I am going to buy some AFR 195 heads for my 400 sb and have been reading the old posts about steam holes. Lots of info but no definative reasons why that I could find as to why they are needed. Knowing GM wouldn't spend the money on the extra steps to drill the 6 holes in each head and block if not necessary I took a real close look at the ones in my heads and block. There appears to be a very small pocket that could trap steam given the proximity of the ports to where the water flows from the block into head. The exhaust side is more open then the intake which is shrouded by head bolt cavitity. The casting in both the head and block are thin in that area so maybe the steam hole relieves just enough temperature to prevent any cracking under normal driving.

In reading the AFR procedure for drilling holes in their heads they state to use an 1/8" drill bit. The factory holes are 11/64" on the intake side and 1/4" on exhaust side on both heads and block. The angle on GM heads was dilled in at 15* but needs to be 30* on the AFR's and drilled much farther to hit the water jacket.

I will follow the manufactures advice and drill the holes per instructions but still do not fully understand why they are needed or even how GM determined the holes were needed.

There is small areas that will trap air if the holes aren't there, most notibly on the exhaust side. The 1/8" holes is all you need.

For an all out race engine, the holes are not needed because of the higher sustained rpm, the coolant circulation is swift enough to prevent steam pockets from forming.
BUT, for a street/performance/driver---------------ADD THE STEAM HOLES!!!!!!!!!!!!!!!!!
Ocassional idling and/or low speed driving will not create sufficient coolant circulation to keep steam pockets from forming.
The steam pockets can form because of the siamesed cylinders in a SB400. Thus, no coolant circulation between the cylinders to flush away any steam that could form at low rpm.

The steam pockets can form because of the siamesed cylinders in a SB400. Thus, no coolant circulation between the cylinders to flush away any steam that could form at low rpm.
To amplify this a bit, consider that in the non-Siamesed engines, steam in the block would rise from the "exhaust" side of the deck--physically lower--and flow like an air bubble across the underside of the deck surface to the "intake" side of the deck--higher up. At that point, the steam can rise through the larger coolant holes into the head.

The Siamesed cylinder bank acts like a dam, preventing the steam from rising to the intake side of the deck surface where it would "bubble out" of the existing coolant holes into the head.

Since the cylinders are in the way, the lower three steam holes on each side allow the steam to bubble up into the head; and from there it condenses back into liquid or is expelled into the intake manifold and then out the thermostat.

I'm going to admit my ignorance; I have no idea why the upper set of steam holes exists.

The last 400 I worked on the steam holes where blocked by rusty muddy crud.

Dave

Drill the suckers and be done w/ it so you won't have to worry!

It's easy enough done! Use a 400 headgasket as a template and watch the second set of holes angle! I want to say, use an 1/8" and a 3/16" bit! (Maybe a 7/32" bit??)..

pdq67

No holes here and no issues.
Also isn't a street car but does see street duty... but very limited and little traffic.

A lot drag cars plug the holes in the block for strength and a larger sealing area, but if you plan on idling at a stoplight drill the holes it is easy to do and doesn't have to be real precise. use the gasket as a template and drill them with a hand drill (MAKE SURE TO CENTER PUNCH SO THE BIT DOESN'T WANDER AND END UP IN THE FIRE SEAL AREA!)

Thanks guys I was planning on drilling them. Just trying to understand how that relatively small are that can trap steam could be a problem.

AFR will drill them for you. There is small labour charge for it.

Dave

What ever the charge it would be well worth not risking something going wrong. Thanks for the info. I assume since they do not sell direct to the public the ordering dealer would have to request it.

AFR will drill them for you. There is small labour charge for it.

Dave

But well worth it, they are a pain to drill :(

Phil,
If you decide to drill them yourself, get some of the best drill bits you can find and plenty of cutting oil. Go very slowly and keep pulling the drill out and cleaning it. The one side is almost a 1" deep hole and it is very easy to snap the bit, ask me how I know !

Thanks Bill I am going to avoid drilling myself if at all possible. According to the instructions on the AFR site the 2 that drill at the 30* angle will be 2.25" deep before hitting water.

A reputable engine builder told me, don't drill them and you may or may not have a problem. Drill them and you won't have a problem. This was in a long rod 400 used on a dirt oval.

Thanks Bill I am going to avoid drilling myself if at all possible. According to the instructions on the AFR site the 2 that drill at the 30* angle will be 2.25" deep before hitting water.
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Hi Phillip,
I just had a set I sold to a gent in Australia drilled for a 400, not a big deal

And on the heads, email me & I will see that you get what you need & will most likely save you a buck or two in the process ;)

Thanks Mike. Email is on the way.