72 K5 with 87-91 K5 Gas Tank Install

[K5 MAX]

My K5 needed a new tank, I have plans for FI or and LS motor down the road so why not go ahead and put a bigger 31 gallon tank in that's set up for fuel injection. I had a hard time finding a complete write up on this tank install, so here's mine to help others who are thinking about this swap. Thanks for looking

Cost

Advanced auto online, had a $50 off coupon so i used that, all with lifetime replacement, free shipping also, total came to $248.21 after tax for the 3 items below

-tank, carquest pn 576-313, retail $135.99
-tank straps, carquest pn 578-011, retail $41.99
-sending unit (no fuel pump), carquest pn HS2542571, retail $103.99

Purchased the rest locally
-misc nuts, bolts for new cross members, ss hose clamps $42
-4' of 3/8" & 5'16" fuel line, inline fuel filter $22
-misc steel stock, 2" , 1.5" and 1" angle ~$50
-1 3/4" I.D. x 20" long fuel filler hose, 1/2" rubber for tank to frame contact and 1/16" rubber for all other contact points $36

Options for purchasing if you don't have these locally
https://www.millsupply.com/rubber-in...960.php?p=4155

https://www.fillernecksupply.com/1-3...oodyear-59201/

https://www.millsupply.com/1-34-flex...954.php?p=9287

Total cost tank install: ~$400

Started by draining the old gas and dropping the tank


Cut the tank filler neck off because i wasn't sure if i'd need it to be welded to the new tank, didn't end up using it though


Had to grind the rivets and punch them out for the rear cross member, bracket that held the rear rubber brake line to the frame and the axle bump stop brackets on both sides. Also removed all the old tank mount brackets, as well as the vent system that runs along the rear body channel


driver side


passenger side, was playing with using the rear cross member in front but ended up making new supports from angle iron. i did not use the rear cross member. it just made things more difficult in my situation


Protected the tank for the mock up phase


Cross member specs and pics

-Lower rear 2" angle x 3/16" thick, sits on the lower portion of the frame rail, goes inside frame to frame, this is the only cross member i drilled new holes for in order to attach it. the tank straps will bolt to this in the rear. used 3/8" bolts to attach this to the frame


-Lower front 2" angle x 3/16" thick, sits on the lower portion of the frame rail, goes inside frame to frame, uses the rear holes of the axle bump stop bracket. the tank straps will bolt to this in the front. used 3/8" bolts to attach this to the frame




-Upper rear (stops the tank from contacting the bed basically, 1" angle x 3/16" thick, sits on the upper portion of the frame rail, uses the upper holes for the old rear cross member location. used 3/8" bolts to attach this to the frame


-Upper front (stops the tank from contacting the bed basically, 1.5" angle x 3/16" thick, sits on the upper portion of the frame rail, uses some existing holes on the top of the frame, right where it starts its upward bend to go over the axle. I did cut 1/2" off of the angle on the side to sit vertically once installed, welded this to the underside (tank contact point) to help the tank sit level. see pics. used 5/16" bolts to attach this to the frame, because that's what size the hole was and i didn't have a way to drill a new hole in that tight location since i didn't take the body off to do this




Overall shot before paint


This is as far back as this style tank can sit without having to relocate the filler neck, that's a cross member for the bed right behind the filler neck. Filler neck was not moved or modified

[K5 MAX]

Next i cleaned up all the cross members, frame in this area and the rear area behind the tank because it was the best time to do so. Then POR-15 degreaser, metal prep and satin black frame paint. Replaced the body mounts at the same time because it'll be impossible to get to the rear most one with the tank in


Tried gluing the 1/16" rubber to the upper cross members (as u can see in the pic) to hold it in place until the tank went in, it wouldn't stay in place long so they were set in place on top of the tank as it when in and was close to contact


Cut 1/16" thick rubber for all the contact points, upper tank stops / supports and tank straps


1/2" rubber x 2" to provide protection between the tank and the frame rail


Put a 3/8" piece of fuel line in place of where the fuel pump would go (if running fuel injection), wired it off to keep it from moving, and wired the plug for the fuel pump up and out of the way. Measured 11 7/8" from the sending unit flange to the bottom of the little lipped sump inside the tank


Set tank in place on the ground, hooked up the 3/8" fuel line to the frame fuel rail, 5/16" fuel line to the charcoal canister line on the frame, capped the return line on the sending unit

[A1971Blazer]

Nice job...good write-up...you might want to consider a filter sock on that pickup tube just in case....I know it's a new tank..... but you never know what might come from a pump at the gas station

[K5 MAX]

Finished shots, tank nor lines contact the bed. The closest thing is the fuel filler but the fuel filler rubber hose is the insulator. I used 5/8" I.D. fuel line for the main vent that went up to the filler neck. Had to widen the hole through the bed floor so that it would pass through



Fuel filler 1 3/4" I.D. (that's the tanks size) to 1 3/8" for the filler spout from the bedside. I was a little worried about the size difference but it clamped down well, used 2 hose clamps in case

before being clamped


and after


front edge shot, rear brake line is zip tied at the moment, when i replace the rubber brake hoses i plan to get a longer rear rubber line and reattach it to the frame further towards the front using the old bracket if possible




bracket zip tied up out of the way as well, for now


Rear axle clearance, my K5 does have a 4"? lift in the rear so if yours is stock height this clearance might be too close. I've seen others mount the tank further back but the tank filler tube has to be relocated further back


Tank to frame clearance, this is where i used the 1/2" x 2" rubber as insulators


Back shot


Drivers side


Tank hangs 4.5" below the homemade bumper the PO had built for this truck


and 7" below the frame rails for reference

[K5 MAX]

Originally Posted by A1971Blazer

Nice job...good write-up...you might want to consider a filter sock on that pickup tube just in case....I know it's a new tank..... but you never know what might come from a pump at the gas station

Yeah i thought hard about it, the sending unit actually came with one to attach to the fuel pump (fuel pump not included with the sending unit pn i went with). my thinking was the stock tank most likely didn't have one and i didn't know if since carb line pressures aren't near that of FI motors it might restrict the pickup flow? Just my thoughts, not a master mechanic or anything, probably dumb. I do have a inline fuel filter right before the mechanical fuel pump at the motor. If i drop the tank ever maybe i'll add one. already done and driving now so not going back now

[A1971Blazer]

Originally Posted by K5 MAX

Yeah i thought hard about it, the sending unit actually came with one to attach to the fuel pump (fuel pump not included with the sending unit pn i went with). my thinking was the stock tank most likely didn't have one and i didn't know if since carb line pressures aren't near that of FI motors it might restrict the pickup flow? Just my thoughts, not a master mechanic or anything, probably dumb. I do have a inline fuel filter right before the mechanical fuel pump at the motor. If i drop the tank ever maybe i'll add one. already done and driving now so not going back now

I understand...my '72 has the 31 gallon efi tank....and I'm running the Holley Sniper EFI....you're all set for a future LS swap...nice work !

[awelle01]

I ended up doing nearly the same exact thing, nice work as well. I was able to get all the angle iron powder coated along with the frame so everything looks good bolted up. I also added a hidden hitch as well. I am doing an LS swap so have put in the proper fuel pump in the tank and plumbed everything up. No clearance issues with the hoses or filler neck. It took some trial and error buying certain hoses/adapters from the filler neck supply, but looks like it's going to work out well. My project is not quite ready to fire up, but getting close!

[timski69gmc]

Great write-up. I need to figure out how to save this.

[TheKorndog]

Reviving an old thread whut whuuuuut!!!!

I’m about to completely tear down my 72 Blazer and want to put this tank mod in it when I got the body off. My question is more or less in regards to the fuel fill neck. With the larger 89 K5 tank, is the fuel neck attachment an issue? Also what about the sending unit? Is it the same 0-90 ohms?

Thanks all. Mostly I read everything on here and never really chime in, because you guys do a great job of documenting your amazing work..
Bump buuuuump!
K

Forgot to mention I’m gonna set the Blazer up for EFi, the whole reason for doing this …

Posted via Mobile Device

[K5 MAX]

Originally Posted by TheKorndog

Reviving an old thread whut whuuuuut!!!!

I’m about to completely tear down my 72 Blazer and want to put this tank mod in it when I got the body off. My question is more or less in regards to the fuel fill neck. With the larger 89 K5 tank, is the fuel neck attachment an issue? Also what about the sending unit? Is it the same 0-90 ohms?

Thanks all. Mostly I read everything on here and never really chime in, because you guys do a great job of documenting your amazing work..
Bump buuuuump!
K

Forgot to mention I’m gonna set the Blazer up for EFi, the whole reason for doing this …

Posted via Mobile Device

With the 1 3/4” fuel filler hose I recommend to use and the exact location of the tank how I did it there’s no issues. Might want to do it with the body on to be sure you don’t have issues once you go to set it back on the frame because everything is tight. Check the specs on the sending unit. I’m pretty sure the ohms are different, I just dealt with it. Seemed the first 1/4 of a tank that you used from full to 3/4 was about half of the fuel in the tank (15 gallons). From 3/4 full to empty is the other 15 gallons. I ran it down to about 1/8 of a tank on the gauge and never had an issue. Good luck, I sold my K5, want another one day

[TheKorndog]

Originally Posted by K5 MAX

With the 1 3/4” fuel filler hose I recommend to use and the exact location of the tank how I did it there’s no issues. Might want to do it with the body on to be sure you don’t have issues once you go to set it back on the frame because everything is tight. Check the specs on the sending unit. I’m pretty sure the ohms are different, I just dealt with it. Seemed the first 1/4 of a tank that you used from full to 3/4 was about half of the fuel in the tank (15 gallons). From 3/4 full to empty is the other 15 gallons. I ran it down to about 1/8 of a tank on the gauge and never had an issue. Good luck, I sold my K5, want another one day

If I wasn’t knee deep in a stroker install I’d have gone with the LS swap. I’ll already have the 4L60e in it, so one less thing once it’s ripe. This fuel tank swap will put me one step closer when it does happen. Thanks for the input!

Most things I’ve read says it’s 0-90 ohms until mid 90s. The only thing that trips me out is that aftermarket companies says these tank sending units are 0-95. Ain’t like it’s gonna be a daily driver, just a weekend warrior. Nice to be able to get back into it after moving around everywhere for work. Now that we’re just about settled, it’s on!
Thanks again!
K
Posted via Mobile Device

[jjzepplin]

YOU SOLD YOUR K5? WHAT THE??? Well you did a nice job with the tank install! Do you have another 67-72?

[hemi43]

Good write up, but squashing that 1 5/8" filler hose down to 1 3/8" is asking for trouble. I had difficulty sourcing 1 3/8" fuel hose, but it is available.

[hemi43]

Originally Posted by TheKorndog

If I wasn’t knee deep in a stroker install I’d have gone with the LS swap. I’ll already have the 4L60e in it, so one less thing once it’s ripe. This fuel tank swap will put me one step closer when it does happen. Thanks for the input!

Most things I’ve read says it’s 0-90 ohms until mid 90s. The only thing that trips me out is that aftermarket companies says these tank sending units are 0-95. Ain’t like it’s gonna be a daily driver, just a weekend warrior. Nice to be able to get back into it after moving around everywhere for work. Now that we’re just about settled, it’s on!
Thanks again!
K
Posted via Mobile Device

If you do an LS swap you will want an aftermarket gauge cluster, so the resistance of the sending unit is not important.

[K5 MAX]

Originally Posted by jjzepplin

YOU SOLD YOUR K5? WHAT THE??? Well you did a nice job with the tank install! Do you have another 67-72?

Yeah it was a sad, sad day, especially now that they are getting to unattainable prices. I bought the one in the thread for $2800 in February of 2018. Put a gas tank in it, some used tires, and a master cylinder and it was reliable enough to enjoy 10-20 miles from my house. We live near Mexico Beach FL where Hurricane Michael hit in Oct of 2018, it was a Cat 5 for those that don’t remember it. The K5 was in my enclosed shop under my house, we evacuated the day before even though it was only goin to be a Cat 2 at that time. We had a 1 yr old and didn’t want to risk staying for power outages and stuff. With it only being a Cat 2 storm surge shouldn’t have affected us. We’ll it jumped from a Cat 2 to a Cat 5 over night the night before it hit. I was 4 hours away so there was nothing I could do but hope our house didn’t get surge. We’ll it did, 3-4’ of salt water where the K5 set. Got salt water in everything but the motor, winched it on a trailer and did every fluid twice after driving it some in between. It was running and driving but the salt surge was accelerating the rust it already had. I wasn’t able to jump on the rust repair anytime soon. Sold it to a cousin that wanted it and could jump on it. He made made major progress after he got it, then caught Covid and died. His son is working to finish it, I have first dibs when he sells it. So maybe they’ll be a good ending to this story. I’ll throw some current pics of it in my build thread at some point.