Fuel Lines, 5/16 or 3/8s? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

68 Suburban · 02-04-2004, 04:50 AM

Which size fuel line did out turcks come with?

Alexis · 02-04-2004, 04:54 AM

3/8

stelth2002 · 02-04-2004, 04:55 AM

5/16ths for the inlines, 3/8 for v8

red71cheyenne · 02-04-2004, 05:59 AM

my 71 had both. 3/8 for fuel supply line from tank and 5/16 going back to the tank for vent. had to change both to 3/8 for the TPI. Jeff.

Tx Firefighter · 02-04-2004, 09:57 AM

Stelth is right. It depends on which engine it had originally.

**CPNE** · 02-04-2004, 11:27 AM

I've found it depends on the carb. All 4bbl carbed V8's had 3/8, whereas the 307 2bbl had 5/16th. All inlines had 5/16th as well.

SWEET7T · 02-04-2004, 11:46 AM

My 2bbl V8 has 5/16...

scotts67 · 02-04-2004, 03:01 PM

I put 5/16 on mine and I run a 350

Luke87gt · 06-27-2025, 06:52 PM

I’m swapping to a new tank in my i6

It will eventually get a 350 so I’m wondering if I should do the 3/8 sending unit, 3/8 lines now.

However the i6 fuel pump and carb is 5/16”

Am I introducing any pressure issues for now if I do the 3/8” lines up to the fuel pump and 5/16” beyond that to the carb?

**Steeveedee** · 06-27-2025, 06:58 PM

^ Nope.

RustyPile · 06-28-2025, 02:29 AM

Quote:

Originally Posted by Luke87gt

I’m swapping to a new tank in my i6

It will eventually get a 350 so I’m wondering if I should do the 3/8 sending unit, 3/8 lines now.

However the i6 fuel pump and carb is 5/16”

Am I introducing any pressure issues for now if I do the 3/8” lines up to the fuel pump and 5/16” beyond that to the carb?

The only issue you'll encounter is the fuel pumps have the same size inlet and outlet - either both are 5/16" or both are 3/8". Run 3/8" line all the way, and install a "reducing adaptor" in the line to the carburetor. While you're at it, Install a 1/4" return back to the tank and install the matching fuel pump and sending unit. The return goes a long way toward preventing vapor lock.

**Steeveedee** · 06-28-2025, 11:07 AM

It's early and I'm not fully caffeinated yet. If a fuel pump has a return line, will that make it so that the pressure at the carburetor will not increase after the engine shuts off and gets heat soaked? I don't know how that recirc works- is it just a return line, or is there some relief valve?

RustyPile · 06-28-2025, 10:59 PM

Quote:

Originally Posted by Steeveedee

It's early and I'm not fully caffeinated yet. If a fuel pump has a return line, will that make it so that the pressure at the carburetor will not increase after the engine shuts off and gets heat soaked? I don't know how that recirc works- is it just a return line, or is there some relief valve?

There is no relief valve. By design, the fuel pump is "self regulating". The return port has an orifice in it. The port comes off the same chamber as the discharge port. The orifice serves to meter the amount of fuel returning to the tank. Since the return line is open at the tank end, there is no pressure build in it.. When the engine stops, the pump discharge immediately drops to zero.

**Steeveedee** · 06-29-2025, 12:03 AM

Quote:

Originally Posted by RustyPile

There is no relief valve. By design, the fuel pump is "self regulating". The return port has an orifice in it. The port comes off the same chamber as the discharge port. The orifice serves to meter the amount of fuel returning to the tank. Since the return line is open at the tank end, there is no pressure build in it.. When the engine stops, the pump discharge immediately drops to zero.

Thanks for this information. I did a little looking around but hadn't found it.

02-04-2004, 04:50 AM	#1
68 Suburban Registered User Join Date: Oct 2000 Location: From Chicago, Live in Phoenix, AZ Posts: 6,802	Fuel Lines, 5/16 or 3/8s? Which size fuel line did out turcks come with? __________________ Just sitting here contemplating contemplation.

02-04-2004, 04:55 AM	#3
stelth2002 Young Gun wth Fast Inline Join Date: Jan 2002 Location: Idaho Posts: 1,475	5/16ths for the inlines, 3/8 for v8 __________________ 1967 Chevrolet LWB with built 292! 415 ft. lbs of torque, 4 speed :O with 27% overdrive too, 3.73 Posi. Frame and body done... getting painted!!!

02-04-2004, 05:59 AM	#4
red71cheyenne Back in the sticks Join Date: Jan 2001 Location: Fordland, MO Posts: 3,188	my 71 had both. 3/8 for fuel supply line from tank and 5/16 going back to the tank for vent. had to change both to 3/8 for the TPI. Jeff. __________________ 1971 Cheyenne C-10 w/700R4 and Tuned Port Injection 1969 K5 Blazer w/Tuned Port 2010 2SS/RS Flaming Orange Camaro 2011 K1500 Suburban 2014 K1500 Pickup 2008 Nissan Altima? The wifes' hoopty

02-04-2004, 09:57 AM	#5
Tx Firefighter Watch out for your cornhole ! Join Date: Jun 2003 Location: Azle, Texas Posts: 14,162	Stelth is right. It depends on which engine it had originally. __________________ I'm on the Instagram- @Gearhead_Kevin

02-04-2004, 11:27 AM	#6
CPNE Resident Curmudgeon Join Date: Dec 1999 Location: NH Posts: 6,662	I've found it depends on the carb. All 4bbl carbed V8's had 3/8, whereas the 307 2bbl had 5/16th. All inlines had 5/16th as well. __________________ Currently on or near the homestead: 67 Chevy SWB 2WD stepside 350/3 on tree (Pat's) 67 GMC SWB 2WD Fleet 402/auto (Brian's under construction) 67 Chevy 3/4 ton 2WD 402/auto (Business Hauler) 67 Chevy 1 ton dually 2WD 396/4 speed (Former business hauler, Needs TLC) 68 Chevy 1/2 ton Suburban 2WD 250 six/3 on tree (Brian's Needs TLC) 70 Chevy 3/4 ton 4WD 350/4 speed (Pat's - Disguised as a 68 GMC) 71 Chevy SWB stepside (Crushed by tree - parts donor) 72 Chevy 3/4 ton 4WD (Parts donor) 72 Chevy 3/4 ton 4WD Suburban (Parts Donor) 72 GMC 3/4 ton 4WD 292 six/4 speed (Mine - Disguised as a 67 GMC) 81 GMC 4WD Dually Dump Body 350/4 speed (Business Hauler) 82 Camaro Z/28 355/Super T-10 (Pat's toy) 93 Caprice 9C1 (Brian's Cop Car) 02 Toyota Camry (Reliable but a souless steel and plastic hulk) 2011 2SS RS Camaro M6 Factory Hurst Shifter Maybe I need to sell some of this crap Yet another Bozo with a sawz-all

02-04-2004, 04:54 AM	#2
Alexis Member since 2000 Join Date: Dec 2000 Location: Mountain View Ca / Mexico Posts: 7,874	3/8

02-04-2004, 11:46 AM	#7
SWEET7T Try spinnin 4 rear tars Join Date: Sep 2003 Location: Salt Lake City, Utah Posts: 757	My 2bbl V8 has 5/16... __________________ SWEET7T 1970 C10, 2wd, LB, 307/TH350/3.08. PS, PB

02-04-2004, 03:01 PM	#8
scotts67 New KISS Band Join Date: Nov 2003 Location: South Carolina Posts: 107	I put 5/16 on mine and I run a 350 __________________ 67 chevy 350 HP 4bolt main turbo 350 3.73 gears

06-27-2025, 06:52 PM	#9
Luke87gt Registered User Join Date: Feb 2024 Location: Pleasanton CA Posts: 472	Re: Fuel Lines, 5/16 or 3/8s? I’m swapping to a new tank in my i6 It will eventually get a 350 so I’m wondering if I should do the 3/8 sending unit, 3/8 lines now. However the i6 fuel pump and carb is 5/16” Am I introducing any pressure issues for now if I do the 3/8” lines up to the fuel pump and 5/16” beyond that to the carb? __________________ 1969 C10 Shortbed

06-27-2025, 06:58 PM	#10
Steeveedee Ram-A-Lam-A-Ding-Dong Join Date: Jan 2013 Location: Simi Valley, CA Posts: 11,903	Re: Fuel Lines, 5/16 or 3/8s? ^ Nope. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

06-28-2025, 11:07 AM	#12
Steeveedee Ram-A-Lam-A-Ding-Dong Join Date: Jan 2013 Location: Simi Valley, CA Posts: 11,903	Re: Fuel Lines, 5/16 or 3/8s? It's early and I'm not fully caffeinated yet. If a fuel pump has a return line, will that make it so that the pressure at the carburetor will not increase after the engine shuts off and gets heat soaked? I don't know how that recirc works- is it just a return line, or is there some relief valve? __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.