Whatever kind of rubber GM used they are 1,000% better than the worthless
made in china so called , rubber repro washers .
Thanks for the detailed and helpful reply as usual!
1000% sounds A-LOT better! It must add at least 20hp.
I ended up using leather, since I have some on hand and don't want to wait for shipping.
Perhaps only 5x better than the knockoff rubber ones
Leather was sandwiched between washers so I could drill a 1/2" hole, then the 1" outside was cut with bandsaw & scissors.
Two layers stack to ~1/4" thick when un-compressed. They softened up after a deluxe spa treatment submersed under oil for a few minutes in the microwave, but did not swell up much.
They seem to fit well.
pictures attached
