Which hole is for what? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Old_iron · 09-03-2019, 10:10 PM

I know one is for heater hose, one for water temp sensor. But not sure which and what the 3rd one is for.

**Steeveedee** · 09-03-2019, 10:14 PM

Hole labeled "A" is vacuum for the power brakes.

GASoline71 · 09-05-2019, 07:39 PM

Quote:

Originally Posted by Steeveedee

Hole labeled "A" is vacuum for the power brakes.

Every 67-72 SBC truck with an auto transmission I've owned or worked on had the modulator for the transmission plugged into that port. The vacuum for the brakes has always came off the back of the carb.

Gary

68 P.O.S. · 09-05-2019, 07:54 PM

Quote:

Originally Posted by GASoline71

Every 67-72 SBC truck with an auto transmission I've owned or worked on had the modulator for the transmission plugged into that port. The vacuum for the brakes has always came off the back of the carb.

Gary

This ^^^

**DeadheadNM** · 09-03-2019, 10:16 PM

350? A is for a vacuum fitting. Temp send unit typically in LH head with the hole in that non OEM water neck plugged up.

**Steeveedee** · 09-04-2019, 10:46 AM

Hole "C" would be for a thermal vacuum switch (emissions control), if the temp sensor doesn't go there (which it shouldn't). What year is the engine?

Old_iron · 09-04-2019, 10:53 AM

Quote:

Originally Posted by Steeveedee

Hole "C" would be for a thermal vacuum switch (emissions control), if the temp sensor doesn't go there (which it shouldn't). What year is the engine?

I don't remember for sure - I'm thinking 68-72?

So, I'm hearing that:
A is for the brake booster
B is for the heater hose
C should be blocked off

If that is right, then where should the water temp sensor go?

EagleChief · 09-04-2019, 10:57 AM

Technically, the temp sensor goes in the block on the passenger side, just below the headers, about midway back.

You could put your temp sensor in location "C" if you wanted to. Just make sure the other location is plugged.

**rechinca** · 09-04-2019, 10:58 AM

Water temp sensor is in the drivers side head between the front two exhaust ports just above the spark plugs.

Andy4639 · 09-04-2019, 11:45 AM

Quote:

Originally Posted by rechinca

Water temp sensor is in the drivers side head between the front two exhaust ports just above the spark plugs.

x2 drivers side between spark plugs 1 and 3.
Make sure the tempature sending unit is for a 71-72 truck are the gauge want read right.

EagleChief · 09-04-2019, 01:31 PM

Quote:

Originally Posted by rechinca

Water temp sensor is in the drivers side head between the front two exhaust ports just above the spark plugs.

Quote:

Originally Posted by Andy4639

x2 drivers side between spark plugs 1 and 3.
Make sure the tempature sending unit is for a 71-72 truck are the gauge want read right.

Hmm... mine is in the passenger side.

TruDom · 09-04-2019, 02:23 PM

Quote:

Originally Posted by EagleChief

Hmm... mine is in the passenger side.

mine is too but thats because the dakota digital sensor won't fit there with the headers.

Old_iron · 09-04-2019, 02:39 PM

I just went and looked and there is nothing on the passenger side. On the driver side between the front 2 spark plugs there is a flat area cast into the block, but no hole was ever drilled. Maybe that's why the PO had that water pump neck with the hole in it?
Can I put the sensor there?

68 P.O.S. · 09-05-2019, 03:10 PM

Quote:

Originally Posted by EagleChief

Hmm... mine is in the passenger side.

The temp switch for the TCS system goes there. Did your truck come with the TCS?

Larry Weaver · 09-05-2019, 07:32 PM

The hole in the water neck is for a ported vacume switch.

EagleChief · 09-06-2019, 09:42 AM

Quote:

Originally Posted by 68 P.O.S.

The temp switch for the TCS system goes there. Did your truck come with the TCS?

TCS? My lack of knowledge automatically makes me think of Traction Control System, but I know that's not what you're referring to.

It took me a minute, but I had to realize my motor is supposedly out of an '88 Suburban, so maybe that's why mine is hooked up different? :shrug:

**71CHEVYSHORTBED402** · 09-05-2019, 01:31 PM

"Which hole is for what?"

That's what she.............Ah, never mind

**rechinca** · 09-05-2019, 11:26 PM

Quote:

Originally Posted by 71CHEVYSHORTBED402

"Which hole is for what?"

That's what she.............Ah, never mind

We can close this now, Tony wins the contest!

siggyfreud · 09-05-2019, 02:57 PM

I've used C for a thermo switch for radiator fans. It works well because it's not kicking the fans on until the thermostat is open anyways, and is "good enough" for the on-off temps in terms of accuracy on actual engine temperature.

68 P.O.S. · 09-06-2019, 09:58 AM

Haha, of course. Transmission Controlled Spark. It was an early emissions thing that didn't really do much. It would stop the vaccum advance from working until the engine was fully warmed up (temp switch in passenger side head) and the transmission was in high gear.

09-03-2019, 10:10 PM	#1
Old_iron Registered User Join Date: May 2016 Location: Idaho Posts: 87	Which hole is for what? I know one is for heater hose, one for water temp sensor. But not sure which and what the 3rd one is for. Attached Images __________________ 1968 GMC 3/4 Ton 4x4 K20 / 2500 1971 Chevrolet Chevelle Malibu

09-03-2019, 10:14 PM	#2
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,591	Re: Which hole is for what? Hole labeled "A" is vacuum for the power brakes. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

09-03-2019, 10:16 PM	#5
DeadheadNM Carpe manana Join Date: Dec 2010 Location: Eau Claire, WI Posts: 12,423	Re: Which hole is for what? 350? A is for a vacuum fitting. Temp send unit typically in LH head with the hole in that non OEM water neck plugged up. __________________ 1972 K5 CST Highlander Blazer: http://67-72chevytrucks.com/vboard/s...d.php?t=708547 1972 K20 Cheyenne Super: http://67-72chevytrucks.com/vboard/s...d.php?t=528308 Members met: ORANGBLAZ, 2003 silverado, MikeCofield, BB72CHEVKT, Duncan&Son, Sameyrasmea72, THENEWMEXICAN, HotRod C/10, brianthelion02, Sport/Truck, ryanroo, michael bustamante, Dirt's72, Already Gone, WestButteTruck, 57taskforce, Moreyel, painterljp, AASmedic, SoCoC10, Lumaestas, carbuff382, Chevyland

09-04-2019, 10:46 AM	#6
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,591	Re: Which hole is for what? Hole "C" would be for a thermal vacuum switch (emissions control), if the temp sensor doesn't go there (which it shouldn't). What year is the engine? __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

09-04-2019, 10:57 AM	#8
EagleChief Registered User Join Date: Sep 2016 Location: Tulsa, OK Posts: 420	Re: Which hole is for what? Technically, the temp sensor goes in the block on the passenger side, just below the headers, about midway back. You could put your temp sensor in location "C" if you wanted to. Just make sure the other location is plugged. __________________ 1971 C10 - Hugger Orange/White My Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=780197 LS Swap Thread: https://67-72chevytrucks.com/vboard/...d.php?t=811460

09-04-2019, 10:58 AM	#9
rechinca Senior Member Join Date: Jul 2018 Location: Vacaville, CA Posts: 529	Re: Which hole is for what? Water temp sensor is in the drivers side head between the front two exhaust ports just above the spark plugs. __________________ 1971 Custom/10 - 350/3 speed, Orange/White roof

09-04-2019, 02:39 PM	#13
Old_iron Registered User Join Date: May 2016 Location: Idaho Posts: 87	Re: Which hole is for what? I just went and looked and there is nothing on the passenger side. On the driver side between the front 2 spark plugs there is a flat area cast into the block, but no hole was ever drilled. Maybe that's why the PO had that water pump neck with the hole in it? Can I put the sensor there? __________________ 1968 GMC 3/4 Ton 4x4 K20 / 2500 1971 Chevrolet Chevelle Malibu

09-05-2019, 07:32 PM	#15
Larry Weaver Registered User Join Date: Jul 2016 Location: Moravia ny Posts: 49	Re: Which hole is for what? The hole in the water neck is for a ported vacume switch.

09-05-2019, 01:31 PM	#17
71CHEVYSHORTBED402 Senior Member Join Date: Feb 2009 Location: Northern Nevada Posts: 7,292	Re: Which hole is for what? "Which hole is for what?" That's what she.............Ah, never mind __________________ Tony 71 Custom Deluxe, SWB, 2WD, 402, A/C. I developed an assembly kit for restoring the (a) truck from the ground up. My build thread, and more on the assembly kit https://67-72chevytrucks.com/vboard/...d.php?t=730025

09-05-2019, 02:57 PM	#19
siggyfreud Registered User Join Date: Jun 2012 Location: Portland Posts: 1,330	Re: Which hole is for what? I've used C for a thermo switch for radiator fans. It works well because it's not kicking the fans on until the thermostat is open anyways, and is "good enough" for the on-off temps in terms of accuracy on actual engine temperature. __________________ Current Build - Project "Ghost of Poseidon" 72 C10 Pro-touring build Previous Build - 66 GMC Pro-touring build. Sold.

09-06-2019, 09:58 AM	#20
68 P.O.S. Registered User Join Date: Jan 2007 Location: Puyallup, WA Posts: 1,662	Re: Which hole is for what? Haha, of course. Transmission Controlled Spark. It was an early emissions thing that didn't really do much. It would stop the vaccum advance from working until the engine was fully warmed up (temp switch in passenger side head) and the transmission was in high gear. __________________ 72 C10 lwb fleetside -stock 350/350 combo



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