Big Block for a 1969

[Scott Lawrence]

I'd like to build a 427 for my '69 and I've found a 396 block with a standard bore (casting number 3855961). My question is can I bore this block to 427 specifications? Any input would be helpful!

[Joe L.]

Re: Big Block for a 1969

Scott-----

For all practical purposes the answer is NO. The overbore required (0.156") will almost always result in going through to the water jacket. Even if the block survives this for all 8 holes (very unlikely), you'd be left with paper-thin cylinder walls which are VERY inadvisable for a host of other reasons. Forget such a conversion and look for a 427 block casting. They aren't that rare as many were used in trucks and sold over-the-counter.

[Joe L.]

Addendum

Scott-----

I should also have mentioned that if you aren't concerned about "numbers" (which, you apparently aren't since you're considering using a 396 block casting) then the block that you look for doesn't have to be a 427. Any 1966-1991 427 or 454 block will do. All of these will be Mark IV blocks and will be a direct retrofit into a 1969. Plus, except for minor nuances of configuration, they'll be the same as those blocks originally used for 1969 big blocks. Any Mark IV 454 block can be converted to a 427 by simply changing the crankshaft (assuming that you want to keep the CID the same as original and forego the benefit of 27 additional cubic inches).

Some Mark IV blocks were produced for SERVICE after 1991, so there are even later-than-1991 Mark IV blocks out there, too.

A Mark V or Mark VI block can be used, also. However, a 427 cid crank for these blocks is rare and these blocks usually (but, NOT always) lack provisions for a mechanical fuel pump. They also don't have provisions for the clutch cross shaft, but an adapter bracket can be utilized. However, the external configuration of Mark V blocks and, especially, Mark VI blocks is significantly different than the Mark IV and they will not look essentially original for a 1969. They are directly retrofitable, though, with the caveats regarding the fuel pump and cross shaft issues.

[Mark #28455]

Re: Addendum

if you want a non numbers matching block, look for a 1 ton pickup motor from the late 1980's (actually pretty common) that has: 1) full length reinforcing strips on either side of the oil pan. 2)valve springs that are each surrounded by a sheet metal shield.
Although they have the "peanut port" heads, they are usually 4 bolt main motors if that's important to you

[scott lawrence]

Re: Addendum

Joe-

Thanks for your help, I really appreciate the input. I have a question about the particular block that I'm looking at. The casting number is 3855961 cast in late '65. This same block casting number has a 427 application in a '66 corvette. Did they re-bore 396s at the foundry for corvettes?

Scott

[Joe L.]

Re: Addendum

Scott----

GM casting number 3855621 was a 396 block. Although I've seen references to this block being used for 1966, if it was used then those engines were 396 cid and not 427. A 396 block cannot be reliably bored out to 427 spec even in SERVICE. I really doubt that the factory ever used a 396 block as the basis for a 427.