Thread: Acceptable Piston to Deck Height Tolerance

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I have a new GMPP – 19419042 SBC, ZZ4/6 Short Block Assembly.

I measured the piston to deck height using a magnetic deck bridge w/ a dial indicator. The following process was used: I set the dial to “zero” on the deck and then moved to the center of the piston along the wrist pin (lifting the bridge). I established TDC of the piston w/ the dial and recorded the readings (below deck). I then measured both sides of the piston along the same pin centerline (for a side to side comparison, parallel w/ the crank bore). To verify my procedure, I then removed the dial indicator and measured the center of the piston with a depth micrometer. (Note: I established the top of deck measurement off the bridge first). I followed this procedure for all 8 cylinders, so I have a high confidence level in my readings, as they were the same for each cylinder (dial vs. depth micrometer).

I came up with the following interesting information:

Odd Piston Bank #1 – #7 measured ~ 0.030 to 0.031”, 2 each below the deck. (~ 0.0305” ave.)

Even Piston Bank #2 – #8 measured ~ 0.027 to 0.028”, 2 each below the deck. (~ 0.0275” ave.)

The best information I know of states: “If the piston to deck height tolerance is within 0.002 to 0.003”, consider yourself lucky.” This tells me don’t screw with it and leave it alone?

You can’t really ask for better tolerances along each piston bank (side). If you average the two sides, they are within 0.003”. Another interesting phenomena is each piston that measured 0.030” on one side, measured 0.027” directly across, meaning the 0.031” & 0.028” measurements were also on the same rod journals, and these measurements alternated along the crankshaft (end to end). However, my four outliers measure 0.004” difference (0.027 & 0.031).

** Is this worth tearing the motor down and having the decks milled to tighter tolerances, say ~ 0.020 to 0.025” on both sides? Would this balancing help the motor run any better? I suspect the crack bore is off slightly, which would result in one side being shorter than the other?

Another interesting phenomenon is the measurements along the wrist pins (side to side – parallel with the crank). Each piston was tilted in the bores along the wrist pin centerline (+/- 0.002”)? Each piston in the even bores were higher towards the back of the engine and each piston in the odd bores were higher towards the front of the engine.

If you average all the measurements (24 total), you have ~ 0.029” deck height. I plan on using an 0.015” head gasket, which will give me an average quench of 0.044” (+/- 0.002”).

I guess the part that bother’s me the most is the top of piston’s are not parallel to the deck (along the wrist pins) along the centerline of the crank.

Thanks,

[QUOTE=ZipTie;590944]I have a new GMPP – 19419042 SBC, ZZ4/6 Short Block Assembly.
I followed this procedure for all 8 cylinders, so I have a high confidence level in my readings, as they were the same for each cylinder (dial vs. depth micrometer).

I came up with the following interesting information:

Odd Piston Bank #1 – #7 measured ~ 0.030 to 0.031”, 2 each below the deck. (~ 0.0305” ave.) What you wrote says 30 thousandths to 31 thousandths

Even Piston Bank #2 – #8 measured ~ 0.027 to 0.028”, 2 each below the deck. (~ 0.0275” ave.) What you wrote says 27 thousands to 28 thousandths

The best information I know of states: “If the piston to deck height tolerance is within 0.002 to 0.003”, consider yourself lucky.” This tells me don’t screw with it and leave it alone? Your written spec'n is 2 to 3 thousandths! BIG DIFFERENCE from your measurements!
You can’t really ask for better tolerances along each piston bank (side). If you average the two sides, they are within 0.003”. Another interesting phenomena is each piston that measured 0.030” on one side, measured 0.027” directly across, meaning the 0.031” & 0.028” measurements were also on the same rod journals, and these measurements alternated along the crankshaft (end to end). However, my four outliers measure 0.004” difference (0.027 & 0.031).

** Is this worth tearing the motor down and having the decks milled to tighter tolerances, say ~ 0.020 to 0.025” on both sides? Would this balancing help the motor run any better? I suspect the crack bore is off slightly, which would result in one side being shorter than the other?

Another interesting phenomenon is the measurements along the wrist pins (side to side – parallel with the crank). Each piston was tilted in the bores along the wrist pin centerline (+/- 0.002”)? Each piston in the even bores were higher towards the back of the engine and each piston in the odd bores were higher towards the front of the engine.

If you average all the measurements (24 total), you have ~ 0.029” deck height. I plan on using an 0.015” head gasket, which will give me an average quench of 0.044” (+/- 0.002”).

I guess the part that bother’s me the most is the top of piston’s are not parallel to the deck (along the wrist pins) along the centerline of the crank.

Thanks,[/QUOTE

Check your measurements to your specs... big difference or did you type it wrong??