Norton Atlas Pistons

So retracing previous posts concerning the possibility that my pistons were " flip top" pistons, as it turns out after removing the oil rings, the horizontal slot connecting the two large slots is NOT present so I am assuming what I have is an engineering revision that addressed the problem. That said, what is so strange about what I do have is that my bore measures perfectly 2.875 from top to bottom but my pistons measure 2.853 with no perceptible scuffing on the sides. How could this be?

Scuffing normally happens on the front and back as the pistons rock forwards and backwards as they go up and down. You only get scuffing on the sides when you get 4 corner seizure due to the piston overheating.

I'm slowly learning. Haha. I should have read the technical data in the manual prior to asking this previous question. According to the data the piston measurement is taken at the skirt. The measurement I provided of 2.853 was taken at the very top and was a uniform measurement 360 degrees around. My skirt measurement is actually 2.870 and my bore is somewhere between 2. 873 and 2.874. My skirt measurement was taken perpendicular to the gudgeon pin. When I measure parallel to the pin the measurement is 2.864. The tech data doesn't tell you which way to measure so the question becomes in what direction do you measure?
Mine would appear to be slightly oval at the skirt but round at the top.The pistons that I have are stamped AE so I assume this precedes the JCC Hepolites. Interesting when I recently asked a vendor here in the States to measure a new standard JCC piston packaged as Emgo, the result was 2.854 at the top and 2.868 at the skirt. Relative to the skirt measurement, I don't know in what direction it was taken. That said if the new JCC's happened to be the same measurement all of the way around the skirt, then it would appear that I would be starting off with a .007 clearance which seems too loose. I was shooting for .0045. Input is most welcome. Thanks in advance

Pistons, and bores, wear more in the front to back direction, due to the centrifugal forces tending to make the pistons fly in the direction the crank rotates, i.e. perpendicular to the gudgeon pin.

Slick

texasSlick said:

Pistons, and bores, wear more in the front to back direction, due to the centrifugal forces tending to make the pistons fly in the direction the crank rotates, i.e. perpendicular to the gudgeon pin.

Slick

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Slick,
That's good news for me in that I'm within spec at plus or minus .004 clearance. s
So it's not uncommon to be slightly oval at the skirt?

Not uncommon for bores with wear. I would expect new pistons to be perfectly round.

Slick

I thought all modern (post-1930) pistons were out of round and tapered.

As TT said, normal piston design is for the top and ring land area to be circular with the skirt tapering out to a larger oval shape at the bottom. All the stock and aftermarket pistons for Atlas and Commandos are like that.

Ken

lcrken said:

As TT said, normal piston design is for the top and ring land area to be circular with the skirt tapering out to a larger oval shape at the bottom. All the stock and aftermarket pistons for Atlas and Commandos are like that.

Ken

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Well that’s exactly what I have. What is the purpose of that design?

Well, in a nutshell, the top of the piston is exposed to combustion and gets quite hot, but the skirts stay much lower temperature, partly because they have a lot of surface area in contact with the cylinder walls to conduct heat away. That means the upper part of the piston has a lot more expansion that the lower, so at operating temperature a lot of the taper goes away, while the piston expands to a better fit in the cylinder. The ovality plays a similar role, but in a bit different way. That's a little oversimplified, but you get the idea. I'm out of time at the moment, but I'll try to send you a good link for a detailed explanation tomorrow.

Ken

I thought all pistons were oval !
I know an old trick for turning pistons in a lathe was to fit a stud and two nuts inside the piston in the gudgeon pin hole and undo the nuts force the piston out a couple of thou before turning

I believe the ovality of the skirt is to do with the gudgeon pin and boss. There is a lot more metal in the boss and supporting area and as such the expansion characteristic of the pistons skirt are not even.

Also the sides of the skirt don’t really have much of a role, hence most modern pistons have more of a slipper design to them, which has the added benefit of enabling a lighter piston design.

So, if the piston sides expands more, and if tight piston to bore clearance is not required on the sides, it is obviously sensible to have more clearance there.

Thats my layman’s understanding, I’m sure there are other factors too.

lcrken said:

Well, in a nutshell, the top of the piston is exposed to combustion and gets quite hot, but the skirts stay much lower temperature, partly because they have a lot of surface area in contact with the cylinder walls to conduct heat away. That means the upper part of the piston has a lot more expansion that the lower, so at operating temperature a lot of the taper goes away, while the piston expands to a better fit in the cylinder. The ovality plays a similar role, but in a bit different way. That's a little oversimplified, but you get the idea. I'm out of time at the moment, but I'll try to send you a good link for a detailed explanation tomorrow.

Ken

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Makes sense Ken. Thanks

I haven't found any really comprehensive piston design documents on-line (probably not searching enough) but JE pistons has a pretty good explanation of piston skirt design here:


But if you really want to get into the details, you need to go to a serious engine design textbook, or even better, hunt up some of the SAE technical papers on the subject.

Ken

lcrken said:

I haven't found any really comprehensive piston design documents on-line (probably not searching enough) but JE pistons has a pretty good explanation of piston skirt design here:

Explained: Symmetrical, Asymmetrical, and Full-Round Piston Skirt Design

There are several different styles of piston skirts, all of which play a vital role in how the engine operates. Learn the difference and make the right call!

info.jepistons.com

But if you really want to get into the details, you need to go to a serious engine design textbook, or even better, hunt up some of the SAE technical papers on the subject.

Ken

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Ok thanks guys