Ironhead math problem, piston rings

[GA_Ironhead]

This?

http://xlforum.net/forums/showpost.p...9&postcount=26

[Iron Mike]

Pehaps that is the one, I seem to remember a bit more detail. It would not surprise me if he had made a few attempts at getting the point of that conversation across. Either way this link gets the point across. Different scenerios need different applications to arrive at the same point.

[RicThompson]

Originally Posted by thefrenchowl

Ric,

With maths like yours on post #5:

""900 standard bore is 3.0, 900 standard ring would be 3.005""

I wouldn't want to buy any engine that's been near you!!!!!!!!!!!!!!

Patrick

Well Patrick I'm so upset. How about you educate me. A 900 HD Sportster has a standard bore of 3 inches. Tell me about rings, what do you know? Please tell me how a 3.00 ring in a 3.00 bore with the proper end gap would seal.

[ryder rick]

Rings are not measured outside the bore. They are machined at a significantly larger diameter then cut so they have tension on the bore.

Many buy oversized rings and hand fit down, so it is easier to butt the ring ends.

Production rings are almost always on the small side (large gap) to prevent butting.

A small Ring end gap is not critical unless you are racing in a limited class against someone with a tighter gap, (splitting hairs) and can't run a total seal ring because of rules. Not sure a total seal ring works well on the street or in a dirty motor like an Ironhead.

Rings seal because compression pushes them down and pressure gets behind the ring and forces it out against the cylinder. This is why many race motor pistons are "gas ported". However I don't believe that works on the street.

[RicThompson]

Originally Posted by ryder rick

Rings are not measured outside the bore. They are machined at a significantly larger diameter then cut so they have tension on the bore.

Many buy oversized rings and hand fit down, so it is easier to butt the ring ends.

Production rings are almost always on the small side (large gap) to prevent butting.

A small Ring end gap is not critical unless you are racing in a limited class against someone with a tighter gap, (splitting hairs) and can't run a total seal ring because of rules. Not sure a total seal ring works well on the street or in a dirty motor like an Ironhead.

Rings seal because compression pushes them down and pressure gets behind the ring and forces it out against the cylinder. This is why many race motor pistons are "gas ported". However I don't believe that works on the street.

Nailed it.

[thefrenchowl]

What Rick said, Ric…

You cannot measure a ring outside the bore, ie a 3.005 diam ring could not possibly seal a 3" bore.

Or look at it another way: machine on a lathe a 3"005 ring, then cut the gap: When you fit it in a 3" cylinder, maybe three points on the ring will actually touch the cylinder… ie NO SEAL despite the spring effect…

A 3" ring is a bigger diameter ring, then cut, then compressed, then machined round at 3", then gapped… that's the only way you'll get more than 3 points touching the bore.

Same logic when you want to redo seriously your drum brakes linings on a lathe: insert shims between shoes and cam, then skim linings on lathe to your drum ID then remove shims and as soon as you touch the lever, all the points of your linings will touch the drum = maximum brake surface instantly…

Patrick

[RicThompson]

I'm not on here to argue with folks. I ask a simple question. It's been answered thank you.

[mrmom9r]

I'm the one who sold you the pistons . The information you seek is:

Hastings 6457-040 cast iron

Hastings 2C 6457-040 moly faced cast iron

[RicThompson]

Originally Posted by mrmom9r

I'm the one who sold you the pistons . The information you seek is:

Hastings 6457-040 cast iron

Hastings 2C 6457-040 moly faced cast iron

Thank you. The pistons and new cylinders with oil returns lowered are in local shop getting fitted. I ordered rings from BMI Karts they will arrive today. Not sure of brand or even if they have 3 piece oil ring. They were cheap so if there not Hastings rings they will go on the shelf and I'll order the ones you have identified.

[thefrenchowl]

All is well Ric, sounds like you know what you're doin'...

Patrick