If the b17 and b18 ls motor have .003 deck height, why if you put an ls crank in the b17 motor the pistons stick out? I thought the b17 is the same block as b16 with just a bigger crank? deck height for b16 is .000 and b17 is .003?

 

Come on people.

 

okay, look. B16 and B17 blocks have the same deck height of 172.5mm. the B16 crank has a stroke of 77mm and therefore a throw of 77/2mm and rods of 134mm in length. the B17 crank has a stroke of 81.4mm and a longer throw of 81.4/2mm, and shorter rods of 131.8mm to make up for the longer crankshaft throw.

B16:
77/2+134=172.5

B17:
81.4/2+131.8=172.5

now, if you put a LS crank into the b17, like you asked, here is what you have: you have a much longer crank throw, 89mm, and you don't have shorter rods. take a look:

B17 block, LS crankshaft:
89/2+131.8=176.3mm

your block is only 172.5mm tall. see the problem? the pistons are going to stick out 4.1mm out of the hole. in other words, if you manage to bolt it all together successfully, when you crank it, you'll break every major part in the engine. the only way you could do this is:
1) just get an LS short block.
2) get custom connecting rods from say Crower that are 4.3mm shorter than stock B17 to counteract the longer stroke of the LS crankshaft
3) get custom pistons that have the wrist pin moved up 4.3mm, rather than in the normal/stock location. this will present big problems, however, because your wrist pins will be in the ring lands. i don't think piston manufacturers can move the wrist pin up the piston by 4.3mm.