I've been using two 1/8 holes with good results. They seem to help a lot with wild temp swings just prior to the thermostat opening

 

using 3/16 holes with good results

 

I have 4 1/8" holes. Blocked off bypass in a smallblock. Works great!

 

Don't drill ANY damned holes. If the holes were needed, the thermostat engineers would have put them there. This assumes you haven't blocked the factory-engineered coolant bypass, or if you did, you don't have an open-flow heater core coolant circuit.

Robertshaw thermostats were made with a small stamped vent, probably about 0.062 or so. 1/8" is too fukking huge, never mind two or four of 'em, or 3/16".

Might as well take the thermostat out.

Don't screw with the factory bypass, don't drill the thermostat.

 

I drill one 1/8 inch hole in mine, it helps in letting the air out of the heads when you're filling the engine with coolant for the first time.

Maybe Grouchy, I mean, Schurky hasn't seen the newer cars thermostats with the vents in them...

 

I've seen thermostats with vents (the aforementioned Robertshaw units, with a single, tiny vent hole. It doesn't take an eighth-inch hole to vent air during coolant-fill. I've seen thermostats with "jiggle valves" that vent until the water pump pressure closes the jiggle valve entirely.

I've never seen a thermostat with enormous 1/8" or larger open "vent" holes as engineered. If you've seen something like that, post a picture, part number, and the application. So far as I know, this whole "1/8" hole(s) in the thermostat flange" thing is a shade-tree attempt to "fix" something that isn't wrong to begin with--or--it's compensation for an improper coolant fill procedure--or--it's because there's a fault in the cooling system and someone is modifying the thermostat to "cure" something else.

True enough, if you defeat the OEM bypass, and you aren't able to use the heater core coolant path as a bypass, the thermostat will need a hole or two popped into it. (but why defeat the bypass system?)

 

I put some drill bits into a Robertshaw thermostat vent stamping, to gauge it's size.

I had to use the second-smallest drill bit that I own-- #79--to get it to fit into the triangular hole. A #79 drill bit is 0.0145, just two thousanths larger than one tenth the size of an 1/8" (0.125) drill bit.

Of course, the actual area of the vent stamping is larger than a #79 drill bit due to the triangular shape--I guess it's approximately equivalent to a 1/16 or 1/32 hole, but hard to actually measure. It's damn sure smaller than a 1/8 drilling. It'll easily vent air, but wouldn't flow water very well at all.

 

It looks like stewart defeats the original factory bypass system that keeps the coolant recirculation restricted to the block and heads and now uses the radiator as part of the bypass. Why bother? What is the advantage?

 

Not arguing with you just showing what I thought you asked for. I believe your thoughts. But removing the bypass hose and not running a heater, the holes in thermostat work for me. Rather cleaner look with out by pass hose IMO.

 

Milodon makes good quality thermostats. I have used my self, installed or recomended it to friends and so far no one has failed and in some cases cured problems for friends with high flow waterpumps.
They look like the Robert shaw design, according to summit when I asked a while back they have Us as country as origin.
http://m.summitracing.com/parts/mil-16400

http://m.summitracing.com/parts/mil-16401

 

Interesting point from PRC:

It isn't often that two widely diverse problems can be resolved with the same solution. What's even more unusual is that the solution is very simple and inexpensive. The first problem is an air pocket created under the thermostat after installing a new motor or simply changing your coolant. This air pocket causes excessive heat build up in the engine until the air in the pocket is heated sufficiently to open the thermostat. This added heat is undesirable for any engine, but can be especially true for engines fitted with numerous aluminum components.

The second problem is thermal shock that may occur at the radiator when hot coolant is released by the thermostat when the ambient temperature is near 32 degrees F. Duane Davis, Owner of PRC, observed cracks developing in radiator cores in the area of the inlet in a few rare instances. He noted that this only occurred with customers located in Northern part of the United States and in Canada and only when the owner drove his car in frigid temperatures.

http://www.streetrodstuff.com/Articles/Cooling/Thermal_Challenges/

Another effect of the holes in the thermostat is that it reduces the loop gain at the crack point of the thermostat.

 

What's interesting to me is how these problems could have continued so long without being discovered by the automotive engineers at FoMoCo, GM, and Chrysler. Apparently it was left up to the shade tree mechanics to discover not only the problems, but the very simple and inexpensive solution to both problems.

Where can I read about "loop gain at the crack point of the thermostat"? Sounds interesting.