Behind the seat tank in a square body.
 

I have a 87 squarebody v10. I want to know if anyone has put the earlier tanks behind the seat?
Write ups or any threads on this or even pictures will help.

Few things before I get the "it is not safe" things.

Tank will have a sealed cap.

Return line and vent line will go through fill neck with return line dropping to bottom.

Flatbed will have headache rack and roll bar that protects cab/tank.

Looking to use a factory tank, 0-90 sending unit, and factory straps to a modified mount.

Re: Behind the seat tank in a square body.
 

you probably don't want to hear what I think about it but I have to ask......why?

Re: Behind the seat tank in a square body.
 

I want to run the exhaust on the outside of the frame rails, a winch in the rear, full 8 foot flatbed, and no fuel below the rails.

It has me building a custom tank behind the cab or using a stock one inside the cab.

Using the stock tank opens the door for poly and stainless tanks as well as parts store replacement parts.

Re: Behind the seat tank in a square body.
 

Moving the tank out of the cab was a huge leap forward in safety, smells, etc All the write ups you will find are how to get it out of the cab.

Old generation tanks won't fit directly. It won't fit well at all unless you limit the rearward seat location. By time you are done with the fab and proper sealing, you could have had a proper tank made, inside the rails.

If you have an electric rear winch, or unless you are running a 10 ton PTO driven one, there should be room between the winch and axle for a pretty decent tank.
Only reason I can think of to run exhaust outside the rails is a wiled motor swap or running stacks by the cab, since there isn't room to sneak the pipes past the tires. Then the area behind the rear wheels, or between the stacks and the wheels, should be available. I bet a set of 16 gallon tanks would sneak in there.... and the fillers would be accesible from the side.

Re: Behind the seat tank in a square body.
 

It won't fit without moving the bench.

Any idea how much it would need to move abouts?. A inch or 2 up or forward I can deal with more then that and it kills the idea.

Re: Behind the seat tank in a square body.
 

I think it is a horrible idea. But with that being said I had a 46 GMC COE that had a gas tank behind the seat that would have been a perfect fit. It was about 6" on the bottom, 3" at the top, 5" wide, flat on the bottom with a passenger side exit. Probably the same as the same gas tank that is in the truck but I'm not certain of that.

Re: Behind the seat tank in a square body.
 

Here are some common issues with behind the seat tanks


The thing rolls and the fuel goes out the cap side down the side of the truck. Possible fire

Fix;
Sealed cap and vent lines running to evap canister.

The cab smells like gas as tank vents
Fix;
Sealed cap and vent lines going to a evap canister under the hood then pulled into engine.

The truck is hit in the side or something thrown through rear of cab and tank ruptures
Fix
Flatbed has a roll bar that extends to the outside of the cab attached to the frame protecting tank and filer neck from this exact situation.

Filler neck rubber cracked or sending unit seal worn
Fix
Maintenance. Periodic checks at oil changes. I will make my fill so that 3 bolts hold it in allowing easy replacement of the hose without moving tank.

Derbies within tank settle to bottom of tank and block fuel line.
Fix
Return line located above tank drain to "turn up" any derbies so that smaller amounts can be filtered before inline pump.

Water,snow, ice form around filler neck.
Fix;
Gas cap inset using hinged gas cap cover that matches close to body profile with spring to keep closed. Weatherstripping around cover should avoid old issues of brushing off snow.



Benefits of in cab tank.


Less tank rust as it is inside the cab out of the salt;

Here is my current passenger tank I am not going back to this setup.
https://preview.ibb.co/fDnjrJ/026.jpg


Less water inside tank.
Tank is inside the cab and insulated better then one with a foot of snow
sitting on top/under it. Sealed gas cap prevents water from entering better then old vented gas cap

Tank is protected from contact with ground or being crushed against frame.

Tank will not receive salt spray like it will if located behind axle.

Mostly vertical tank has less chance of starvation then a mostly horizontal tank.

Tank is out of the way allowing easier maintenance.
Easy to replace sending unit and inline pump means once secured the tank should not need to be removed for years.

Drivetrain options.
Route my exhaust any way I want. Run longer springs changing wheelbase if I want. Shorten the frame behind spring perches If I want.








With the exhaust I want to build some fender well headers to a flexible section and v bands. It should allow me to unbolt and "pull out/down" the headers slightly making maintenance and engine removal easy. I don't have it all figured out yet. That is down the line in this. But I believe I can route the primaries under the frame protected by bump stops and follow the frame back then run to a flexible section to under the cab on the outside of the frame without much issue.

Re: Behind the seat tank in a square body.
 

Years back I had a 86 chevy that I wheeled and did not want the saddle tanks hanging, and the way I bobbed the truck there was no room for a between the frame tank or fuel cell. I used a behind the seat tank from a 69 GMC and mounted it inside the box between the box sides up against the front of the box. The filler neck was perfect height to be just above the box side.
If you are going to put a flat bed with a roll bar on it why not mount it between the front bars of the roll bar?

Re: Behind the seat tank in a square body.
 

I think a Blazer tank which is 25 gallon tucked in right will give you enough room for a rear winch.

On my 4x4 I wish I never sold. I mounted the winch to a hitch mount, and made jumper cables to power it. I only tested it on stupid things, but it worked well.

Re: Behind the seat tank in a square body.
 

Keeps the tank out of the environment snow rain etc. I don't like the idea of the tank sitting in 4" of snow covered in icicles or the sending unit/pickup full of water/mud. Should lead to a little less condensation inside the tank.

Putting it inside the cab allows the headache plate that everything is pushed up/thrown against on the flatbed to be within 3" of the cab for a full 8 foot bed to be maintained.


I ordered a galvanized tank from LMC with eec tonight. I will pull the seat, rebuild the floor,(3/16 steel floor while eliminating rockers), and set the thing in there figuring out where I will need to add brackets and figure out if I can inset the gas cap into the cab.

One of those get it in there and figure things out at this point.

I seen a post on here where the person ran the vent line up to the top of the cab on the passenger side then did a loop before going into the floor and will use that method.

Re: Behind the seat tank in a square body.
 

:fry::fry::drama:

Re: Behind the seat tank in a square body.
 

I was hoping to see more info on guys doing this here. The tanks take a beating hanging down there and typically the passenger side box side without fuel hole cheaper.. I am redo'ing my 87 as per new tanks and pumps just cause I'd like the factory TBI to work perfectly. My 85 however running a typical mechanical pump can draw from anywhere... don't ever use behind the seat storage and would like both tank fills on one side - factory driver and a behind the seat tank or immediately in front end of box. Inside, as the fellow discussed (his reasons were bang on), is my preferred with any return and vapor stuff plumbed back to factory. Burn the factory and then the BtS aux when I am looking for a gas station. The 30 ohm sending unit fine with me as well. It will only read 1/3 tank when full but as long as it reads, I got gas till I fill the 20 Gal main. Anyways, my 2 cents... this summer has been horrible so next summer I can tackle a conversion and let you guys know what I find. Thanks for trying to get some discussion going on this. Brian

Re: Behind the seat tank in a square body.
 

I had a custom tank built for my flatbed (1969 though) It sits behind the cab, between the frame rails and above the driveshaft. Dimensions came out to 47 gallons. I had it made out of stainless by Ricks Tanks in Texas.

Re: Behind the seat tank in a square body.
 

A huge factor in the 85 is that its a refresh that is pretty dated and has achieved day to day duty with goal reliability and being cheap as cheap can possibly get. A custom tank is no where in the cards. The 20 gal driver side was changed when refreshed (so in good shape except not reading level!... sigh) and the 16 on pass side my last spare tank and she looked rough... not to mention the tank carriers rotting out. Cheap bastard and using junk easily obtainable how it flows. Used back seat tank looking like new with sender - $50... 1/3 price of new bare saddle tank alone. Luv my squares and 87 TBI 1 ton is where real money is spent (new sending units, one new tank with one almost new)... $$$... ouch...

Re: Behind the seat tank in a square body.
 

Cerial: That's exactly what my 1 ton pick up driver side tank looked like... jeez they take a beating. Agreed with all your points. Please do post any updates on config for TBI system you do. I also have an 87 pick up, knock on wood, fuel system working perfectly (one tank just started to leak)... Thanks in advance if you did the project and share. Brian