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Thermostat........Drill A Hole Or Not?

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Old 08-11-2015, 01:09 PM   #1
toobroketoretire
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Default Thermostat........Drill A Hole Or Not?

I have never drilled holes in my thermostats and I have never had any problems. After reading many posts in Chevrolet forums about the "need" for a hole I decided to do it last winter just to see what difference it made. I drilled two 3/32" holes 180 degrees apart and reinstalled it. Well, right off the bat it needed a full 10 miles of driving before my heater began to work so I came to the conclusion the "need" for drilling a small hole or two is an urban myth.

First of all most intake manifolds are counter bored about 3/32" deep which is about 3 times the thickness of the thermostat flange. Which means coolant leaks around the edges of the thermostat anyway which also means a hole is just making the amount of leakage worse. And then coolant also leaks thru the center of the thermostat so what in the heck is the hole supposed to accomplish other than making the "hole driller" feel needed?

So what are the opinions of the forum "experts"? Drill a hole or not?
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Old 08-11-2015, 01:13 PM   #2
JimLentz
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The last thermostat I purchased had a hole already in it with a check valve of sorts. I believe it works by allowing the hole to be open for air, but gets closed when water/coolant presses against it. Do you need the hole? I can't say for certain.
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Old 08-11-2015, 01:17 PM   #3
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I do but I also delete the bypass in the block at the water pump. The bypass in the block gives #2 cylinder a little extra cooling. I like to run a small hole in the t-stat and two 1/4 lines from the rear of the intake and that equals about the same area as the bypass. I'll also use a hi-flow T-stat, because the engine pumps more than just air/fuel & exhaust.
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Old 08-11-2015, 01:18 PM   #4
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You do not "need" a whole - if you did, it would come with one. I am of the "Urban Myth" camp, but do whatever makes YOU feel good!
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Old 08-11-2015, 01:30 PM   #5
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You DO need the extra holes, and yes they hinder bitter winter warm ups, no way around that issue.....

Years ago there was 2-3 hole diameters in the stat passage ways....had nothing to DO with the opening temp 160/180/195 BUT the 195 passage way was about 1/4 inch smaller diameter then 160-180 stats, so when in summer the thing is open, fine, but the 195 stat would limit the flow, as do ALL of them now....

I drill 2-3 holes in the rim to aid flow in Florida, the changes in stat making came around maybe a decade ago.....

So if in a warm/hot climate, you need the holes to increase flow with a modern/new stat....thank the EPA for this BS.....
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Old 08-11-2015, 01:49 PM   #6
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Gene i just cant disagree more on this. Maybe the new t-stats do have holes in them and i cant explain that. But as far as i know the holes that were drilled by owners were the fix for those that didnt know how to fill the coolant system on a sys refill. Owners kept trying to fill the block by using the radiator only. They couldnt figure out that if they popped up the t-stat and filled the block from there they wouldnt have such a low volume of coolant after filling the radiator at the cap. I dont know how many of those radiator fill necks have sold because owners cant figure out once they fill behind the t-stat they can fill the rad and sys right up to the rad cap. Radiator fill necks are a cure for lack of understanding as they somewhat work - more so if install at the high point in hose/sys - but untill the t-stat opens up the block is still gonna be full of air - now holes would help for that.

I quit drilling t-stat holes long ago and always fill the block behind the t-stat.
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Old 08-11-2015, 02:01 PM   #7
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But as there's so much leakage around the edges what do the holes accomplish other than more leakage? Are you suggesting the college degreed automotive mechanical engineers just don't know what they're doing? As my un-drilled thermostats have never given me any problems why change anything? And why are only the C3 Corvettes in need of these holes?

Here's how I see it: The thermostat prevents the coolant from circulating thru the radiator until the coolant in the engine block and heads reaches the minimum desired temperature at which time the thermostat opens and allows the coolant to begin flowing thru the radiator. As the thermostat already leaks what good do the additional holes do other than allow it to leak more?

Please explain to me what the holes do to improve the operation of the thermostat.
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Old 08-11-2015, 02:02 PM   #8
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doesn't the cooling system already have a bypass? what would a little 3/32 hole accomplish that a 3/4" bypass hose does not? (that's a rhetorical question...unless you modified your small or big block, there is already a bypass in place from the factory)
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Old 08-11-2015, 02:02 PM   #9
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Quote:
Originally Posted by cardo0 View Post
Gene i just cant disagree more on this. Maybe the new t-stats do have holes in them and i cant explain that. But as far as i know the holes that were drilled by owners were the fix for those that didnt know how to fill the coolant system on a sys refill. Owners kept trying to fill the block by using the radiator only. They couldnt figure out that if they popped up the t-stat and filled the block from there they wouldnt have such a low volume of coolant after filling the radiator at the cap. I dont know how many of those radiator fill necks have sold because owners cant figure out once they fill behind the t-stat they can fill the rad and sys right up to the rad cap. Radiator fill necks are a cure for lack of understanding as they somewhat work - more so if install at the high point in hose/sys - but untill the t-stat opens up the block is still gonna be full of air - now holes would help for that.

I quit drilling t-stat holes long ago and always fill the block behind the t-stat.
Same here, but added a small single hole will allow the air the purge a little better.
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Old 08-11-2015, 02:44 PM   #10
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Quote:
Originally Posted by cardo0 View Post
Gene i just cant disagree more on this. Maybe the new t-stats do have holes in them and i cant explain that. But as far as i know the holes that were drilled by owners were the fix for those that didnt know how to fill the coolant system on a sys refill. Owners kept trying to fill the block by using the radiator only. They couldnt figure out that if they popped up the t-stat and filled the block from there they wouldnt have such a low volume of coolant after filling the radiator at the cap. I dont know how many of those radiator fill necks have sold because owners cant figure out once they fill behind the t-stat they can fill the rad and sys right up to the rad cap. Radiator fill necks are a cure for lack of understanding as they somewhat work - more so if install at the high point in hose/sys - but untill the t-stat opens up the block is still gonna be full of air - now holes would help for that.

I quit drilling t-stat holes long ago and always fill the block behind the t-stat.
I just replaced the ALL cooling system components in my '66 big block (except the radiator), and the lasts thing I did was fill the system through the radiator without the upper hose or thermostat installed. Since I had even drained the system with both plugs removed from the block, I knew that it was totally empty. As I saw the antifreeze rise up from inside the block into the thermostat housing opening, I knew that no air pocket would form and it was full. I then installed the upper hose and thermostat housing, along with a 180* thermostat with NO bleed hole. After a long drive ( heater running) and sitting overnight, it needed no additional coolant added. And the 427 ran "cool" as it always does.
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Old 08-11-2015, 02:45 PM   #11
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If you have a bypass hose it should not be an issue on our top mounted t-stat engines. More of an issue with them mounted lower on the engine. However I have seen initial start-up heat issues when there was no hole but have never seen an issue when a hole was in the t-stat flange. All it takes is one 1/16" hole to insure all the air can purge allowing water all the way up around the thermostat so it can work properly. That will make virtually no difference in warm up time or cause any other ill effects. Do what works for you. I'll drill the hole.
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Old 08-11-2015, 03:24 PM   #12
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No hole here, and no problems even when filled for first time after overhaul.
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Old 08-11-2015, 04:59 PM   #13
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You should only need a hole if you are running a high flow water pump like the Stewart without a bypass.
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Old 08-11-2015, 05:05 PM   #14
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My engine's thermostat came w/a hole.....

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Old 08-11-2015, 05:20 PM   #15
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I have drilled a small hole in the past, but most of the ones I have used lately already have a vent. It is just easier to fill, without having to go back and check. It made no difference in the cooling ability or how quickly it came up to temp. If you don't like it - dont do it.
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Old 08-11-2015, 05:55 PM   #16
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An easy way to bleed the air from the system when refilling is to remove the heater hose from the intake manifold and fill through the radiator till the coolant is at the heater hose nipple, then put the hose on and continue to fill.
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Old 08-11-2015, 07:09 PM   #17
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Unless you race or hit the track occasionally there's absolutely no need to drill a hole. If you have overheating problems best check your cooling system. If you're pushing more power then stock it's time for a cooler thermostat or more importantly a larger radiator.

I've raced for 6 years now and discovered that drilling a hole can help but too big of hole or too many can have a reverse effect never allowing the coolant enough time to properly cool off in the radiator.

A tiny purge hole would help the novice mechanics but really isn't necessary.

Last edited by vetpace78; 08-11-2015 at 07:14 PM.
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Old 08-11-2015, 07:33 PM   #18
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Quote:
Originally Posted by big block ken View Post
I just replaced the ALL cooling system components in my '66 big block (except the radiator), and the lasts thing I did was fill the system through the radiator without the upper hose or thermostat installed. Since I had even drained the system with both plugs removed from the block, I knew that it was totally empty. As I saw the antifreeze rise up from inside the block into the thermostat housing opening, I knew that no air pocket would form and it was full. I then installed the upper hose and thermostat housing, along with a 180* thermostat with NO bleed hole. After a long drive ( heater running) and sitting overnight, it needed no additional coolant added. And the 427 ran "cool" as it always does.
This is SOP at a well know Corvette shop I hold in high regard, fill the system from the intake manifold thermostat opening. I will always drill a 3/32 hole in a Tstat if I can't observe a vent opening in one but, I don't drive my Vette in winter snow either
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Old 08-11-2015, 08:03 PM   #19
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I drill the hole on the Vette, right/wrong don't know but it works for me.
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Old 08-11-2015, 08:06 PM   #20
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My vote is, no hole.
Years ago, I had a smallblock powered pickup that took over a half hour to warm up in the cold weather. When I opened it up, I discovered that someone had drilled two 1/8" holes in the 'stat. Replaced it with an unmolested one and the truck had heat in less than 4 minutes.
Most engine wear takes place in the first 5 seconds after a cold start, before the oil has had a chance to get anywhere useful. Chevrolet determined that the rest of the wear occurred between then and when the engine was fully warm. Once an engine is fully warm, NO measureable wear takes place.
Drilling more than one TINY (like, 0.040") hole just increases the warmup/high wear part of the engine running cycle.
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