Stock head stud threads are M10x1.5

Why barrel nuts and not flange nuts?
 

The barrels seem like the 1964 solution to not having enough room for a 17mm socket of standard OD.

Witness the three barrel nuts on each cam carrier, where the semi-circular relief is tight.

So perhaps reduced head flange nuts were not available in 1964 or earlier when the engineers for the 911 gained their knowledge of such details? Or it wasn't clear at that time that they were as strong? Or something.

I started wrenching on a motor scooter in maybe 1953 or 4, but I didn't start contemplating strengths of materials and similar engineer-like things until the late 1980s when I read Carroll Smith's Nuts and Bolts book. So I have no clue as to when flange nuts came into play in any mechanical field.

I do know that some fastener parts, which Smith declared to be worthless, are in use to this day. For instance, split washers. Smith ridicules the notion that these will a) if the fastener elongates under load, keep the clamping force which the original stretch gave, or b) did their cut endges into the nut or bolt head on one side, and the clamped piece on the other side, so as to provide mechanical resistance to loosening of the fastener beyond that provided by the clamping force.

Of course, he didn't think Schnorr washers were useful either.

So maybe engineers, like the rest of us, focus on what hasn't worked, or what they think can be made to work better, and forget about optimizing anything else. Like the barrel nuts: if everything is right, they certainly work. Only with the ever increasing HP/liter has anyone thought that the reduced impeding of cylinder head air flow of a barrel nut compared with a flange nut was worth thinking about. And maybe that is negligible anyway?

Ah, thanks for the correction Walt- that'll make things much faster and easier to either find locally or add it to my "hurry up and order it from PP" list.

Whatever the reason was for barrel nuts, I guess Porsche did the best anyone could have done in 1964- certainly better than I would have done at that time!

Speaking of Schnorr washers, what do you think of the idea of using those under the head nuts in stead of the stock flat washers (assuming I can find some with the correct dimensions)? Or is that overkill and the main reason for re-torquing the heads has more to do with the "gasket thingies" settling in?

What about this washer? Its a little large for the OD but you can turn them down to 21mm so they fit, they are about 2mm thicker than the originals.

McMaster-Carr

I like having extra options! Thanks Craig!

Curt

Another option (albeit more expensive than just new nuts) is Supertec headstuds.
they also have an open-ended nut.
Good quality stuff.
With Supertec studs you don't need to worry about how much stud is sticking out of the case. 135, 137, 145, all good, screw in and fasten.