized/semi finished pistons and turn them to near final size, then finish them in the cam grinding machine. That shop had an older geared head Reed & Prentice engine lathe that the pistons were rough-turned in. That shop used to do a lot of work on a wide variety of engines ranging frm old industrial and heavy equipment engines to cars and motorcycles. They had a variety of old engine rebuilding machine tools including a van Norman crankshaft grinder which could handle diesel engine crankshafts. That shop used to make a lot of parts from semi-finished blanks or frm scratch.
I do not know if automotive engine rebuilding machine shops still cam grind pistons. However, I think I would try to find an automotive machine shop which did re-size pistons and see what they do nowadays. A modern-day turbocharged light truck engine is a high performance engine compared to the type of engines my old friend's automotive machine shop was working on.
03-07-2006, 08:15 PM #18
Out of curiosity, how did you end up shortening the connecting rods?
03-07-2006, 08:45 PM #19
Sounds to me like the simple answer is to bore your block oversize to suit the pistons. That way you get a fresh bore, and it is easy for the shop to finish the bores (hone) to match the pistons. Turning the pistons to suit the bore is mickey mouse. Don't do it. There, I said it! [img]smile.gif[/img]
Also, new pistons probably have a coating (tin, lead?) to help prevent scuffing when new. Some are anodised.
Piston technology is unbelievably clever stuff! If you heat up a piston in hot water it will not fit into the cylinder. Typically the aluminium expands twice as much as cast iron.
The obvious answer of increasing the clearance to allow for expansion doesn't work because of the noise when the engine is cold (loose fit).
And yet a modern compensating piston can be fitted with less clearance than a cast iron piston! It could be as low as 0.0002" per inch
The "compensating" part relies on the top of the piston being much hotter than the skirt, and it is possible to design a piston whose clearance increases as the load increases.
As for piston shapes - some of those used are also pretty astounding, "versine" is one - sort of peanut shaped. It was popular because it is easy to generate - a flat follower around the outside of an eccentrically mounted circular disc.
No doubt things are different now in the CNC age, my info comes from an excellent book "Repco Engine Service Manual", 1977.
The AE pistons may well have been single point turned (diamond?), but still probably cam controlled.
03-07-2006, 09:10 PM #20
Joe mentioned resizing pistons, This was done to make the piston bigger, not smaller. It was like knurling on the skirt at the major diameter of the piston. Hone the cylinder, resize the piston new rings and SELL the car.
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