im going off all stock pistons. the lm7 5.3 in the nbs (00-07) use a dished -8cc piston where as the newer ones, LH6,LY5, and what ever started using the flattop pistons that the 4.8l used and those are like -2cc volume.

so the newer 5.3l had around a 10:1 compression but the 4.8l with the same pistons but shorter stroke gave it a lower compression of like 9.1:1.

deciding to mill them is up to what you are trying to accomplish and what compression you want. i had to replace my cracked 706 castech heads so i went with the 799 heads because they are one of the best flowing stock heads made that would fit. they have a 65cc chamber vs. the 62.5cc chamber on the 706 heads that were on mine. so if i were to just drop them on without milling that would of dropped my compression from 9.5:1 to around 9.2:1. milling them only cost like $50 so might as well bump the compression a little to get even more hp/torque for cheap.

another option would be to get a thinner head gasket from cometic. doing this would tighten up the quench distance and could help with preventing some pre-detonation but you really need to see how far out of the hole the piston comes to get a good answer on how thin to go. when i was looking they say getting a quench of around .030-.035" would be best.

doing that would cost more as they are like $90 each but would you would mill less of the head.

 

The compression ratio of the newer NNBS 5.3's is 9.65:1 with the stock .051 gasket. Also, I was told to go for about .040 quench on a stock rotating assembly to play it safe.

 

*Correction* My calculations weren't right lol. It appears to be around 9.8:1. This is with my -2cc valve relieves, 3.920 gasket bore with .051 compressed thickness and 64.43 cc chambers. For those of you that have actually measured, how far out of the whole are your pistons? I checked my front and back ones today and I got .004, .005, .005 and .002 all out of the whole? I'll double check again tomorrow to make sure I got these right.

 

from what i can see your right they are still around the 9.6:1 range. but for some reason when i put it in the compression calculator they are closer to 10:1.

 

Ya, the factory list it at 9.6:1. I get 9.8:1 with the 3.780 bore, 3.622 stroke, 64.43 chambers, .051 compressed/ 3.920 bore gasket and piston being .006 out of the whole. Idk how the factory gets 9.6:1. It also appears that I forgot how to properly measure piston deck clearance. I'll double check with my instructors Monday haha.

 

The factory used to list the Gen IV 5.3 at 9.9:1 which falls inline with everyone elses calculations and even your own. This of course varies a bit with the head castings but not much. The Gen IV 4.8 is listed at 8.8:1.... good for boost lol.

Here is the L20 ( the 4.8) Powertrain Products

I found anywhere from 9.6:1 to 9.9:1 for the LC9 as an example from varying sources. I know I have calculated bang on 9.9:1 using junkers I have torn down for reference. Pistons do vary though too in them.

I have had Gen IV's with perfectly flat top pistons in them with coated skirts to Gen IV's with the pebbly face pistons with the cross cast in the middle.

 

This is where I got the 9.6:1. Mines is a LMG GM 5.3L Liter V8 Vortec LMG Engine Info, Power, Specs, Wiki | GM Authority

 

40 here with no issues.

 

My heads are milled about .050" with no issues. Intake alignment won't be a problem. There's a lot of wiggle room with the dry intake and thick gaskets that are used on the LS motor.

 

I would keep the rpm in check with that tight of quench. That's not leaving much room for stretch. I wouldn't go any tighter then .040" but that's IMO. .040-.050 is a good range.