T-stat to drill a hole or not to drill a hole? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

1969 Project · 02-22-2018, 03:57 PM

I am getting ready to replace my thermostat and was wondering if it is necessary to drill a 1/8 hole in it before installing , if so what are the benefits? I heard that by drilling a hole it could cause the temp to fluctuate. The PO did not drill holes in the current thermostat and the temp is at a constant 190.

**leddzepp** · 02-22-2018, 04:04 PM

If it ain’t broke, don’t fix it. I’ve never drilled a hole in one, never had any issues either.

Rich69shortfleet · 02-22-2018, 04:20 PM

Necessary? No. Helpful? Yes. It allows the engine and radiator to be completely filled with no air pockets being trapped. Drilling one or or two 1/8" holes in a thermostat creates no issues or fluctuating temps that I've ever seen. Many thermostats nowadays come with holes already in them....

Toms67 · 02-22-2018, 04:30 PM

X2.

demian5 · 02-22-2018, 05:12 PM

Quote:

Originally Posted by Rich69shortfleet

Many thermostats nowadays come with holes already in them....

They have the jiggle thing in them though.

mikecaddy500 · 02-22-2018, 05:42 PM

Always drill two holes in them, it stops air pockets.

**Grumpy old man** · 02-22-2018, 05:45 PM

Never drilled holes in any of them over 45 years ,So now it's the thing to do ?

demian5 · 02-22-2018, 06:13 PM

Quote:

Originally Posted by Grumpy old man

Never drilled holes in any of them over 45 years ,So now it's the thing to do ?

Millenials...

weq92f · 02-22-2018, 06:32 PM

Apparently it's the in thing to do now. This thread is the first I've heard of it!

-klb

K20super · 02-22-2018, 06:50 PM

It also helps with aluminum heads because it is not as much of a shock due to temperature change when it opens.

harpo231 · 02-22-2018, 07:38 PM

My t-stat has patina ...

**jocko** · 02-22-2018, 08:05 PM

I have heard of this before (recently), but have never done it. It came up in a discussion about the lack of wp bypass passage in the L31’s vortex heads and this was brought up as a possible mitigation. Having said all that, I HAVE had an air pocket in a rad and, if this would prevent that. I’m all for it. But I do wonder why, if it’s such a good idea why they didn’t come like this originally and still don’t as far as all the replacements I’ve bought.

What is this jiggle thing you speak of Demian??

Hey! I used to live in Lake Forest!!

Toms67 · 02-22-2018, 08:58 PM

I had a Corvette years ago. And they are notorious for running hot. The person that built my motor. Put a bleed hole in the thermostat. And it did run cooler. I think maybe it helps with extra circulation. But it seem to work for me. Just my two cents.

hjewell2 · 02-22-2018, 09:09 PM

Quote:

Originally Posted by weq92f

Apparently it's the in thing to do now. This thread is the first I've heard of it!

-klb

Just a tad overkill - but yes one 1/8 inch hole drilled helps for the purpose of air pockets

garyd1961 · 02-22-2018, 09:46 PM

A bypass hose is what you need not a hole in your thermostat especially on a vortec. The bypass hose keeps the water circulating through the heads eliminating hot spots. I have never know a SBC to have a problem with air pockets, these old trucks ran with air in the radiator.

**HO455** · 02-22-2018, 09:55 PM

It isn't anything new. It is an old racer modification. I was doing it back in the 80's. The reason is that it makes it easier to vent air out when refilling the water. Used to be you were allowed to dump the water out of your radiator between rounds and refill it to cool the motor down for the next round. Some guys used to put little valves in the top of the intake water passages to vent air too.

1972RedNeck · 02-22-2018, 10:01 PM

Quote:

Originally Posted by Rich69shortfleet

Necessary? No. Helpful? Yes. It allows the engine and radiator to be completely filled with no air pockets being trapped. Drilling one or or two 1/8" holes in a thermostat creates no issues or fluctuating temps that I've ever seen. Many thermostats nowadays come with holes already in them....

$10 says the PO's thermostat has a little notch in the seat to let air through. If such is the case, no holes necessary.

1969 Project · 02-23-2018, 01:27 PM

Thanks Fellas! I don't have any issues with my engine running hot and if the cooling system is bled correctly wouldn't you not have to worry about air in the system?

6T7 C10 · 02-23-2018, 11:46 PM

FWIW I drilled one very small hole in mine. The heater core looks to be higher than the top of the radiator and I did it to purge air pockets faster. Some thermostats already have a hole with a rivet looking piece that accomplishes the same thing. I have heard of guys propping a thermostat open with an aspirin on reassembly to let air out then the aspirin melts away and the system goes back to working normally. Without a hole I imagine air could accumulate around the thermostat and it would not open as soon but once it did it would purge the air. Probably over thinking it a bit but love the trucks.

**Steeveedee** · 02-23-2018, 11:57 PM

I haven't ever seen an actual flow diagram of the coolant circulation system of a Chevy engine, but I suspect that the bypass port in the SBC water pump or the bypass hose in a BBC probably covers this issue.

**special-K** · 02-24-2018, 09:50 AM

Is it necessary to drill a hole in the thermostat? No, or they would come with the hole already drilled. Is it an effective way to keep a truck running cool as that thermostat allows in a wider set of circumstances? Yes. Is there any negative affect from drilling al 1/8" hole? No, only good. I never did it in all my years till a friend who is an ace mechanic suggested it when my truck seemed to run a tad hot after fresh heads bumped my compression. I'm a baby boomer and he is generation X. If it's a millennial thing to do, there's at least one good thing we can credit them

. But I think it's an old school track trick that has found it's way into the mainstream.

CANUCIT · 02-24-2018, 11:16 AM

Drill

BRL · 02-24-2018, 02:49 PM

Years ago I rebuilt a SBC and upon its first run my 50/50 mix kept spewing out of the radiator overflow and fill hole like a coffee pot peculator!

Assumed it was an air pocket trapped in there somewhere and was causing steam.

Removed the thermostat and drove a few days and eventually it settled down, so I put the stat back in.

I read someplace that the hole causes the engine to take a bit longer to warm up but can be helpful if the stat should seize closed.

davepl · 02-24-2018, 02:56 PM

Small block Chevrolets have an internal bypass in the water pump, do they not? Seems redundant...

BRL · 02-24-2018, 03:09 PM

Could very well be, that was my first rebuild and really don't know what was causing that problem.

ETA: Oh I think I see what you're saying, didn't know SBC water pumps had a bypass.

...oops!

02-22-2018, 03:57 PM	#1
1969 Project Registered User Join Date: Sep 2016 Location: Brentwood CA Posts: 180	T-stat to drill a hole or not to drill a hole? I am getting ready to replace my thermostat and was wondering if it is necessary to drill a 1/8 hole in it before installing , if so what are the benefits? I heard that by drilling a hole it could cause the temp to fluctuate. The PO did not drill holes in the current thermostat and the temp is at a constant 190.

02-22-2018, 04:04 PM	#2
leddzepp Moderator Join Date: May 2007 Location: Southern Cal Posts: 19,979	Re: T-stat to drill a hole or not to drill a hole? If it ain’t broke, don’t fix it. I’ve never drilled a hole in one, never had any issues either. __________________ 1972 C/10 Cheyenne Super SWB. Restored, loaded, slammed. 1968 C/10 50th Anniversary LWB. Unrestored, stock, daily driver/work truck. RIP ElJay RIP 67ChevyRedneck RIP Grumpy Old Man

02-22-2018, 04:20 PM	#3
Rich69shortfleet Registered User Join Date: May 2016 Location: Waverly, NE Posts: 334	Re: T-stat to drill a hole or not to drill a hole? Necessary? No. Helpful? Yes. It allows the engine and radiator to be completely filled with no air pockets being trapped. Drilling one or or two 1/8" holes in a thermostat creates no issues or fluctuating temps that I've ever seen. Many thermostats nowadays come with holes already in them.... __________________ 1965 Malibu SS L79 Restoration Page 1965 Two-Door Wagon Project Progress Page 2006 Pontiac GTO Preservation Project Rich Cummings Last edited by Rich69shortfleet; 02-22-2018 at 07:27 PM.

02-22-2018, 04:30 PM	#4
Toms67 Registered User Join Date: May 2016 Location: Sackets harbor ny Posts: 282	Re: T-stat to drill a hole or not to drill a hole? X2.

02-22-2018, 05:42 PM	#6
mikecaddy500 Registered User Join Date: Jan 2015 Location: howard city mi Posts: 83	Re: T-stat to drill a hole or not to drill a hole? Always drill two holes in them, it stops air pockets.

02-22-2018, 05:45 PM	#7
Grumpy old man Senior Member Join Date: Dec 2013 Location: Gods country East,Tn Posts: 8,545	Re: T-stat to drill a hole or not to drill a hole? Never drilled holes in any of them over 45 years ,So now it's the thing to do ?

02-22-2018, 06:32 PM	#9
weq92f Registered User Join Date: Oct 2016 Location: Dallas Texas Posts: 2,860	Re: T-stat to drill a hole or not to drill a hole? Apparently it's the in thing to do now. This thread is the first I've heard of it! -klb Attached Images __________________ 67 C10 fleet fuel injected '70 402, 700r4, 3.73 posi 07 335 sport turbo 6sp 94 Trans Am GT LT1 6sp posi -- sold after 22yrs 99 540 sport V8 6sp -- sold 73 240z L24 4sp -- given to friend 68 C10 step 350/350 3.73 open -- sold

02-22-2018, 06:50 PM	#10
K20super Registered User Join Date: Nov 2016 Location: Center ND Posts: 72	Re: T-stat to drill a hole or not to drill a hole? It also helps with aluminum heads because it is not as much of a shock due to temperature change when it opens.

02-22-2018, 07:38 PM	#11
harpo231 Registered User Join Date: Nov 2014 Location: Lincoln City DE Posts: 1,292	Re: T-stat to drill a hole or not to drill a hole? My t-stat has patina ... __________________ Stan 67 swb BLU MULE posted via flip-fone

02-22-2018, 08:05 PM	#12
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,931	Re: T-stat to drill a hole or not to drill a hole? I have heard of this before (recently), but have never done it. It came up in a discussion about the lack of wp bypass passage in the L31’s vortex heads and this was brought up as a possible mitigation. Having said all that, I HAVE had an air pocket in a rad and, if this would prevent that. I’m all for it. But I do wonder why, if it’s such a good idea why they didn’t come like this originally and still don’t as far as all the replacements I’ve bought. What is this jiggle thing you speak of Demian?? Hey! I used to live in Lake Forest!!

02-22-2018, 08:58 PM	#13
Toms67 Registered User Join Date: May 2016 Location: Sackets harbor ny Posts: 282	Re: T-stat to drill a hole or not to drill a hole? I had a Corvette years ago. And they are notorious for running hot. The person that built my motor. Put a bleed hole in the thermostat. And it did run cooler. I think maybe it helps with extra circulation. But it seem to work for me. Just my two cents.

02-22-2018, 09:46 PM	#15
garyd1961 Registered User Join Date: May 2016 Location: Henderson NC Posts: 975	Re: T-stat to drill a hole or not to drill a hole? A bypass hose is what you need not a hole in your thermostat especially on a vortec. The bypass hose keeps the water circulating through the heads eliminating hot spots. I have never know a SBC to have a problem with air pockets, these old trucks ran with air in the radiator.

02-22-2018, 09:55 PM	#16
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 10,787	Re: T-stat to drill a hole or not to drill a hole? It isn't anything new. It is an old racer modification. I was doing it back in the 80's. The reason is that it makes it easier to vent air out when refilling the water. Used to be you were allowed to dump the water out of your radiator between rounds and refill it to cool the motor down for the next round. Some guys used to put little valves in the top of the intake water passages to vent air too. __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban the WMB,1991 S(stink)-10 Blazer,1969 GTO, 1970 Javelin, 1952 F2 Ford 4X4, 29 Model A, 72 Firebird. 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

02-23-2018, 01:27 PM	#18
1969 Project Registered User Join Date: Sep 2016 Location: Brentwood CA Posts: 180	Re: T-stat to drill a hole or not to drill a hole? Thanks Fellas! I don't have any issues with my engine running hot and if the cooling system is bled correctly wouldn't you not have to worry about air in the system?

02-23-2018, 11:46 PM	#19
6T7 C10 Registered User Join Date: Feb 2015 Location: TX Hill Country Posts: 150	Re: T-stat to drill a hole or not to drill a hole? FWIW I drilled one very small hole in mine. The heater core looks to be higher than the top of the radiator and I did it to purge air pockets faster. Some thermostats already have a hole with a rivet looking piece that accomplishes the same thing. I have heard of guys propping a thermostat open with an aspirin on reassembly to let air out then the aspirin melts away and the system goes back to working normally. Without a hole I imagine air could accumulate around the thermostat and it would not open as soon but once it did it would purge the air. Probably over thinking it a bit but love the trucks.



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02-23-2018, 11:57 PM	#20
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,081	Re: T-stat to drill a hole or not to drill a hole? I haven't ever seen an actual flow diagram of the coolant circulation system of a Chevy engine, but I suspect that the bypass port in the SBC water pump or the bypass hose in a BBC probably covers this issue. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

02-24-2018, 09:50 AM	#21
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,862	Re: T-stat to drill a hole or not to drill a hole? Is it necessary to drill a hole in the thermostat? No, or they would come with the hole already drilled. Is it an effective way to keep a truck running cool as that thermostat allows in a wider set of circumstances? Yes. Is there any negative affect from drilling al 1/8" hole? No, only good. I never did it in all my years till a friend who is an ace mechanic suggested it when my truck seemed to run a tad hot after fresh heads bumped my compression. I'm a baby boomer and he is generation X. If it's a millennial thing to do, there's at least one good thing we can credit them . But I think it's an old school track trick that has found it's way into the mainstream. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ Last edited by special-K; 02-25-2018 at 08:36 AM.

02-24-2018, 11:16 AM	#22
CANUCIT Registered User Join Date: Nov 2017 Location: JOPLIN MO Posts: 59	Re: T-stat to drill a hole or not to drill a hole? Drill __________________ 1962 chevy short and wide 2015 z71 1967 SS Chevelle

02-24-2018, 02:49 PM	#23
BRL Registered User Join Date: Jun 2014 Location: Tahuya WA Posts: 510	Re: T-stat to drill a hole or not to drill a hole? Years ago I rebuilt a SBC and upon its first run my 50/50 mix kept spewing out of the radiator overflow and fill hole like a coffee pot peculator! Assumed it was an air pocket trapped in there somewhere and was causing steam. Removed the thermostat and drove a few days and eventually it settled down, so I put the stat back in. I read someplace that the hole causes the engine to take a bit longer to warm up but can be helpful if the stat should seize closed.

02-24-2018, 02:56 PM	#24
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: T-stat to drill a hole or not to drill a hole? Small block Chevrolets have an internal bypass in the water pump, do they not? Seems redundant... __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

02-24-2018, 03:09 PM	#25
BRL Registered User Join Date: Jun 2014 Location: Tahuya WA Posts: 510	Re: T-stat to drill a hole or not to drill a hole? Could very well be, that was my first rebuild and really don't know what was causing that problem. ETA: Oh I think I see what you're saying, didn't know SBC water pumps had a bypass. ...oops! Last edited by BRL; 02-24-2018 at 03:28 PM.

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