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Factors that affect piston rock

4547 Views 8 Replies 8 Participants Last post by  Deleted member 4436
Is there is an acceptable spec on piston rock?

I'm in the process of assembling my sbc 400(6" rods, so fairly short skirts). About to measure piston to deck height and was wondering what is an acceptable number for piston rock and what all factors affect it.

Any normal ball park figures?
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if it hits the valves, or head, it is a bit much...this is a hard question..unless you can heat the parts up..trying heating a piston in the oven, minus rings..see how your combo handles it..lots of fun games to play, while learning at the same time..remember, its the difference in temps that counts, not the "as-running" temp...70 degree block, 200-300 degree piston...

i'd say anything over .010 is ridiculous..cold..
I have run then as loose as .012 and had no issues. It did worry me running that loose but I ran it a full season and the pistons looked perfect.
I asked a similar question a couple of months ago and what I was told is this. An acceptable piston rock and piston to head clearance is based off of piston material/compression surface design, rod manufacturer, bore diameter, valve angle, and combustion chamber design. What I took away from that was, there isn't a defining answer on what is ok.
The more important issue is the piston to cylinder head distance you have at full rock on both sides of the piston. Piston to bore clearance, skirt length,and rod ratio affect piston rock. Longer stroke motors tend to have alot of rock at BDC.
I've ran the engine before and, piston to head/valve clearance is/was good(same combination of parts). The reason for teardown was oil burning, which I think i've determined was a combination of bad valve guides and rocker studs leaking into the intake ports. Piston to deck height just wasn't a measurement I had recorded from the initial build.
Reher Morrisons book is great for these kind of questions when building a motor.
It's all about skirt design! Think about it. :D
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