Atlas Cylinder Bore

The bore on my Atlas cylinder measures out a consistent 2. 874 which is great. Here' the question. The barrel sat around for an undetermined about of time and had some minor rust in one side only so I Flex Honed it with a 73MM hone. It cleaned up really well with almost no dimensional changes to the bore however right at the very beginning of the bottom of the bore, say within a half an inch off the bottom there is some slight pitting in one area the size of a quarter and at the top with 1/2 of the top, again two small individual pits just bigger that head of a pin. Do I re-bore or will it suffice. Does the Commando piston rise to the very top of the barrel with a stock base gasket? Looks like the bottom of the first compression ring is roughly 1/4" from the top of the piston. I've got a brand new set of standard Hastings rings. Thanks

Post a picture and remove all doubt, I would ignore it from your description but pictures would be mor definitive.

kommando said:

Post a picture and remove all doubt, I would ignore it from your description but pictures would be mor definitive.

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Ok will do

jms said:

Ok will do

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First picture is at the top about 1/2" down. Second picture is 1/2" up from the bottom. I think with some continued honing most of it except the two pits at the top should clean up

jms said:

First picture is at the top about 1/2" down. Second picture is 1/2" up from the bottom. I think with some continued honing most of it except the two pits at the top should clean up

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Oh and what does a "B" stamped on the gasket flange on the top of the barrel mean

I looked at the bore pics. I'd have that set of barrels bored .040 over to clean them up. That said listen to kommando's advice.

I have no idea what the B stands for.

Is it possible to assemble and measure where the rings would be at the bottom of the stroke? I had rusted barrels that I had to have bored, was lucky to find some .040 over NOS 7.5:1 pistons for my N15CS on eBay, low price ($100?) and perfect. .040 did the trick. Here it is after having the spigots milled off, as I had one good later non-spigot head and one buggered exhaust thread 'proper' one.

Not an expert!

jms, there is no need to hone away the pitted area, as it's not affecting piston ring seal. Just make sure there are no corrosion "peaks" which may cause scuffing of the piston skirt.
Corrosion products have different elemental compositions than the original grey iron, and may have a substantial hardness.

- Knut

gpzkat said:

Is it possible to assemble and measure where the rings would be at the bottom of the stroke? I had rusted barrels that I had to have bored, was lucky to find some .040 over NOS 7.5:1 pistons for my N15CS on eBay, low price ($100?) and perfect. .040 did the trick. Here it is after having the spigots milled off, as I had one good later non-spigot head and one buggered exhaust thread 'proper' one.

Not an expert!

Click to expand...

Thanks looking at the video it would appear that the first compression ring would certainly be touching the cylinder walls 1/2" down.

mdt-son said:

jms, there is no need to hone away the pitted area, as it's not affecting piston ring seal. Just make sure there are no corrosion "peaks" which may cause scuffing of the piston skirt.
Corrosion products have different elemental compositions than the original grey iron, and may have a substantial hardness.

- Knut

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Ok Thanks. There's none of that

Cylinder block grade A = 2.8746/2.8750 and grade B = 2.8750/2.8754

marinatlas said:

Cylinder block grade A = 2.8746/2.8750 and grade B = 2.8750/2.8754

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Norton could not bore as accurately as the piston makers could turn on the piston OD (including taper etc), so the piston makers made 2 slightly different sizes of piston ie grades. Norton then measured the machined bores and selected the piston that gave the best fit. A workshop borer can hold better tolerances than a factory borer so no need for Graded aftermarket pistons.

When I worked for a company making shell bearings one customer wanted 7 grades of bearing thickness with a difference of 3 micron between each grade. They measured their cranks off the line and selected the grade of bearing to fit. Nothing to do with keeping oil pressure, they stated it was to reduce vibration by controlling the clearance between crank and bearing.

1 micron = .00003937 inches

Ignore the bottom corrosion, rings will never go near it, the top will cause a very small bit of extra oil to be burnt if the ring goes over it and leaves some oil in there. But if you are only going to do a few thousand miles a year then a few extra drops of oil is nothing.

If that top rust corrosion feature is actually 1/2 inch below the deck, at a minimum your top ring will go over it every stroke on a stock hepolite piston. I went out and checked on the hepolite pistons I have in a box and a set of iron barrels on the bench.

Looks like the consensus is put it back together as is.

gpzkat said:

Is it possible to assemble and measure where the rings would be at the bottom of the stroke? I had rusted barrels that I had to have bored, was lucky to find some .040 over NOS 7.5:1 pistons for my N15CS on eBay, low price ($100?) and perfect. .040 did the trick. Here it is after having the spigots milled off, as I had one good later non-spigot head and one buggered exhaust thread 'proper' one.

Not an expert!

Click to expand...

Be an expert. I think you've earned it.

The rings are higher up in the stroke than the crater field at the bottom of the bore. It would bother me leaving any of what jms has shown like it is. I might be a touch anal about keeping things clean though.

kommando said:

Ignore the bottom corrosion, rings will never go near it, the top will cause a very small bit of extra oil to be burnt if the ring goes over it and leaves some oil in there. But if you are only going to do a few thousand miles a year then a few extra drops of oil is nothing.

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Why should the cylinder with corrosion at the top cause more oil to be burnt? The oil scraper is supposed to stop oil contamination of the combustion chamber, and at .5" below upper deck level, the oil scraper will do its work unaffected. A possible effect of the corrosion is a tiny increase of gas leak at TDC. However, maximum pressure occurs after approx. 20 degree after TDC, and bearing in mind the second compression ring doesn't hit the corroded area, there should be no noticeable effect of the corroded area, provided rings and bore mate as they should.

- Knut

mdt-son said:

Why should the cylinder with corrosion at the top cause more oil to be burnt? The oil scraper is supposed to stop oil contamination of the combustion chamber, and at .5" below upper deck level, the oil scraper will do its work unaffected. A possible effect of the corrosion is a tiny increase of gas leak at TDC. However, maximum pressure occurs after approx. 20 degree after TDC, and bearing in mind the second compression ring doesn't hit the corroded area, there should be no noticeable effect of the corroded area, provided rings and bore mate as they should.

- Knut

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Its called an Oil Control Ring not an Oil Elimination Ring for a reason.


Oil control rings are located in the bottom piston groove and their function is to control the supply of oil to cylinder wall to lubricate the piston skirt– and keep excess oil out of the combustion chamber.

Coming back to this as I amclose to assembling now, when measuring the piston to determine the bore clearance, i have the following questions.
1. Measure the piston at the very bottom of the skirt perpendicular to the wrist pin?
2. Measure the bore how far down from the top of the barrel?

Thanks