[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:

[Ron H]

Great info Tom, thanks. One question I have.

Is there another aftermarket block available from other brands or companies made with a 4.8" bore spacing?

[Engo]

Originally Posted by Ron H

Great info Tom, thanks. One question I have.

Is there another aftermarket block available from other brands or companies made with a 4.8" bore spacing?

BBC stock bore spacing is 4.84"

[adam woodmancy]

thanks tom.. good read, and i agree.

[goatless]

That's all fine and good, Tom, but racers being like they are, are going to then just use that newfound bore spacing allows them to increase bore size until there is marginal sealing surface.

[68gtoMN]

Facts...Schmacts... Wait, what?

<uhoh>

[PONTIAC DUDE]

So if I had a IA II and wanted to go 4.400. how much offset bore can I get away with and still use the std Aftermarket Pontiac head design/chamber?

Which way for each cylinder and still not shroud the valves?

Valve size Intakes? 2.250 or 2.300 or larger?

What about rod offset with std aftermarket location journal spacing?

[Half-Inch Stud]

PMD's V8 crank is oh say 7" shorter than their OHC-6 crank, so we gots plenty of bore spacing to run 5" bores.

I figured the real HP occurs in the 5-6" diameter bore. so let's see action.

[Dragncar]

If the Pontiac engine was allowed to grow and develop like it should have we would have seen a lot of great things. But is was not allowed.
All that has happened with the aftermarket Pontiac blocks and heads is the fans of the brand have done it themselves. On their own dime.
Do it, move the bores out , call it a Pontiac and go racing.
Do not get all the "its not a Pontiac BS". BBC guys do not have one problem calling a Anderson a Big Block Chevy.
Just build it and people will buy it.
Pontiacs engineers were forward thinking people. The 1970 427 SOHC Hemi was proof. It would have turned the racing world upside down if they were allowed to finish it.

BTW, does anyone know what the bore spacing is on the Pontiac block Lynn is spacing out ?
And could you get what you needed for space if you just moved the outside cylinders "out" and made the space in the middle ? So it was not some sort of "stretched block" so you could run standard Pontiac cranks and heads. If that is the case it should have been done yesterday.

[KEN CROCIE]

A Quibble: Tom V , in his argument, seems to assume there were no sealing issues with Pontiac bores up to 4.211" In fact , we noticed 'between cylinder' leakage when tearing into stock unmolested engines. The 'fix', upon reassembly was to place a straitened piece of paperclip between the cylinders. This procedure was conveyed to our customers through the High Output newsletter.

[Tom Vaught]

Originally Posted by Engo

BBC stock bore spacing is 4.84"

I intentionally made the proposed Pontiac Bore Spacing 4.8" vs 4.84" so that you could say that you have not copied a STOCK 396 CID BB Chebby Block Bore Spacing. Note: almost every Aftermarket BB Chebby Block is larger in Bore Spacing vs the factory 4.84" number.

Admit it, we currently are limited on boosted engines to a smaller (more dependable) bore size around 4.250" to maintain a .375" gasket width between the cylinders.

With a 4.8 Bore Spaced block we could use a piston with a 4.425" bore vs a 4.250" bore
(almost .200" BIGGER). The 482 boosted engine would now be a "521" Boosted engine
(100 cubic inches more than the old 421 engine) and WITH BOOST!

Tom Vaught

[Ron H]

Will the current cranks and heads work with a 4.80" spacing?

If so, why not do it?

If not, you are looking for alot more than our community can support, imo.

[slowbird]

Big block Mopars have a 4.800 bore spacing.

[bignate]

Why not go to 5" like the Caddy 500?

[Tom Vaught]

Originally Posted by Ron H

Will the current cranks and heads work with a 4.80" spacing?

If so, why not do it?

If not, you are looking for alot more than our community can support, imo.

Again, in my opinion, if you step up to a "spread bore" Aftermarket Block design, you will most likely be using a Custom Crankshaft anyway. Moldex can make a 4.8" Bore Spacing Billet Crankshaft just as easily as a 4.625" Bore Spacing Billet Crankshaft.

Tom Vaught

[Tom Vaught]

Originally Posted by KEN CROCIE

A Quibble: Tom V , in his argument, seems to assume there were no sealing issues with Pontiac bores up to 4.211" In fact , we noticed 'between cylinder' leakage when tearing into stock unmolested engines. The 'fix', upon reassembly was to place a straitened piece of paperclip between the cylinders. This procedure was conveyed to our customers through the High Output newsletter.

Was this the average 455 engine Ken or an occasional exception?

My Uncle's Chief Mechanic would fix a troublesome engine using a similar piece of steel welding rod in a similar manner many years ago. He taught WW-II soldiers how to build engines. So how did you hear of this fix?

Tom Vaught

[fiedlerh]

I believe Pontiac bore spacing is 4.620
Mfr Bore Spacing
Packard 5.000
Cadillac 5.000
CN Blocks 5.000
Ford 4.900
Chevrolet DRCE 4.900
Chevrolet 4.840
Oldsmobile DRCE 4.840
Mopar 4.800
AMC 4.750
Buick 4.750
Ford 4.630
Oldsmobile 4.625
Pontiac 4.620

[Ron H]

Unless this new engine design has more in it than other aftermarket engines, why would it sell? You could call it based on whatever original brand you want. It is still just an aftermarket engine unless existing Pontiac parts can connect to it?

Pontiac head designs were adapted to chebbie short blocks in the past. Wouldn't this be the same thing?

[Craig Hendrickson]

Originally Posted by fiedlerh

I believe Pontiac bore spacing is 4.620
Mfr Bore Spacing
Packard 5.000
(snip)

You bring up a good point that I was talking to Tom Vaught about tonight.

The Packard V-8 was brought out the same year as the Pontiac and Chebbie, i.e., 1955. The Packard was a very physically large engine that was "designed" to eventually be bored/stroked out to 500CID. As it turned out, the largest production displacement was 374CID in 1956 and there was a 1957 prototype extant that was supposedly 440CID.

Another interesting point between Packard and Pontiac is that John Z Delorean worked for Packard from the late 1940s until sometime in the early to mid 1950s. He was primarily a transmission engineer, but he certainly would have seen Packard's plans for their V-8 while "hanging around" the engineering department.

There are other similarities between the Packard and Pontiac V-8s such as the center Siamesed exhaust ports, a separate valley pan, Delco electrics, etc. I once had a conversation with Pete McCarthy about these similarities, which Pete explored, and he came to the probably correct conclusion that there was no "cross intellect" going on between Packard and Pontiac during that era despite Delorean's mutual employment.

If Packard had continued beyond 1956 and "fixed" some of the design problems with their V-8 then no doubt it would have been one of the outstanding wedge valve V-8s of the early 1960s. It certainly had the ability for large displacement, but then again so did the Caddy 500CID.

It's all moot at this point, but thanks Tom for bringing this subject up.

[john marcella]

Tom,
You know as well as anyone that building race motors is about manipulating every part and component of the engine to just shy of its yield point. It will always be this way.
And those are the people that will be fast.
You go 4.800 BS and there will be a 4.600 bore in it.

If you want bigger bore spacing you can buy that, and it comes with way better parts for making power as a bonus. BUT IT WILL NEVER BE A PONTIAC. JMO