Thoughts on piston to deck measurements varying about .010? I'm putting together an aluminum 5.3 from a 2009 Silverado for boost. I'm using Summit brand forged rods, pistons, and crank. I was mocking up the bottom end and discovered I have some variance in how far the pistons are down from the deck when at TDC.

Cylinder number one is very nearly zero deck, about .001 in the hole. It's mate, number two (on the same rod journal) is .011 down in the hole. I moved rods and pistons around and wasn't able to make any measurable changes. I repeated the mock up with a stock crank, and got the same .010 variance from cylinder one to cylinder two.

Cylinder 7 and 8 (the opposite corners) are very close, .006 in the hole and .007 in the hole.

I'm curious if I should go through the hassle of taking it back to the machine shop, explaining what's wrong, listen to them tell me why I am wrong, then mock it up in front of them to demonstrate, then take it apart, watch them mock it up to confirm, and finally leave the block there for a week or so, etc... Should I mess with all that, or just press on with the build?

Ultimate power goal is mid 4 digits; enough to put a 3000lb foxbody into the high 4's in the 1/8 mile.

 

You won't notice it, but if your machine shop decked it, I wouldn't put much faith in them fixing it right.

 

Gotcha. I'll probably go ahead with the assembly then.

Thanks!

 

Just to add to my comment, obviously it would be nice if it were right, but the small change in compression, valve clearance, and lifter preload will be negligible. They most likely took a set amount off the existing deck surface so the original build was probably off by roughly the same.

 

What's strange to me is how 7/8 are so close and 1/2 are .010 off. You said you moved rods and it didn't change?

 

Sounds like the decks aren't accurate to the crank centerline, which isn't all that uncommon. What's concerning is that its usually easily corrected, it's making them square that's a little tougher.

 

Yes, moving individual rods and piston combinations from hole to hole didn’t make enough difference for my measuring gear to note a difference. My stuff is only accurate to the third decimal, so it’s entirely possible that the numbers moved a few ten thousandths when swapping rod/piston/cylinder; but nothing significant enough to make much difference.

 

About 3% variance in CR eg 10:1 - 10.3:1

 

Do you have pressed pins in the rods or floating pins?

Tom V.

 

Sorry, I know "old thread" but quest for knowledge is forever!
Interesting question, Tom. How would that make a difference? What is the typical tolerance on "floating"?

 

Sorry for not responding earlier. They are floating pins.

Like Saltfever, I'd appreciate you expanding on the wrist pin deal.

 

When you have the Compression Height on the Piston and you have a given Connecting Rod Length with a Pressed Pin you are basically stuck with the assembled combination vs the Block Deck height.

When you have a "Floating" pin with Piston Pin Locks you can move the pistons around in relation to the crank stroke and deck height and can sometimes reduce the Deck to Piston height.

With swapping Pistons around maybe you can get to .005 - .oo6 below the deck on all of the bores.

Tom V.