.040 Pistons in .020 bore -is it even possible?

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Dmftoy1

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Ok, just finished the complete disassembly of my 68 Norton and the "prior owner" mysteries continue. I don't have the proper machinist tools to do perfect measurements but I appear to be about .005 shy of what would be a .020 over bore, but the Pistons are GPM and marked 74 which to my research would be .040 pistons. . .

So am I smoking dope? Is this even possible?

Here's a pic of the piston

https://www.dropbox.com/s/ly898c32j21zqoe/Photo Oct 31, 6 24 59 PM.jpg?dl=0
 
Just to be clear, are you saying the pistons measure within a few thou of 73.5 mm at the bottom of the skirt, or are you referring to some other measurement?

The "74" stamped on the pistons might lead one to expect them to be for a 74 mm bore, but if it measures close to 73.5 mm at the skirt, it for sure isn't a +.040" piston.

Just trying to make sure we're all talking about the same sort of measurements.

Ken
 
I was measuring across the crown of the piston and seemed to get 73.75mm. Should I be measuring across base Of skirt? My assumption about it being a .040 piston is on andover's site the show 74mm piston for .040 and these are stamped 74. (Sorry if this is a stupid question)
 
Hi Dave

Unless you have proper measuring gear it is very hard to get accurate measurements.

I you have an engine rebuilder nearby I would take the barrel and Pistons to them and get them accurately measured and then you can decide your next step.

Cheers and good luck...
Dean.
 
Yeah, that's the next step. Really need to get myself a set of proper mic's too
 
Dmftoy1 said:
I was measuring across the crown of the piston and seemed to get 73.75mm. Should I be measuring across base Of skirt? My assumption about it being a .040 piston is on andover's site the show 74mm piston for .040 and these are stamped 74. (Sorry if this is a stupid question)
Click to expand...

You definitely have a set of 74 mm(73 mm +.040") pistons.

As Rohan pointed out, pistons are always measured at the bottom of the skirt, usually 1/2" or so above the lowest point, perpendicular to the pin, when measuring for clearance purposes.

The crown of the piston is always smaller than the measurement at the bottom of the skirt, to allow for the extra expansion of the crown, which runs much hotter than the skirt. The diameter at the top land (the crown) is typically something like .005" smaller per inch of bore, depending on the piston material. For a 74 mm bore, that means the top would measure something like .015" smaller than the skirt. The actual amount varies between manufacturers, and depends on detail piston design.

I've just gone out in the shop and measured the diameters of a stock, slightly used, Hepolite Powermax 73 mm +.040" piston, and it is 73.54 mm at the top land and 73.88 mm at the skirt. If new, the skirt diameter would be slightly larger, as the skirt always collapses slightly in use.

Ken
 
Yeah, measuring as you describe I was seeing 73.90mm. I spent some time this morning ordering a decent micrometer that'll measure to 3" and some bore gauges, this won't be the last bike I tear down (knock on wood) and it's nice to know roughly where I'm at before going to the machinist. They can use their starett and mitoyubo's to get exact.
 
Just thought I'd close this one out as I just got everything back from the machinist. It looks like what happened is that the prior rebuild was done by a machinist that didn't know WTH he was doing. The GPM 74mm pistons were fitted but with almost zero clearance. In fitting the new pistons (also 74mm) we had to bore it out .004 (thousandths of an inch) in order to get the new pistons to have proper cylinder/wall clearance.

Guess that explains the serious scuffing on the sides of the pistons . . :)

Anyway - don't feel like I'm going insane anymore. :)

have a good one,
Dave
 
Glad to hear you got it all sorted out without too much grief.

Ken
 
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