It is a bit confusing because Bill J was given some early aluminum blocks that some called "Chaparral" blocks so he can build and test them.
These were 427 4.250in bore and similar to the production ZL-1 block. Some say it was a little stronger then the ZL-1 block but it was legal in NHRA SS/C as it was very close to the ZL-1 block
Bill said the 427 aluminum blocks "moved too much" and would lose power due to loss of ring seal.
He even tried using a water heater rigged to keep the block warm all day at the track so it didn't heat cycle.
I heard Bill talk about this at Carlisle when he was there at a vendor booth and I was able to ask him some questions and get an autographed die cast model of Grumpy's Toy IV.
In 1970, with the new Pro Stock class rules, they allowed the Chevy 430 Can Am block
4.440 in. bore 3.47 in stroke siamese cyls design.
The Chevy 430 was legal in Pro Stock but not legal in SS/C.
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