[Z Code 400]

When I was pressure washing this block, a LOT of water was coming out of that little 3/16" hole; much more than was coming out of the larger openings. Additionally, when I looked inside my 6X-4 heads recently, there were sections of the casting that would impede water flow into that area between the valve seats.

I definitely think the modification has merit. It would be nice if Steve Barack would chime in on this topic...Robert

[LStathas]

Does anyone have any thought or experience with adding the extra 4 cooling holes shown in this picture. I have read about the center hole and know it needs to be added to the head also. These extra 4 holes are already in the cylinder head and wondered if it would be worth adding the passage to the block deck also. I'm making a torque plate and want to put these hole in the plate to use as a fixture to drill the deck.

[Z Code 400]

LStathas,

The one in the middle, between the two center cylinders, is the '421' cooling hole mentioned throughout this post. I don't have an answer for you on the other holes, but I don't hink they are beneficial or required....Robert

[Billz428]

How thick is the deck when drilling the hole"421 style" that matchs the E-heads? I am thinking about doing this while everything is still apart.

[Z Code 400]

Billz428,

My 1970 400 Block is over 5/8" thick in this area...Robert

[Billz428]

Thanx, I have a 428 YH block from 1968, just checking before I start drilling.

[Z Code 400]

Billz428,

Drill to 3/16" because that is as big as you can go without drilling into the head bolt boss. Use a 45° carbide bit to countersink the hole in the head out to 5/16" (gasket opening) as this will promote fluid flow up from the block and into the heads. Leave the hole in the deck of the block 3/16"...Robert

[firebird400]

*edit*

Never mind, talked to the pros and got what I needed

*edit*

[General Z]

Does anyone do the three hole plug, like Steve Barcak describes, or do most just drill the block and head holes and leave the holes that he plugs alone?

Thanks.

[Schurkey]

Originally Posted by General Z

Does anyone do the three hole plug, like Steve Barcak describes, or do most just drill the block and head holes and leave the holes that he plugs alone?

I did. I did no testing, I figured if it was good enough for Steve B, it was good enough for me.

[Region Warrior]

Didn't plug others on mine. Figured more thru of the heads the better?....

[Rodney R]

I did the three hole plug after talking to Steve. It's hard to argue with experience. He said that in his experience, the back two cylinders on each bank were the ones that ended up overheating whenever an engine got hot because of the way the cooling is designed.

With the large holes in the front of the block, water tends to circulate around the front 4 cylinders better than the back 4. By plugging the 3 holes in front, you're forcing water to circulate the whole block rather than the front 4 cylinders. I live in AZ and have had great success with this mod.

[Region Warrior]

Thanks RR
Can visualize that. Since mines apart, put that on to do list.

[General Z]

Thanks guys.

[TRADERMIKE 2012]

Originally Posted by Rodney R

I did the three-hole plug after talking to Steve. It's hard to argue with experience. He said that in his experience, the back two cylinders on each bank were the ones that ended up overheating whenever an engine got hot because of the way the cooling is designed.

With the large holes in the front of the block, water tends to circulate around the front 4 cylinders better than the back 4. By plugging the 3 holes in front, you're forcing water to circulate the whole block rather than the front 4 cylinders. I live in AZ and have had great success with this mod.

Mikes Reply:

Let's see if Mike gets this, we drill a hole between the two Exhaust valves ports in the deck and head side to match and bevel @ the location depicted by a certain gasket. Then we drill and tap and plug the 3-deck holes at the front of the Engine in certain places that match a particular Head gasket.

Result: The water fills from the block up into the Heads. The water stays longer in the Heads because it can't drain down until it gets to the center Exhaust area and rear of that area. So, when you move forward while in motion the water mixes in the head's front to rear and down the drain holes mixing into the block to an equal Temp. and out the water pump to the thermostat opens allowing the heated mixture to flow through the Radiator top to bottom where it is 30 degrees cooler and flows to the rear of the Radiator where this hose has a spring so it can't collapse under vacuum. Say you're using a 180-degree Thermostat opens all the way by 190and you have the 3 bypass holes drilled. Now you have optimum running Engine Temp in most places.

[tooski]

Originally Posted by TRADERMIKE 2012

The water stays longer in the Heads because it can't drain down until it gets to the center Exhaust area and rear of that area. So, when you move forward while in motion the water mixes in the head's front to rear and down the drain holes mixing into the block

Those are not drain holes. Water from the pump pressurizes the block. The water then flows up through those holes into the head, then forward to the water crossover and out the thermostat. Those three holes essentially short circuit the cooler water from going to the back of the block. By blocking them, more cooler water is forced to the back of the block, up to the heads and then back to the front through the heads.

[turbo69bird]

Not saying it’s not valid but it’s interesting Pontiac chose not to do this on the RAIV or RAV or the SD RAIV for 70. Etc. I agree w others it seems odd they didn’t employ this in special engines later on. Again not to say a bean counter couldn’t have nixed it for mass production but why not reappear in special engines ?

[tom s]

on conventional pontiac heads,MOST head gasket failures are on the 2 center cylinders mostly because the 2 exhaust valves are together.Anything that can be done to help cool the engine in that area cant be a bad thing.Note,I have never done the cooling hole on later engines.Would not have been necessary on Tunnel port head builds.JMHO,Tom

[25stevem]

Any folks know if the plug mod in post 50 works in conjunction with the block being filled up to the water pump holes?

[TRADERMIKE 2012]

Originally Posted by tooski

Those are not drain holes. Water from the pump pressurizes the block. The water then flows up through those holes into the head, then forward to the water crossover and out the thermostat. Those three holes essentially short circuit the cooler water from going to the back of the block. By blocking them, more cooler water is forced to the back of the block, up to the heads and then back to the front through the heads.

Mikes Reply: Yes, I can visualize "How the flow of water the factory delivers compares to the 421 fix now". Good explanation.Mike-out.