Very Early 1966 427 blocks
I am a long time observer of Corvette engine blocks, and I have never seen or heard of a 3869942 block cast earlier than September1965. NCRS books refer to the possibility that 3855961 blocks were used for very early 427s, but I believe it was really 3855962 blocks that could have filled this gap.
We know that 3855962 blocks cast in the 1965 model year had the same internal water jacket dimensions as a 427 and could be machined as a 427 block. I have compared the space between cylinders, observed through the freeze plug holes, between 427 blocks and 962 blocks cast between February and June 1965 and found them to be the same. I have owned '65 962 blocks bored to 427 + .060 over (4.31" bore) and run until they were worn out. These thick wall 962 4 bolt blocks are easy to spot because they have two small gussets between the timing chain cover and the intake manifold like ZL-1 aluminum blocks. The 1965 model year 2 bolt blocks were 3855961 castings, and they had the same gussets between the manifold and the TC cover. However, I do not think they had the thick walls. (If anyone reading this has a 65 model year 3855961 block laying around, please measure the distance between the outsides of the cylinders thru the freeze plug holes.)
In his book about Zora's 1965 L-88 test car, Ken Kayser reported that while GM was deciding which cars would get a 427 in 65 (none it turned out), it had blocks being cast with thick walls for the 427 and blocks being cast for 396's with thin walls. To add to the confusion, he identifies the thick wall blocks as 3855961 2 bolt blocks, and the thin wall blocks as 3855962 4 bolt blocks. However, we have measured the 3855962 blocks and know that they are thick wall blocks identical to the later 427 blocks. Therefore, the casting numbers must have been transposed in his book. Also it would be very inconsistent to make the highest performance 427 engines with 2 bolt blocks and the lower horsepower 396 engines with 4 bolt blocks.
The most likely source of the idea that early 66 427s used 3855961 blocks was Noland Adams't book. In it, he cites two 66 Corvettes with 961 blocks. One is car #828 with a 961 block cast F185 and stamped T0625LF. LF is the code for a 66 Chevelle 399/325. This engine is simply an early 66 396 Chevelle engine and says nothing about 427 Corvettes. The second car is #4092 which had a 961 block dated J15 and stamped T1013IK. I find this engine to be suspicious. Even the 962 blocks used in Chevelles had reverted to thin wall castings and surely by October 1 of the 66 model year 961 blocks would have had thin walls and would not have become factory 427s. Also, the IK code is suspicious. This would have been a rare M-22 car which invites more skepticism.
I recently learned of a 962 short block with a casting date of H225. It would fall into the 1966 model year, and I had thought the 66 model year 962 blocks used in Chevelles no longer had the front and rear gussets or the thick walls. However, this block had the gussets and it was wearing 66 production GM 427 L-72 pistons (without the part numbers found on service pistons). This shows that August 962 blocks still had the internal water passages of a 427, and were capable of being used as 427s prior to the casting of 942 blocks. Unfortunately, this block had been decked so there is no proof whether it was an early 66 427 or a 66 Chevelle block early enough to have thick walls which was bored later to 427. At any rate, if there are any 66 Corvettes with 427s made prior to the 942 blocks, I think it is much more likely that they would have August 962 blocks than any 961 block.
We know that 3855962 blocks cast in the 1965 model year had the same internal water jacket dimensions as a 427 and could be machined as a 427 block. I have compared the space between cylinders, observed through the freeze plug holes, between 427 blocks and 962 blocks cast between February and June 1965 and found them to be the same. I have owned '65 962 blocks bored to 427 + .060 over (4.31" bore) and run until they were worn out. These thick wall 962 4 bolt blocks are easy to spot because they have two small gussets between the timing chain cover and the intake manifold like ZL-1 aluminum blocks. The 1965 model year 2 bolt blocks were 3855961 castings, and they had the same gussets between the manifold and the TC cover. However, I do not think they had the thick walls. (If anyone reading this has a 65 model year 3855961 block laying around, please measure the distance between the outsides of the cylinders thru the freeze plug holes.)
In his book about Zora's 1965 L-88 test car, Ken Kayser reported that while GM was deciding which cars would get a 427 in 65 (none it turned out), it had blocks being cast with thick walls for the 427 and blocks being cast for 396's with thin walls. To add to the confusion, he identifies the thick wall blocks as 3855961 2 bolt blocks, and the thin wall blocks as 3855962 4 bolt blocks. However, we have measured the 3855962 blocks and know that they are thick wall blocks identical to the later 427 blocks. Therefore, the casting numbers must have been transposed in his book. Also it would be very inconsistent to make the highest performance 427 engines with 2 bolt blocks and the lower horsepower 396 engines with 4 bolt blocks.
The most likely source of the idea that early 66 427s used 3855961 blocks was Noland Adams't book. In it, he cites two 66 Corvettes with 961 blocks. One is car #828 with a 961 block cast F185 and stamped T0625LF. LF is the code for a 66 Chevelle 399/325. This engine is simply an early 66 396 Chevelle engine and says nothing about 427 Corvettes. The second car is #4092 which had a 961 block dated J15 and stamped T1013IK. I find this engine to be suspicious. Even the 962 blocks used in Chevelles had reverted to thin wall castings and surely by October 1 of the 66 model year 961 blocks would have had thin walls and would not have become factory 427s. Also, the IK code is suspicious. This would have been a rare M-22 car which invites more skepticism.
I recently learned of a 962 short block with a casting date of H225. It would fall into the 1966 model year, and I had thought the 66 model year 962 blocks used in Chevelles no longer had the front and rear gussets or the thick walls. However, this block had the gussets and it was wearing 66 production GM 427 L-72 pistons (without the part numbers found on service pistons). This shows that August 962 blocks still had the internal water passages of a 427, and were capable of being used as 427s prior to the casting of 942 blocks. Unfortunately, this block had been decked so there is no proof whether it was an early 66 427 or a 66 Chevelle block early enough to have thick walls which was bored later to 427. At any rate, if there are any 66 Corvettes with 427s made prior to the 942 blocks, I think it is much more likely that they would have August 962 blocks than any 961 block.
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