I have never heard of plugging the steam holes so I cant say, but they are there because the cylinders are siamesed,in other words no gap between them in water jacket.

 

Yep....if they're plugged, redrill them.

They're very necessary.

 

The only time you should plug those holes is if you're only driving it a quarter-mile at a time, then shutting it down until it cools off. Driving a plugged 400 on the street is just asking for overheating problems and block crackage.

 

I never drill the heads for use on a 400 block intended for racing only, which I assume a Lefthander modified would qualify. At high water pump speeds, they are not required. For street use, the holes in the head and the block are required.

 

I am building a 406" SB and have received mixed opinions on this subject, just like here. I'm using Dart 215 heads and while drilling the "necessary" holes, I ended up snapping a drill bit. My bad, but I took the head to the machinist that did the prep on my block and he says he never drills the holes (in the heads) for the race (over 3,000 rpm) engines that he builds and said he couldn't take the bit end out. This was a 'upper' hole, intake side. So now I had a problem; to drill or not to drill the rest of the holes. My engine will be used on the street with weekend strip use. I looked at the situation closely and came up with this logic: The block deck surface is at a 45 deg. angle to horizontal. Any steam pockets formed in the lower (exhaust) side would tend to stay right there and cause problems. It would take a lot of water movement to remove it.
The intake (upper) side is totally opposite. Any steam pocket formed would tend to move away (rise) from the point of origin with any engine vibration at all, BUT a hole still exists in the block which would capture the steam.
My solution (hopefully) is to keep the exhaust side holes as drilled by the factory and drill the heads to match. Intake side I'm plugging the holes in the block (to eliminate the pocket) and not drilling the remaining holes in the heads. We'll see how my logic pans out but as long as I don't idle the engine for lengthy periods I think I'll be OK...
With your roundy-round car and aluminum heads you shouldn't have any problems with the plugged holes.
In addition, Dart doesn't drill their little M blocks at all and they're siamezed. It's a factory Chevy thing.

 

I am building a 406" SB and have received mixed opinions on this subject, just like here. I'm using Dart 215 heads and while drilling the "necessary" holes, I ended up snapping a drill bit. My bad, but I took the head to the machinist that did the prep on my block and he says he never drills the holes (in the heads) for the race (over 3,000 rpm) engines that he builds and said he couldn't take the bit end out. This was a 'upper' hole, intake side. So now I had a problem; to drill or not to drill the rest of the holes. My engine will be used on the street with weekend strip use. I looked at the situation closely and came up with this logic: The block deck surface is at a 45 deg. angle to horizontal. Any steam pockets formed in the lower (exhaust) side would tend to stay right there and cause problems. It would take a lot of water movement to remove it.
The intake (upper) side is totally opposite. Any steam pocket formed would tend to move away (rise) from the point of origin with any engine vibration at all, BUT a hole still exists in the block which would capture the steam.
My solution (hopefully) is to keep the exhaust side holes as drilled by the factory and drill the heads to match. Intake side I'm plugging the holes in the block (to eliminate the pocket) and not drilling the remaining holes in the heads. We'll see how my logic pans out but as long as I don't idle the engine for lengthy periods I think I'll be OK...
With your roundy-round car and aluminum heads you shouldn't have any problems with the plugged holes.
In addition, Dart doesn't drill their little M blocks at all and they're siamezed. It's a factory Chevy thing.

The answer is no , wrong and all that. Steam pockets will form, and if your thinking that high speed coolant will blow them out think again. Once steam pockets form they are very hard to move and in this case the cause of formation is constantly renewing itself.

The holes arn't there for cooling the heads, they're there to vent steam trapped against the lower outside of siamesed cylinders where they join the deck. Normally steam and entrapped gases rise to the highest point in the cooling system, except for the very end cylinders this means that without a vent back to the head, there is no place for this steam to go but form larger bubbles. Steam bubbles large or small offer no cooling and the outside of the cylinder wall where it joins the deck overheats. This leads to scored pistons and rings, and cracks in the upper cylinder to deck area.

Chevy, in it's infinite wisdom put holes there to let this trapped steam out. That requires additional tooling and machining which adds cost to a cost adverse company, do you really think if it wasn't needed that they would just go thru the expense of this as a fun exercise. At the least you can look at it as they we're doing CYA in case this was a problem that would add to warranty expenses for the company. You can bet your bottom dollar that they had problems with test engines and took this step to correct the problem.

So Dart doesn't put em in, one might conclude that they either core a small passage in the top of the siamese casting to allow a little coolant to pass on to the upper side of the water jacket, and or they are casting a lot thicker than the factory does so their blocks can structurally withstand the added thermal stress. That ain't grounds for the same assumptions on a thin wall factory block that you know doesn't have any secret passages between the siamese cylinders and hardly has enough material to keep the cylinder block connected to the crankcase.

Bogie

 

I never drill the heads for use on a 400 block intended for racing only, which I assume a Lefthander modified would qualify. At high water pump speeds, they are not required. For street use, the holes in the head and the block are required.

Correct, it can be an issue without them below 3000 RPM on the street.

 

The answer is no , wrong and all that.
Chill out, man. It's MY frikken block!

I don't think I asked a question here. Just stated an opinion and what my course of action is going to be.


"The holes arn't there for cooling the heads."

Duhhhhh!~ Never said they were.

So, tell me, Old Guy, just how many 400's have you had first hand experience with overheating?
I think I'll take advice from an experienced engine builder, thank you.

 

Some builders tap these and other deck holes and fill them with plugs, mill flat, then redrill the water passages. The idea is to stabilize the deck as much as possible on a factory engine. The steam holes are not redrilled.

I think this arguement has gone on many times on this site as well as the overheating thing. I have two factory 400 blocks, both with steam holes, one turbocharged with drilled Dart heads, one with drilled OE heads. They both run without heating problems.

I will say drilling the Darts is very unnerving as you will have to drill about two inches deep on one of them to get through.....hard to do on a new set of heads. They do need to be at a certain angle, but it has been too long for me to remember what that is.

 

i have a 406 with eldy heads and only drilled the steam holes on the exhaust side the instructions that came with the heads said not to drill the intake side and i have not had any over heating issues

 

Got great response, lots to "chew" on. I'm using dart 220s and super victor with massive porting. They did jacket all the water ports in the heads before they decked it. Looks rather impressive.
Since I posted this, I went back to machine shop and wanted all the details. They assured me that steam holes are not needed and they have been blocking them off for 30 years (a little bragging there). One of the reasons sited is the operating rpm of the engine. They assured me, unless I'm spending alot of idleing time, no steam will be there at all. I'm using a restrictor instead of thermostat on a triple bypass radiator and my operating rpm is 4500-8000 rpm (probably 7K). 1 of the options they gave me was filling the block to the bottom of water pump jackets but told me to wait til I go .040 over would be fine. They say that 350s they do for oval, 1/8 or 1/4 miles, they usually use block filler but with my siamesed cyclinders, I can wait til 040.
Thanks for all the experience/opinions. And, clearing it up, 1/2 asphalt oval track racing, not street.

 

i have a 406 with eldy heads and only drilled the steam holes on the exhaust side the instructions that came with the heads said not to drill the intake side and i have not had any over heating issues

Now THAT'S what I'm talkin' 'bout!! Somebody with first hand experience.
You can pump out all the theory you want but until you've done it, it's worthless.

 

"I will say drilling the Darts is very unnerving as you will have to drill about two inches deep on one of them to get through.....hard to do on a new set of heads. They do need to be at a certain angle, but it has been too long for me to remember what that is."

The angle is 30 degrees from vertical on the intake side and yes, it is unnerving. AFTER snapping a drill bit, I learned from the machinist that I need to use lots of lube and go slow, vewy.. vewy.. slow.. and remove the chips often. I had to get a extended bit to get to the water jackets and that's where it broke, while breaking through (on the next-to-last hole).

Also, the hole in the block on the intake side is 1/8th" and the exhaust side is at least 1/4".

 

Block filler can be necessary on even stock bore blocks. I have a section of cylinder wall from a stock 400 block that was .030 overbore and the piece of cylinder wall is .040 indicating that as new, the wall was only .055, not enough for much horsepower. Any serious engine based on a stock block should be sonic checked first.

 

"I will say drilling the Darts is very unnerving as you will have to drill about two inches deep on one of them to get through.....hard to do on a new set of heads. They do need to be at a certain angle, but it has been too long for me to remember what that is."

The angle is 30 degrees from vertical on the intake side and yes, it is unnerving. AFTER snapping a drill bit, I learned from the machinist that I need to use lots of lube and go slow, vewy.. vewy.. slow.. and remove the chips often. I had to get a extended bit to get to the water jackets and that's where it broke, while breaking through (on the next-to-last hole).

Also, the hole in the block on the intake side is 1/8th" and the exhaust side is at least 1/4".

Yeah, I talked to some folks before doing it. I used a regular length drill chucked shallow and did go very slow. I think it took a couple hours just to drill the holes if I remember correctly. I distictly remember sweating bullets the entire time worrying about exactly what happened to you happening to me.

If you are going to run the holes only on exhaust side you can have someone use TIG to fill 1/4" or so of the intake holes and the one with the bit then have them touched off with a mill......like they were never there.