Over the last couple of issues (
Part I, Part II), we’ve taken an up-close look at Chevy’s tall deck bow tie blocks. This time around, we’ll complete out examinations of the big boy setup. As you can see in the accompanying photos, there’s more to these cylinder blocks than first meets the eye. And in the future, we’ll examine other cylinder blocks (as we get our hands on them). On to the photos:
Look closely at this photo. What you see is the main oil gallery. On late model big blocks such as this, the oil gallery is placed up high, next to the cam tunnel. On earlier big blocks, the main oil gallery was positioned between the oil pan rail and the main journal tunnel. In that case, the block was limited to the amount of stroke it would accept (clearancing the block for big strokes wasn’t entirely possible since the builder would eventually “hit oil” – more later). There’s more, too: If a connecting rod departs on an early block, it usually takes out the oil gallery. Here, there’s a chance it won’t, since the gallery is so high in the casting.
Brass core (or “frost” plugs) are installed in the CNC bow tie blocks. Brass is superior in that it doesn’t rust in this application. It’s also better looking. In circle track racing, the soft plugs are either pinned (two or three pins on the circumference) or epoxied in place so they don’t pop out. That practice isn’t necessary in a street-driven vehicle.
Chevy Performance Parts Bow Tie blocks, for the most part, are based around “siamesed bores”. That means there are no water passages between the adjacent cylinders. The maximum core diameter of these blocks is (+ or -) 5.060-inches (big block cases), which allows for big bores. Additionally, these blocks have reinforced decks, which improves head gasket sealing. What about cooling with siamesed bores. The bottom line is, it’s not an issue.
Previously we touched upon long strokes and oil galleries. Here’s what we were talking about. This block has been clearanced for a long stroke crankshaft. This means that the block is ground internally so that the crankshaft counterweights and connecting rod caps don’t hit the block casing. This particular bow tie block will accept a stroke of 4.50-inches, with no further modifications. Coupled with the as-delivered bore size of 4.560-inches, that means this big block can conceivably have a final displacement of 588 cubic inches. Boring the engine block to maximum increases the potential displacement to 598 cubic inches while additional block work (more clearancing) and an increase in stroke to 4.75-inches can result in a final displacement of 632 cubic inches.
This is a dead giveaway a specific block is a superior piece – all heavy duty Chevy Performance Parts blocks come with bow tie cast into the assembly, adjacent to the oil pan rail. The first big block bow tie cases saw service in 1983-1984. HD blocks from Chevy are really nothing new.
One thing that is new is the one-piece rear main seal. Chevy bow tie blocks can typically be purchased either way – one-piece rear main seal or old style two-piece rear main seal. In the case of the big block, the seal type has nothing to do with the crank flange bolt pattern, but it does require a dedicated crankshaft.
Like it’s earlier Mark IV special high performance counterpart, the bow tie block casting is designed to accept an external oil cooler. The cylinder block is also dry-sump oiling system friendly (the back of the block has a direct port to the main oil gallery, just behind the intake valley). In addition, these blocks will accept a vintage style clutch ball stud (some earlier Gen V blocks from Chevy did not have the boss drilled).