[Tom Vaught]

A lot of members in the race section know that the Traditional Pontiac Bore Spacing is 4.625" between the cylinders.

A little History first:

1) When the Traditional Pontiac V-8 Engine was first designed (as a 287 cid engine) the Bore Spacing was 4.625" and the Piston Bore was 3.75" so the distance between the cylinder bores that the Pontiac Designers
and Engineers designed for was .870" so there was a lot of meat between the cylinders.

2) Pontiac Engineers, when they changed the bore dimension, decided to increase the bore to 3.9375",
a 5% increase in the bore size. This changed the design distance between the actual cylinder walls was
now .683" (still well over .500" of meat to seal the gasket.

3) Pontiac Engineers changed the bore again in 1958 to 4.0625" and it stayed there for all of the 3.00"
main engines until 1967. The design distance between the actual cylinder walls was now .558" (again,
over .500" of meat between the cylinders. You rarely ever heard of a Pontiac engine with a head gasket
failure).

4) The large main engines, beginning with the 421 CID engine, increase the bore diameter a few more times. The 421 Engine had a bore of 4.093" and a design distance between the actual cylinder walls was .527".
We are STILL over .500" of meat between the holes!

5) The numbers for the 428, 455, 455 +.060 engines were: 428= .500", 455= .465, and the 455 +.060"= .410" so with the worst case factory engine we have reduced the gasket sealing capability of the engine from the 317 engine distance of .683" to the 455+.060" distance to .410" or a reduction of 40% and the 370 engine distance of .558" to the 455+.060" distance to .410" or a reduction of 26% of gasket sealing capability.

6) So now we enter the "Aftermarket Block" phase and and bump the bore up to a very common 4.350"
Bore size. Now we have .270" of meat between the cylinders to seal the head gasket material. To repeat,
the Pontiac Engineers, (who were very smart Engine Engineers) started out at .870" and now the Racers
have reduced that dimension over the years to .270"!!!!!!!!!!! (AND THIS IS BEFORE BOOST!)

7) No Pontiac Engineer would have allowed this to happen. The last engine Pontiac Engineers signed off on
was designed with a minimum of .410" of meat between the cylinders under worst case over-bore conditions.

8) I think that Mac McKeller would have said: The Gasket Sealing distance can be no less than .410" or I will
not sign off on it". Using that logic, with the new aftermarket blocks, the Bore Spacing would have to grow to 4.8" (with a max piston bore of 4.390") to meet the guidelines.

Let the racers continue to bump up the bore to 4.5" or bigger but the Pontiac Enthusiast who wanted a Durable Block Design would have gone to the 4.8" bore spacing at some point if Pontiac Engineering had been allowed
to do their jobs.

So my vote would be, ALLOW THE BLOCK BORE SPACING TO GROW TO 4.8" and forget about the 'rule" that the bore spacing must be 4.625" to be a "Pontiac Engine." I would love to have a 4.8" Bore Spacing, Short Deck, 4.4" bore, 3.5" boosted engine to play with.

Everyone today is running 'Pontiac Heads" that do not resemble a previous design Pontiac head, CV-1 heads are a good example. One could say, "if you can't screw it down with stock Pontiac Head Bolts (at the Pontiac Bolt Length) it is not a Pontiac Head."

That deal would disqualify much every aftermarket Pontiac Head out there.

So my vote would be (as we move into the Boost World, where Ford has been forever) allow the Aftermarket Block guys the option to offer a 4.8" bore spacing block, call it a "Next Generation" Pontiac Engine Design, and let's go racing.

Tom Vaught



























Doing a bit of math with different Bore Diameters: