I did the dance with the 2 piece neoprene seals (pre-BOP) twice and failed both times. First time was with a Ford 460 neoprene seal and the second time was with the '63-'67 Cadillac neoprene seal which I believe the current BOP one is based on.
On the first build, I trimmed the ends .010" proud of the block dry, set it in silicone clocked with the ends away from the block/cap interface. Leaked like a sieve. When I removed the seal, it looked like the crank never even touched the seal even though it took noticeably more torque to rotate the crank after installing indicating it was sealing.
On the second build I used the Cadillac neoprene seal per the late Jim Taylor's (Pontiac Guru not the forumite) recommendations. I hand-cut radial anti-rotation slits in the block groove for retention, installed the top and bottom seal in separate operations, trimmed meticulously, meticulously set it in silicone making sure to fill the retention holes in the block first and siliconing the ends and out across the caps before torquing. Looked perfect, tilted with no leakage overnight and then leaked like a sieve within hours of running.
I pulled the motor, installed a graphite Best Seal and never had another leak.
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Triple Black 1971 GTO
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