Need one last bit of info to calculate static compression ratio
Need one last bit of info to calculate static compression ratio How far down in the bore is the stock LS1 piston when at TDC? I would assume about .010 to .020, but I don't know for sure. Also, when armed with this information, how do I determine how far down in the bore (or out of it) a particular aftermarket stroker piston/rod combo will be. I realize that the compression height of the piston determines this and it is based on the length of the rod, but if all I know is that my aftermarket piston's compression height is 1.315" and the rod is 6.125", how do I determine where this puts me in relation to my block deck? 
Thanks in advance for any help provided.
Thanks in advance for any help provided. OK, so I forgot....I guess I need two other pieces of info as well. What is the compresed thickness of the stock graphite coated head gasket and what is it's bore diameter? Thanks.
TECH Apprentice
Joined: Jan 2004
Posts: 337
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From: Frisco TX (Dallas Area)
Your rotating assy height @ tdc is 9.27 (3.66/2+6.125+1.315). The nominal deck is 9.255 (blueprint is 9.250). The stock gasket is .053 IIRC, and I wouldn't worry about the gasket bore, I would just use 3.905 (stock 3.900 honed for rebuild). That should give you within 2 decimals of the actual CR. At 9.27 rotating assembly TDC height, the piston is .015 above the nominal deck and .038 below the head surface. HTH 
Thanks for the help guys. I assume this equation is Stroke/2 + Rod length + piston compression height. When I take the stock dimentions, 3.622/2 + 6.098 +1.340, I get 9.249. If the nominal deck is 9.255, that means the stock piston should be .006 down in the bore, correct? Just curious about a few things....What do you mean when you say "blueprint is 9.250". Also, where did you obtain the info on the deck height? Is there a good way to measure it? Once again, thanks for the outstanding help!
TECH Apprentice
Joined: Jan 2004
Posts: 337
Likes: 0
From: Frisco TX (Dallas Area)
OEM's add a little matl to the top of the block for tolerance and rebuild allowance (9.255) vs. the actual design dimension (9.250). When an engine is "blueprinted" the machine work done is designed to achieve all the design dimensions as closely as possible. The deck height is from the center of the crank journal bearing to the top of the block (the deck). You measure it with several instruments designed to locate the center of the journal and a depth gage to the top of the block. Gee, I can't remember where I got the dimension, probably from reading all the specs on the LS1. The old SBC had a deck height of 9.000". My guess is that GM increased that to 9.250 to allow some more room for a fatter, stronger bottom end. HAND (have a nice day).
OK, after reading a few other posts, along with what DaleMX said, I'm still a little confused. According to what I calculated, the stock LS1 should sit at 9.249. (See my post above). So if the stock block is 9.255, that tells me the piston should be .006 down in the bore (9.255 - 9.249 = .006). Above, DaleMX said that it is .005 - .010 OUT of the hole, and I found many other posts that said it is .006 OUT of the hole. What am I missing here? According to that, I seem to be off by .012. Am I using the wrong compressioin height for the stock piston or something?
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Both my LS6 block and my LS1 block have pistons 0.004" & 0.005" in the hole, not out! You may want to measure to be safe.
One thing that people forget to do when the measure piston height is they only take one measurement and this can be very deceiving. The proper measuring procedure is to set up your measuring equipment right next to the bore wall on the piston top and then rock the piston both ways in the bore (pushing it toward the bore at the measuring point and then directly opposite the mic push it down toward the bore), add the measurements and then divide by 2!
One thing that people forget to do when the measure piston height is they only take one measurement and this can be very deceiving. The proper measuring procedure is to set up your measuring equipment right next to the bore wall on the piston top and then rock the piston both ways in the bore (pushing it toward the bore at the measuring point and then directly opposite the mic push it down toward the bore), add the measurements and then divide by 2!
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Verdad Gallardo Thanks John. So, what do you think causes these discrepancies between motors. Maybe variences in the deck heights of the blocks? To your knowledge, was I using the proper compression height for stock LS1 pistons? I was basing that off of the numbers given for stock replacement pistons (for use with stock crand and rods). I have since found another post where someone said the stock deck height was 9.240, not 9.255. Thats a pretty big difference. 
