Piston rock and zero decking

[73 Mike]

I've never really figured out what went wrong on my last engine so am a little paranoid about thinking through every detail. One thought came to me last night.

I know that there is a certain degree of rock in the piston though don't have much of an idea of how much. I would presume that the rock would be greatest at the top and bottom of the stroke. I do know that my last block was zero decked and it was as close to zero as I was able to measure. Given that this is the case, you are assuming that the rock of the piston is less than the gap produced by the head gasket which, as I recall, was 30 thou.

Is this a problem? I presume not because people try to target a small quench height, but I'm trying to make sure I cover all the bases before I have the machine shop start cutting on the new block.

 

[damoroso]

If I understand your description correctly, the "rock" of the piston would be increased or decreased by the piston to cylinder wall clearance wouldn't it? Understanding some clearance is necessary, there's going to some movement, but the rings and piston skirt should be able to handle it. Just another one of those many "critical" measurements to check and re-check.

 

[mrvette]

This topic getting into the kind of shit I just can't get wrapped around, how a high volume factory can assemble engines all that close to work well for a typical 300k miles, and yet the assy techniques and all this clearance stuffs has to be measured and REmeasured to get a lousey ONE piston in right....

which is why I have always farmed it out...:crutches:

 

[rugmuffin]

The cylinders in our toys are essentially parallel walled with no taper top to bottom. Some engine builders set them up a bit looser at the bottom than the top, but it's a very small amount. In a straight (parallel) wall cylinder the piston rocks the same top, bottom and everywhere in between, depending on the piston clearance to the cylinder wall.

Don't sweat the rock on the piston. Unless the piston diameter is grossly smaller than the cylinder diameter, the amount of piston rock is not going to cause any issues.

 

[TimAT]

MY $.02

A given piston has .00x of clearance to the cylinder wall. That's total clearance between the piston and cylinder. 1/2 of that will be on each side so what you 'd be looking for would be .00xx. Putting numbers in let's say a piston has .005 total difference between the measured diameter of the piston and the cylinder wall. So the piston will rock in the bore .005. Which would be .0025 on each side. As I see it, to check the deck height, you'd need the average between the piston rocked over all the way one direction and then all the way the other. OR measured directly above the pin. I'd be a bit wary of measuring directly above the pin because if the piston was down on one side it'd have be higher on the other and could possibly be above zero deck.

May not be right, but it's how I see it.