The Stovebolt Site Forums The Shop Area Engine Shop Hole in the Thermostat ?

I remember reading something here on the SB Page about drilling a hole in a thermostat. What was the reasoning behind it ?? Good idea, Bad idea ??

Bill,

I don't really know for sure, but:

The 261 engines had a special thermostat with a hole in it that allowed fluid to always circulate. This was supposed to help to keep hotspots and vapor pockets from forming. The risk of these hot spots and vapor pockets was localized overheating (and cracking) of the head/block.

I trust that others will correct me if I am dreaming.

Tim

GMC Syclone Typhoon says "Prevent Overheating: ...you can drill a small hole in the thermostat to prevent an air pocket from forming when you refill the coolant system, thus causing the thermostat not to open."
Stant Thermostats actually sell a thermostat with a hole in it and a "jiggle pin" for venting.
Viper Club "If you are servicing your thermostat at this time, drill a small (3/32" or so) hole in the new one, which will greatly aid the first fill, as it will allow air to get by the thermostat, even though it is closed."

This is a trick used by lots of fellas on old tractors, specially ones with "stat" in upper hoses like my MM has. Seems it helps in "bleeding" when filling and also allows some flow so as not to have hot spots in system as stated above.

The other thing you can do is tap a bleeder screw into the thermostat housing. My BMW has one from the factory. Makes filling the system really easy.

My hot rod Lincoln (90 Mk VII - which now has a positive displacement supercharger on it) has a hole in the thermostat as well. I added one to the 160 degree thermostat I just stuffed in there. Trying to keep heat down to avoid detonation under boost.

I use a Robert Shaw thermostat in the '76 with a couple of 3/16-1/4" holes in it (Drilled myself). It dosen't warm up real good in the winter ('have to put some masonite in front of the radiator) but it definately helps in the summer when I'm towing with the a/c on.

I was having trouble with my '72 Chevy C-10 six-250: while driving from a cold start, the temp gauge would go way over to the hot mark (within one mile of driving), then suddenly the 'stat would open and the gauge would drop back to normal.

Needless to say, this got my attention. I replaced the 'stat several times, with the same result each time.
Finally, a friend suggested that there was an air pocket, so I drilled a 1/8" bypass hole in the 'stat flange, figuring that would allow enough circulation to bleed any air bubbles and release any trapped steam.

So far it's worked great, although that little 1/8" hole adds a surprising amount of time to how long it takes the engine to reach "operating temp". But, it's a nice gradual rise , no nasty spikes to hot accompanied by the unmistakable aroma of anti-freeze!

Frank 54 is right, 1/8" is large enough.

This is an old hot rodder trick and works well on a fresh engine that is being filled up with coolant for the first time. Sure wouldn't want a steam pocket to form when you're trying to break in the cam . Small Block Chevy's are famous for this problem, as decribed above "while driving from a cold start, the temp gauge would go way over to the hot mark (within one mile of driving), then suddenly the 'stat would open and the gauge would drop back to normal."

Frank, I know you were talking about a six, but I have seen this too.

Drilled a hole (1/8") in the 'stat on my '51 GMC. It does slow down the warm up, but it still comes up to 170-180 degrees and stays there.
Overall, a worthwhile trade-off .

GMC 248,

Yeah, this one had me "buffaloed" for a spell!

I even tried two different brands of t-stat (Stant & Robt. Shaw), with the same results.
This was in the winter and the weirdest thing was, even as the gauge was climbing for "Hot" , the heater was blowing cold air!
This struck me as odd, since the six is supposed to have a by-pass cooling system so that coolant can still circulate in the block while the 'stat is closed.

So anyway, it's behaving now; next time around, I'll drill a 1/16" hole and see if strikes a better balance between warm-up & "spiking".

Just drill the hole and forget about it! It's easier than shopping for a new head, and it sure won't hurt a thing. So just do it!!

this old trick is still alive and well. I just changed a stat in a Yanmar 3 cyl diesel last week. It had a check valve, or jiggle pin as some call it, in the hole to speed the warm up. I saw this type stat in another piece of equipment sometime in the last year or so too. Just can't rember what it was in right now. A burp tube will do the same thing, but drilling one hole in the stat is a lot faster. Mike