I R Gunnr

I am getting ready to put heads on my fresh short block and I was checking the piston deck clearance today to decide if I would use the .040 or 0.45 Cometic head gasket. I have checked every piston and keep coming up with the piston being in the hole .002-.004. I have stock crank, stock rods, wiseco +4 cc dome pistons with a compression height of 1.335".

The only thing I can think of is the machine shop they had to hone the pin end of the rods to make the pins fit that came with the wiseco pistons, would the larger pin be enough to sink my piston that far in the hole? The Wiseco piston pin diameter is .945”.

Really appreciate any help. This is my first LS build and I need to get this engine in the vehicle!

KCS

iTrader: (20)

How are you measuring?

I R Gunnr

Magnetic deck bridge with dial indicator. Put the deck bridge on the flat part of the block and zero the gauge. Then I slide the deck bridge out over the piston onto the flat part that meets the flat part of the head (at the top of the piston). At the point on the flat part closest to the center of the piston I get -.003-.004, at the edge of the piston I am getting -.001 to .002. I thought I may be reading the negative/positive indicator incorrectly, but I played with it a bunch of times and I'm reading IN the hole.

KCS

iTrader: (20)

Are you rocking the piston up and down?

I R Gunnr

Like by rotating the crank or by pushing on the piston? I tried rocking the piston and didn't see any movement, I'm moving the crank around until I get the highest reading on my dial indicator.

KCS

iTrader: (20)

Maybe try pushing harder, or trying it without the rings on the piston. A forged piston should be able to rock about .020" at TDC. If it's .003" in the hole one way, and .017" out the other way, you're sitting at .007" out.

I R Gunnr

So I realize I may have been measuring in the wrong spot. I also was not rocking nearly hard enough. A pic of how I have it oriented now is below. I had it 90 degrees off from that before. For the rocking, when I push as hard I can on one side, the other side goes up to .010 OUT of the hole. When pressure is relaxed it sits at about .008 OUT and the opposite side sits .016 IN the hole.

Does that make any more sense? So this would put me around .004? That's a complete guess, I have no idea the formula or how to calc that.

I R Gunnr

KCS

iTrader: (20)

I measure here:



I'll measure the other side above the valve reliefs as well to check how square the deck is.

I R Gunnr

I went and measured all four corners of the block and also talked to Wiseco. According to Wiseco, the 6.0L truck blocks are out of spec all the time and it's normal for the piston to end up in the hole like this. Also, the honing of the wrist pin hole is very common. I have a local machine shop that can deck for me today, just need to figure out what my actual quench/piston head clearance currently is. Block needs cleaning either way, since I had a brain fart and used a scotchbrite pad on the block for about 30 seconds before remembering the warning I read on here about them.

Should I average all these numbers and that is my actual clearance?

Or is the highest point what I should base my head gasket choice from?

I R Gunnr

I R Gunnr

Block is now in the hands of machine shop, who unlike me, actually knows what they are doing. I will post back here with the outcome. I think part of my inability to get consistent results is my crappy harbor freight digital dial indicator. Shop is going to take the block down to zero deck or what it takes to level it, which is ever is less. I had no idea these iron blocks had such a bad rep for being out of spec.

KCS

iTrader: (20)

In your defense, the decks usually aren't that straight on these engines. They tend to have a little twist side to side and a dip around the middle two cylinders. Once you surface the block, it's not too difficult to get everything within +/- .001".

DavidBoren

Is there any reason that being "in the hole" would be a bad thing? Just use a thinner head gasket to achieve your desired quench. Who cares if you are below deck as long as you can get ~0.035" squish without dry decking your heads? If anything, it just ensures your rings will always have plenty of cylinder wall to seal against, even at TDC.

KCS

iTrader: (20)

The main drawback is that aftermarket gaskets are more expensive than the GM gaskets. Is rather have the deck cut square and straight than have to buy thinner gaskets.

rpturbo

iTrader: (47)

To me, this sounds like your opening up a whole can of worms. Unless the same machine shop is decking this, that did the rest of the work. If not, I know how this goes. The new shop blames everything on the old shop, and you end up paying to "redo" all of the old work, at minimum. I get that it's out ~.005-.006, but in the end, I haven't seen any proof of that really causing issues in this instance. JMO

Killer-Miller

So, usually the piston is above deck .008. that is with a stock OEM rotating assembly, and stock rod length being 6.098" inches. MOST aftermarket rotating assemblys use a 6.125" rod length. Thus, most aftermarket pistons have their wrist pin location for the slightly longer rod length. If you used a piston that was made for the 6.125 rod, on a stock rod, that would answer why the piston is lower than normal. Difference is .027", making TDC from .008" above deck to now .021" below deck. Appropriate. Hope this helps. Probably why you had to mill the wrist pin size.

G Atsma

If you are addressing the OP, he hasn't been here for over 3 years. The thread is over 6 years old.
Good info, though!