Piston rocking in bore VS deck height / compression ratio

[Matt@RFR]

We are putting together a SBC 383 with Eagle rotating assembly and Mahle pistons. These pistons have the friction coating on the skirts, and Mahle suggested to our machinist to use .005" clearance to the coating, not the piston itself. The tech at Mahle also told our machinist that these pistons are going to be "noisy".

Issue #1: We are mocking the engine up, and while checking deck height noticed that each piston rocks in its bore .029". We checked deck heights at the top and bottom of each piston with it rocked up as far as it will go. The thinking there is that we are checking for the worst case scenario. We have deck heights that range from -.002" to +.005". Is this the correct way to check deck height?

Issue #2: I am thinking that compression ratio and squish are dictated by theoretical deck height (piston perfectly square in the bore) since both are controlled by volume, not distance. How far off am I? If I'm on the right track, that means that our theoretical deck heights (piston rock / 2 + measured deck height) range from .016" to .011". We were shooting for a .005" deck height, but if I'm right, we're going to need to machine our heads a lot more than we thought to achieve our compression ratio goal of 13:1, which also means cutting the pistons because as of right now, we are RIGHT on the minimum .100" PV on the exhaust.

Any help or guidance will be appreciated!

[kirkwoodken]

When the piston is hot, it doesn't rock much.

[ProPower engines]

if the block is square to the main line I would suggest checking the rods for length.If you have +.016 I would be checking and juggling the rods a bit to get the heights closer. The pistons are going to be within.0005 so the rods are the next thing to look at to reduce the height difference

[Mark O'Neal]

The piston is .005 at the gauge point. The skirts taper and the ring lands are probably .020 smaller than that. If the top land is over about .300 it will taper, or step, smaller than that.

The top of the piston is hotter than the bottom of the skirt, so it grows more.

[Matt@RFR]

Kirk and Mark, thanks for replying, but I'm not asking WHY the pistons rock in the bores. I'm asking how one takes that rock into consideration when calculating deck height and compression ratio. There doesn't seem to be a way to calculate what the piston rock will be at operating temperature, so how does this all correlate?

ProPower, thanks. We will measure the rods and see if we can't get everything to match, or atleast be closer than it is now.

[governor]

Put enough masking tape evenly on both piston shirt's to reduce the rocking and you will be able to better ck when calculating deck height and compression ratio with the piston more stable.

Gov

[mtkawboy]

Im no engine expert but have built a number of NHRA stocker engines. What I did was either take a measurement at each side of the piston when rocking it and use the average or use feeler gauges on each side to stabilize the piston. Thats when mocking it up with no rings. Ive never had an engine fail a teardown inspection that way.

[Wolfplace]

Kirk and Mark, thanks for replying, but I'm not asking WHY the pistons rock in the bores. I'm asking how one takes that rock into consideration when calculating deck height and compression ratio. There doesn't seem to be a way to calculate what the piston rock will be at operating temperature, so how does this all correlate?

ProPower, thanks. We will measure the rods and see if we can't get everything to match, or atleast be closer than it is now.

If a dome piston & you cannot support the piston on center with tape or feeler gauges
Put the indicator on the flat of the piston at any convienent location close to the centerline front to rear
Rock the piston from skirt to skirt a few times to be sure you get repeatable numbers
Divide by two & subtract the answer from either side
IE 014 down, 010 up 24/2=.012,
.014-.012=.002 down
Or .010-.012= -.002 or .002 down

If a flat top measure in the middle of the piston directly

Any of these should get you within ± .001