I was told at the time when I bought them that the service Boss heads had a different casting number than the production heads.
I was told that Ford took the hydraulic lifter head version and had them machined for the Boss solid lifter valve train.
I can't confirm either story. I didn't make the things.That source is generally very reliable.
I think the original 4 bolt block was different that the service block as werll?
The original was a 4 bolt version of the D0AE block and the service part for it is the D2AE-CA block.
The D1ZZ balancer also was never serviced. It was replaced by the D2ZZ balancer.
The original '71 Boss engineering numbers were only available in the assembly line version.
Distributor also.
Might have been a D1ZZ casting number on the heads rather than a ZX?
I'm not going to go calling anyone wrong and go argue with an entire web site, but let me put it to you this way, show me a bunch of pictures of heads off of real Boss 351 production cars and I will accept that casting number.
It was an issue that cost me some money and a lot of grief a long time ago. It is fairly etched in memory even 40 years later.
