85 gn block


Jul 25, 2009
Is an 85 GN block the same as an 87 GN block.besides the one drain hole that would be needed to drill for the drain back for the turbo.any info would be great. Thanks joe
No, the two blocks are not identical. The 86-87 block has a lower deck height and has countersunk head bolt threads. Lots of guys use 84-85 blocks in place of 86-87 blocks, but the piston compression height is different between the two blocks to make up for the difference in deck height.
The deck heights are different....however, I thought the pistons and entire rotating assembly were the same...i thought they lowered the deck height simply so they could run a thicker head gasket.
I beleive they are slightly different.......but I can check, I have lots of 84 -85 pistons and 86-87 pistons in my inventory of Buick stuff. I'll compare them just for confirmation.
Nope, you were right. Same Compression Height. The top ring placement on the 86-87 piston is higher up compared to the 84-85 piston.