[Z Code 400]

Sometime ago, I posted a question about this modification. I received an e-mail from a gentleman in Arizona who has performed this operation with great success, but lost his address.

The 421s had an extra cooling hole in the block that supplied additional water flow to the siamesed exhaust valve area of the heads.

All Pontiac head gaskets have this hole, even though the later blocks no longer use it.

I have looked at the cooling system and I definitely believe there is some benefit to these holes being present. The question is, how large should they be?

The close proximity to the head bolt leads me to believe the holes should be kept small. I am thinking that 1/8" (like 400 Chevrolet) would be sufficient.

Any thoughts on this modification???...Robert

ZCode400@aol.com

[Z Code 400]

Sometime ago, I posted a question about this modification. I received an e-mail from a gentleman in Arizona who has performed this operation with great success, but lost his address.

The 421s had an extra cooling hole in the block that supplied additional water flow to the siamesed exhaust valve area of the heads.

All Pontiac head gaskets have this hole, even though the later blocks no longer use it.

I have looked at the cooling system and I definitely believe there is some benefit to these holes being present. The question is, how large should they be?

The close proximity to the head bolt leads me to believe the holes should be kept small. I am thinking that 1/8" (like 400 Chevrolet) would be sufficient.

Any thoughts on this modification???...Robert

ZCode400@aol.com

[ponchopete]

Steve Barcak, who owns Pontiac Heaven in Chandler, AZ, taught me about this modification. He said he has done it to all of his engines, and he has logged hundreds of thousands of miles on some of them with no cooling system problems. He told me to get them as close as practical to the actual hole size of the gasket. I drilled the extra holes in my 412 to .250" on both the deck of the block and the cylinder head. With this size hole, I came just to the outer edge of the bolt boss inside the block without actually removing any material from it. It should be running here in a couple of weeks, so we'll see if it runs cool!

"My car would be fast if it had an engine, transmission, and rear axle in it."

[fiedlerh]

Did the same to mine but haven't run it yet. I noticed if you hand drill them, the bit will tend to wander. Use new sharp bits and pre-drill with a small bit.

http://fiedlerh.home.att.net/

[Half-Inch Stud]

I'm on the fence with that...Might do it.

"tbdET/1xxMPH/1.xx 60foot/28"/3.54:1/10"/469 #48/Flat HYD/DualQ-Jets/Offyl/3xx0Lbs
11.00/123MPH/1.50 60foot/29.5"/4.10:1/10"/469 #48/FlatHYD/DualQuad/Wenzler/3250Lbs
12.00/112MPH/1.61 60foot/26"/3.31:1/10"/469 #48/FlatHYD/Q-Jet/Torker/3650Lbs
12.70/103MPH/1.71 60foot/28"/4.10:1/12"/469 #48/FlatHYD/Q-Jet/Torker/3950Lbs"

[Z Code 400]

15/64" is the largest I can drill the hole and not cut into the Head Bolt Boss on my 9799914 1970 400 Block.

I drilled the heads to 15/64" and used a 45° carbide countersink to bevel the mouth of the opening (in the head only) out to match the 1/4" hole in the head gasket. The countersink is about .015" deep.

Since water flow is upward into the heads, this should promote good flow characteristics and maintain structural integrity of the block and head around the bolt boss.

I hope this information is helpful...Robert

[Z Code 400]

Here is a photo of the '421 style' cooling modification as performed on my 1979 6X cylinder heads.

The hole is 3/16" in diameter, which just clears the bolt boss without removing any material from this critical area.

The 45° countersink measures approximately 5/16" and fits within the 3/8" hole in the Fel-Pro gasket with room to spare on each side.

I feel this is the best way to modify these heads as drilling the hole any larger would cut into the bolt boss in both the head and the block.

I hope this information is helpful...Robert

Photo by Anthony Bejarano

[tikiklubracing]

Use an aircraft drill. Some call it a stubby, but they come with a 135 degree split point which is self centering. I sold em for 15 yrs. Besly-Bendix is the best, but Dormer makes a good one too.

BIGGERISBETTER-78 Catalina, 3650#, 13.567@100.51, 400 .060 .260 domes, 6x8, RPM, 750cfm 4150, 3.73 spool. 15 yrs in NHRA tech ret.

[Z Code 400]

Here is a link to a company in Michigan that offers the 45° countersink burrs.

http://severanc.brinkster.net/catalog/cutters.asp

Hope this is helpful...Robert

[Junkyard Dog]

(1) Anybody ever try this on Edelbrock, or any of the other aluminum heads out there?

(2) Any empirical benefits to doing this?

[Skip Fix]

Edelbrock heads come with this already drilled. My current pump gas motor I drilled the block for it . Runs cool all day long maybe 10 degrees coller than last motor,but never had a problem with other motors either.

[Junkyard Dog]

So if I have Edelbrock heads, all I need to do is drill the block, correct?

[Skip Fix]

Be sure the head gasket has a hole. I think the "Butler" ones don't , but I could be wrong. FelPro and Cometics do.

you might check inside the head that the hole doesn't have andy casting flashing around it so it flows better.

I did drill my new block also.

[Frank_Zeller]

This the info I have on the mod. Not sure where I got it from.

"Cool tip- Ever notice that hole in the middle of the head gasket below the bore centerline? Here isanother way to make your Pontiac more detonation resistant. On Pontiac's legendary SD 421, that hole in the head gasket was a water passage from the block to the heads to cool the center of the head, which is hotter than anywhere else on the head by a large margin. Using a 5/16 drill, open that passage up by drilling through the deck of the block. Then, drill the heads. If you have a set of Edelbrock heads, this hole is already drilled for you, but may have some casting flash obscuring it. On a stock cast iron head, just drill through the deck surface, using a head gasket as the template. The Fel-Pro Printoseal head gasket has the hole already located so it makes a good template. Be careful! Use a SHARP bit and mark the hole first with a centerpunch so the drill doesn't "walk" all over the deck surface. When the holes are drilled, lightly chamfer the holes so there are no sharp edges. I have also heard that another hole above the bore centerline can be drilled to match the lower hole for an even greater coolantflow"

[goatasaurus rex]

good idea for a motor thats right on the "edge". may make the difference between pump gas and not!

[76transam76]

My engine will be right on the top edge of being able to run pump gas, so would this mod be possible on my 76 400 block??

[Mr. P-Body]

We've also been doing this modifcation for a long time. It was the SD engines that had it from the factory. Some 421HOs may have had it as well, can't say for sure. It only takes a few minutes and is well worth it on ANY engine. We use a 3/8" drill.

The Butler gaskets we have in stock do NOT have the hole. We use them only when the intake runners are "maxed" on E-heads, as it cuts into the head bolt hole, and can cause a tremendous vacuum leak, internally. The Butler gasket does not have the relief in the gasket for oiling the early heads, sealing the bolt holes completely. We learned this one "the hard way".

Jim

[Skip Fix]

One of my E head sets has the head bolt holes tubed.

[76transam76]

Do kre heads have this cooling hole?

[Junkyard Dog]

Don't know . . .