Piston-to-head clearance w/ piston rock?

[jt1]

Interesting.

 

I would never have imagined that the clearance at the pin end sides of the pistons would be enough to allow that much rock. It would be cool to talk to someone onvolved in the piston design and see what's the reasoning behind that.

 

I'm glad the pin clearance is OK, that had me worried. Sorry for the false alarm.

 

John

[dr_hunt]

If your rod side to side clearance is .004 then it's way too tight. Should be .016 to .024. What kind of rods do you have?

 

Pistons are made the way you describe, piston to wall clearance is usually mearued at the wrist pin centerline. What brand of pistons are you using? If you ever try JE, you'll probably never use anything else. Try measuring the dimensions and balancing with a set of Ross or Lunati's and then do JE's you'll see what I mean.

[dmyntti]

Pistons are not round on the top they are actually slightly oval. This is because of the geometry of the piston in the pin area causes the piston to grow differently along its axis. Forged pistons have more growth then cast or other types of pistons. Your pistons are probably small on the top compared to the bottom due to the fact that that is where the heat would be concentrated and would grow more there. By having the bottom of the piston have a larger OD you can get tighter clearance at the bottom of the piston and still allow the top of the piston enough room to grow with heat.

 

My experience with piston design is with large reciprocationg engines and compressors and not automotive type but these are my thoughts.

[pparaska]

Great replies all!

 

I've learned a lot lately about piston design, pushrod length methods, etc.

 

I understood the barrel shape of the piston design for material growth due to heating, but the numbers seemed like they showed more variance when I was measuring them at 230AM . I was surprised they were cone shaped top to bottom, instead of barrel shaped. But I guess with such a short skirt, that's to be expected.

 

These are Probe SRS forged pistons.

 

Oh, I was wrong. The builder included a build book.

Here are the side clearances he measured:

rod side clearance #1 .023

#2 .021

#3 .020

#4 .021

(the crank end play is 0.004")

They are Cat Power I-beam rods, with ARP bolts. I've heard some bad things about Cat stuff lately, but the builder was reasonably impressed with them. They certainly look plenty hefty, but who knows about what the void content, treatment, etc. Next time I build an engine for myself, it will have US material and machined rods from Callies, Manley, Carillo, etc. I'm tired of making the Chicoms rich.