Ironhead math problem, piston rings

[RicThompson]

LOL, I don't feel so bad now almost 500 people have viewed this thread and no one has taken a stab at the math question.

[ryder rick]

Originally Posted by RicThompson

I have new S&S 4 and 5/8 stroker pistons and no rings.

It'll run better if you put some rings in it!

Even a one piece oil ring is better than none.

[needspeed]

Originally Posted by RicThompson

LOL, I don't feel so bad now almost 500 people have viewed this thread and no one has taken a stab at the math question.

I thought the math question was : How does Dennis Kirk get the numbers given in post one for the ring sets he sells? The numbers given are for 1000 sportsters. I thought the answer as to how they are computed was pretty clear in post #4.

[RicThompson]

Originally Posted by ryder rick

It'll run better if you put some rings in it!

Even a one piece oil ring is better than none.

Actually no, a one piece oil ring on these short forged pistons would not likely control oil very well at all. A cast one piece oil ring will work with the tight tolerances of a cast piston but set up loose for a forged piston not so well.

[ryder rick]

Ya, I can see how a one piece would not work well on a short piston.
I noticed in other reading that 1 piece rings don't always work well even with a long piston.

[needspeed]

Originally Posted by RicThompson

…. A cast one piece oil ring will work with the tight tolerances of a cast piston but set up loose for a forged piston not so well.

This is somewhat of a misconception. Take two pistons with two different rates of expansion. The one with the higher rate of expansion needs more clearance when the engine is cold, but if the clearances are figured right both will have about the same running clearance when hot at operating temperature. So the loose one is no longer loose.

True, shorter skirts don't help with ring seal.

Anyway, I agree, the 3 piece oil ring is better.

[RicThompson]

Originally Posted by needspeed

This is somewhat of a misconception. Take two pistons with two different rates of expansion. The one with the higher rate of expansion needs more clearance when the engine is cold, but if the clearances are figured right both will have about the same running clearance when hot at operating temperature. So the loose one is no longer loose.

True, shorter skirts don't help with ring seal.

Anyway, I agree, the 3 piece oil ring is better.

I have to disagree with you. A cast piston does not thermally expand significantly during normal operation. They can be fit for very close tolerance. I am only concerned with high performance applications utilizing long strokes, short skirts, high heat, piston rock and slap. Not the application for cast pistons nor single cast iron oil rings.

[Iron Mike]

There was an interesting post made by Dr Dick about the difference between cold set up clearances and actual running clearance. it was a very interesting read and it spelled it out pretty clearly. I will try to find it instead of trying to explain through my own recollection.

[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

[Iron Mike]

Originally Posted by Iron Mike

There was an interesting post made by Dr Dick about the difference between cold set up clearances and actual running clearance. it was a very interesting read and it spelled it out pretty clearly. I will try to find it instead of trying to explain through my own recollection.

Well I cant find it. Basically regarding the proper BENCH clearances used, taking in account expansion rates of given materials, heat applied and derived from power output ect ect, arrives at desired predetermined RUNNING clearances. Maintaining the proper warmup and and initial desired state of tune holds these RUNNING clearances. Straying from one of these variables throws all of it out the window. It does not matter what material you are starting with, it is what clearances you want to maintain in operating conditions.