I am building a 406 sb for my 87 IROC-Z. I bought a set of Air Flow Research 220cc heads without steam holes because i planned to just have them machined by a shop. I called an engine builder, who happened to be an AFR dealer, said he wouldn't drill the steam holes in them because they are different than other heads. I then talked to an AFR tech and he said they are no different than any other head, the only difference being that you would have to drill farther into the head to reach the water jacket. I want to know if this was the case, if anyone has experience with these heads. It's not that i don't believe the AFR tech, but it's better to have a second opinion on this.

 

Normally whenever a non 400 head is installed on a 400 block, you simply use a head gasket as a template and drill the steam holes into the head. 1/4" drillbit.

Although may people will say that the steam holes are not required, it doesn't hurt to drill them. The downside is the heads probably won't seal properly if installed on a non 400 block after the holes have been drilled. I guess it all depends on which gasket is used.

 

I'd go with the manuafacturer's information. This is straight from AFR's website:
https://www.airflowresearch.com/faq.php
Does my 400ci Block require steam holes

 

400 blocks ABSOLUTELY require steam holes for street driving. Steam pockets will build up any time the RPM stays low for awhile (say, sitting at a traffic light idling), and result in strange overheating behavior while the temp gauge reads fine, because the places where the steam gets trapped under the deck and accumulates will get VERY hot.

For a PURE RACING application, where the engine never gets all the way hot and the RPMs never drop low enough for the water to flow slow enough for that to happen, not so important; but CRITICALLY NECESSARY for a street setup.

You can drill them in any heads. Here's a pair of Dart IEs with them added.



I usually use a smaller drill bit than ¼"; maybe 3/16" or so; but it really doesn't matter. I see AFR recommends 1/8" which oughtta work just as well too. All ya gotta do, is provide a path for any steam bubbles that might get trapped at the top of the siamesed cyl walls, to continue upwards and let liquid water back in there. You're not trying to promote "flow" there or anything, just bleed off any gases.

The ones on the intake side need to be angled away from the bolt hole; pretty easy to see why, looking up into the water jackets. There's a HUUUUJJJJJE lump of metal the bolt hole goes through, which if you drill the hole straight (i.e. parallel to the head bolt), you'll NEVER get out of until the bit emerges in the rocker gallery. You can see in the pic that they look somewhat oval, from the angle they're drilled at.

Just lay a 400 head gasket on the head, align it to the dowel pin holes with a coupla drill bits or some such, mark em with a center punch, and use a hand drill. Shouldn't take 30 seconds a head in aluminum ones, might take a whole minute in cast iron.