2618 Piston Street Use Longevity

[4vpc]

I don't see anything unusual about any of it.
If double checking the clearance first thing I will do is check clearance at the other end of the skirt.
The clearance at the top of the skirt is MORe important.
The bottom clearance may be more or less just depending on the severity of the taper and also the length of the skirt, and also how flexible the piston is.
The clearance at the top of the skirt is a more meaningful measurement IMO, but the bottom has become the standard point to measure probably because... it is easier?

An iron piston in an iron cylinder. An aluminum piston in an aluminum cylinder. Which one needs more clearance?

??? I don't know actually, but thinking about it you have to consider a lot of factors.
I would propose that if we assume the pistons are very conservative designs with full skirts, BOTTOM of the skirt is near the same temp as the cylinder in both cases then they both can run very tight.
But the clearance at the TOp of the skirt, has to have something to do with hot much hotter the piston will get than the cylinder, which is very much application specific.

Open deck aluminum block engines can run zero clearance, and siamese open deck blocks benefit from oval top pistons.
Cool stuff indeed! Different pistons for different holes.

Now you're thinking; good news
I thought bore size is always taken from the widest point of the piston (skirt), usually about 1" up from the bottom on say a 3" long piston, no?
Do you mean aluminum piston in iron bore? Either way it does need careful consideration.
There must be a ratio of crown diameter to skirt diameter dependent maybe on length of piston, I bet someone has got it written down in a book somewhere. I wonder if the ratio changes if it's a modified turbo (hotter) engine or not?

You've also headed me off at the pass, preempted my next suggestion. I was going to recommend putting a piston in the oven and measuring it when hot, but as you point out it will be hotter at the top, cooler at the skirt so that wouldn't tell us the full story. It would show us some degree of ovality at the crown though. Maybe if you were quick with the mic you could get it out the oven, take your measurements, put the skirt into cold water, check temp with infrared gun then measure again. Very unscientific and fiddly, but then that's why i'm suggesting look at those who have already done the calculations for us and take some hints from there.....

Flexibility I never considered, are you talking about a difference in design or material or both? I suspect cast is more rigid. And that if a piston does flex then needs more bore clearance?

Thanks all for your suggestions, you're getting me thinking.

[MadBill]

Ovality on the top land is to accommodate piston rock without leaving unnecessary crevice volume at the front and rear.

[4vpc]

You don't think it will become round when it's at running temp Bill?

[Cougar5.0]

Interesting discussion. I ran these Probe SRS 2618 pistons about a half mil tighter than recommended in a SBF 306 without consideration of the 9 psi non-intercooled (but meth/H2O inj) Kenne Bell boost (~0.003" IIRC) . Someone had to do it . I measured about 1" up from bottom of skirt as recommended, the comment about measuring maybe closer to the top of the skirt is interesting based on the wear pattern (about 10k miles, many trips to drag strip). Pic isn't that great, but the wear was from the middle of the skirt to the top of the skirt.

2618 piston.JPG

[modok]

Bill's probably right, I thought you meant it was oval ring lands the other direction, now I wonder!

Some pistons the bottom half will become more round under running conditions, but I am not sure if that is true of the slipper skirt forged pistons we are mainly discussing.

[MadBill]

You don't think it will become round when it's at running temp Bill?

No, the top land, together with the crown, essentially forms a solid disc which despite any local temperature variations, will expand pretty uniformly in all directions. The need for lateral 'rocking clearance' can be seen in the top edge wear patterns on some loosely-fitted pistons.

From the standpoint of the piston supplier, more top ring land height and clearance reduces the chance of customer complaints re breaking/scuffing, but for the builder reducing crevice volume to the absolute minimum pays surprisingly large power dividends. (math available on request)

[Matt80]

Good info. So what is the more durable design on street the narrow x forging or a wide round skirt?

"I" would stay away from an X forging for the street. I don't see any benefit. Race engine yes on the x forging. I am only one person and one opinion. Others may have a different opinion and that is fine.

Still have doubts, what would you use guys on an high boost engine 50% street and 50% track & mountain races?

And also, isn't the round design going to make the piston to rock less, thus decreasing the wear/increasing life versus X or Box?

[4vpc]

You don't think it will become round when it's at running temp Bill?

No, the top land, together with the crown, essentially forms a solid disc which despite any local temperature variations, will expand pretty uniformly in all directions. The need for lateral 'rocking clearance' can be seen in the top edge wear patterns on some loosely-fitted pistons.

From the standpoint of the piston supplier, more top ring land height and clearance reduces the chance of customer complaints re breaking/scuffing, but for the builder reducing crevice volume to the absolute minimum pays surprisingly large power dividends. (math available on request)

You're right, I was wondering if maybe there was a difference of expansion if the crown was maybe shaped to suit a pent roof high CR chamber as compared to a flat top or simple dish, but possibly not.
I did a test last night, I put a cast full skirt piston in the oven and took it up to a max of 250'c (475f) stopping off a few times at lower temps and measuring it all the way at the bottom and top, as you say it stayed elliptical around the crown.
The strange thing however was it never went round at the bottom either, it grew equally on both axis and stayed oval.

[digger]

And also, isn't the round design going to make the piston to rock less, thus decreasing the wear/increasing life versus X or Box?

the real contact area is only part of the skirt such that most of the extra skirt in a full skirt piston wont touch anyway. it seem the clearance and piston height would be the biggest inhibitors of rock

pg 8 and 9 shows the skirt bearing surface

https://cdm.ing.unimo.it/files/progetto ... mbH%29.pdf

[4vpc]

And also, isn't the round design going to make the piston to rock less, thus decreasing the wear/increasing life versus X or Box?

the real contact area is only part of the skirt such that most of the extra skirt in a full skirt piston wont touch anyway. it seem the clearance and piston height would be the biggest inhibitors of rock

pg 8 and 9 shows the skirt bearing surface

https://cdm.ing.unimo.it/files/progetto ... mbH%29.pdf

+Pin offset too.
I don't really understand why some run zero offset, there isn't any advantage.

[modok]

Some say zero offset will have less friction, which is probably true although difference might be trivial in a lot of cases.

If you could heat the top of the piston higher than the bottom I think then the bottom will become closer to round, but not entirely. it may pivot about the pin bosses, being stiffer. It's probably never supposed to be round entirely as some oval is needed to account for the force on it.
Remember that "four corner scuffing" is the tell-tale of having overheated (overexpanded) the piston or not having enough clearance for how hot it was.
usually skirt collapsed also, so which happened first? I think first it ran out of clearance and then it collapsed from getting too hot. Maybe, chicken or egg?

[Matt80]

But maybe Round design is going to make the bottom skirt closer to round as temps go up?

[Mark O'Neal]

This is a fun read........

[4vpc]

This is a fun read........

Unlike your unhelpful sarcastic comments.

[Mark O'Neal]

I guess that's true, if you only read one post.