I’m wanting to swap a 305 with a 327 in an El Camino, had a 68-69 327 long block but the block turned out to be cracked so I kept the crank (2 piece rear main seal), pistons, rods and heads from it and I believe it should be a large journal crank. I was wondering what year they stopped using the 2 piece and switched entirely to 1 piece rear main seal and if the crank journal size stayed the same until then also. Just trying to figure out how old of a block I need to look for for every thing to work.
They stopped using the two piece seal in 1986. The 86 SBC was an odd one in that it still came with the perimeter bolt heads, yet went to a one piece rear main. Here is a graphic showing the differences compared to the years of engine manufacture:
You'll notice the rear crank flange is very different on a two piece oil seal than it is on a one piece.
As far as the journal sizing goes, GM changed the size of the journals in 1968 to accomodate a design change in the crankshaft. Prior to '68, the main journals were 2.300" while the rods were 2.000". After the change, the journals were upped to 2.450" and 2.100", respectively.
What does this mean for you? Looks like you're looking for a block between 1968 and 1985. There are only about a billion of them out there, so shouldn't be hard to find one to suit your needs. Ensure you pull the casting number off of the back left corner of the block, then do a little research. Don't take someone's word for it before you purchase. Do your research.