modok wrote: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?
modok wrote:??? 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.