47-55.1 Yet another gas tank issue/question

[TX3100Guy]

I've pulled the gas tank from the cab and want to replace it with a tank under the bed, if possible. I say "if possible" because I have a unique rear axle/differential. It is a Hallibrand quick change rear end, that I need access to the back cover if I ever want to change the gear ratio.

I know there have been tons of threads about gas tanks and I've read most, if not all of them. Like many folks, I don't want to remove the rear cross member, and I suspect that limits many of my options. Here is a photo of my open bed and axle so you can see my situation.

Any suggestions and/or specific tank options that would allow me to get the tank below the bed will be greatly appreciated.



PS. The hoses running along the rear of the truck are new lines run from the carb and fuel pressure regulator (yes, the truck has a 261 cu/in inline 6 with a McCulloch supercharger, thus the return fuel line to the tank)

[jweb]

Could you use the side mount style from a 47/48 truck? Looks like you would have enough room if you don't mind the filler location.

Could be an issue with exhaust routing depending on your plans.

[Dan in Pasadena]

Originally Posted by TX3100Guy

....Like many folks, I don't want to remove the rear cross member, and I suspect that limits many of my options....

I removed my rear cross member and moved it back to the very end of the frame rails. It's bolted in so it is removable if I ever need. We also fabricated a cross member that is mounted approximately in the location the original was in when we felt the top flange of the frame rail MIGHT be stressed there since I continue to use the original leaf stack, just flipped. Hope this helps some.

Best, Dan

[mr48chev]

Part of having a quick change is being able to have people behind the vehicle see the quick change. I don't remember what you have for power but that particular one isn't noted for it's strength. Far more popular behind a flathead than a big 400 hp V8.

Outside of a 68/72 Suburban tank you pretty well have to modify/move the rear crossmember for most rear tank installations. That burb tank hangs down right where you don't want it on your truck.

[TX3100Guy]

Originally Posted by mr48chev

Part of having a quick change is being able to have people behind the vehicle see the quick change. I don't remember what you have for power but that particular one isn't noted for it's strength. Far more popular behind a flathead than a big 400 hp V8.

Outside of a 68/72 Suburban tank you pretty well have to modify/move the rear crossmember for most rear tank installations. That burb tank hangs down right where you don't want it on your truck.

Thanks for the comments. The engine in this truck, installed by the previous owner - now deceased) is a 1959 261 cu/in inline six, with a McCulloch supercharger. I don't think this combo will rip the rear end apart and I completely agree on the value of letting the quick change be visible.....

My concern about moving the cross member is that it appear to be factory riveted to the rear leaf spring shackle mounts (see photo) and I'm reluctant to using the angle grinder to cut them off, then having to drill new mounts for the cross member in that stout frame. Thoughts?

[e015475]

Like Dan, I took the factory cross member out, but instead of relocating it to the end of the frame, I mounted a .25" wall square tube at the end of the frame. It's a little heavy but it offers a measure of protection for the rear mounted gas tank from a Gen 2 Camaro.

The new rear cross member is held in with the bolt that connects the bed to the frame. This bolt is in double-shear and goes all the way through the frame horn.

Here's what you see from the rear of the truck


Here's the Camaro gas tank - the filler is under the license plate


I don't know that there's any off-the-shelf gas tanks that will give you sufficient access to your quickchange diff - you may have to make one. I've made a few for off-road vehicles out of 6061 .080". I usually have it sheared to size and then it is just a lot of welding. I've bought all the bungs and parts from "The Chassis Shop".

[TX3100Guy]

Originally Posted by e015475

Like Dan, I took the factory cross member out, but instead of relocating it to the end of the frame, I mounted a .25" wall square tube at the end of the frame. It's a little heavy but it offers a measure of protection for the rear mounted gas tank from a Gen 2 Camaro.

The new rear cross member is held in with the bolt that connects the bed to the frame. This bolt is in double-shear and goes all the way through the frame horn.

I don't know that there's any off-the-shelf gas tanks that will give you sufficient access to your quick change diff - you may have to make one. I've made a few for off-road vehicles out of 6061 .080". I usually have it sheared to size and then it is just a lot of welding. I've bought all the bungs and parts from "The Chassis Shop".

Wow, very nice truck! My concern about the cross member isn't necessarily removing it as much as its mounting also serves as the mounting for the leaf spring shackles and while I do plan on replacing the old leaf springs and shackles, I wasn't planning on moving or working on their mounting point.

[e015475]

Looks like a custom tank might be in your future.

You might also look at "Tanks Inc" website and see if there's something you can use. They've got quite a few universal tanks.

When I lived in Phoenix, there were a few places that would custom make a fuel tank. You might try Googling to see if there's any in your area.

And thanks for the nice words

[_Ogre]

Jweb might have the best cure for your problem. TF panel trucks had a similar tank hanging next to the driveshaft, unfortunately no vendor repops the panel fuel tank.

I'm on stock springs in back, I removed the x-member and welded my fabricobbled one back in. Unless you plan on filling your bed with gravel or stacking firewood over the cab, I wouldn't worry about it, 36,000 miles and 12 years hasn't affected mine.

[TX3100Guy]

[QUOTE= I'm on stock springs in back, I removed the x-member and welded my fabricobbled one back in. Unless you plan on filling your bed with gravel or stacking firewood over the cab, I wouldn't worry about it, 36,000 miles and 12 years hasn't affected mine.[/QUOTE]

So what are your thoughts on me cutting the cross member off flush with the frame, leaving the rivets in the frame and shackle mounts (A - in the photo below), and then welding in two steel braces at the back of the frame (B - in the photo below), thus giving me ample room for a rear tank?

Admittedly, while I have the tools to tackle this task, my reluctance is in making sure I don't negatively affect the frame rigidity and/or the shackle stability.

[leegreen]

maybe box the rear half of frame to tie area supporting shackles to your new rear cross member?

[_Ogre]

Originally Posted by TX3100Guy

So what are your thoughts on me cutting the cross member off flush with the frame, leaving the rivets in the frame and shackle mounts (A - in the photo below), and then welding in two steel braces at the back of the frame (B - in the photo below), thus giving me ample room for a rear tank?

Admittedly, while I have the tools to tackle this task, my reluctance is in making sure I don't negatively affect the frame rigidity and/or the shackle stability.

Attachment 2225837

what your suggesting only supports the bottom flange, anything you put in there should support both top and bottom flanges. i believe most of tanks require relocating the brace 6" rearward, no need to move it further than needed. not sure what your issue is with cutting out the stock brace, many people relocate it. not sure any of the rivets are common to the shackle mounts, they are not on the 55-59 trucks. the rivets are pretty soft, replacing rivets with grade 3 bolts will probably be equivalent

[TX3100Guy]

Originally Posted by _Ogre

what your suggesting only supports the bottom flange, anything you put in there should support both top and bottom flanges. i believe most of tanks require relocating the brace 6" rearward, no need to move it further than needed. not sure what your issue is with cutting out the stock brace, many people relocate it. not sure any of the rivets are common to the shackle mounts, they are not on the 55-59 trucks. the rivets are pretty soft, replacing rivets with grade 3 bolts will probably be equivalent

I'm coming around to just cutting the rivets off so that the entire cross member can be moved rearward. That will mean using bolts in the two holes on each side for each shackle that have rivets for the crossmember. Hopefully, I can get my angle grinder in there to cut one end of the rivet off and then use a hammer and a punch to get the rivet out. After moving the crossmember, I will then bolt them back to the frame (top and bottom). Lots of cutting, pounding, and drilling in my future.

[nvrdone]

if I remember correctly, that frame has a taper to it - it gets wider at the rear. If so, your crossmember may not be wide enough to fit. you may have to add some shims.

[TX3100Guy]

Originally Posted by nvrdone

if I remember correctly, that frame has a taper to it - it gets wider at the rear. If so, your crossmember may not be wide enough to fit. you may have to add some shims.

Actually, given the arced nature of the support given its past life as the spare tire bracket, I've decided that when I cut it out, I won't be using it. I'm going to get a 2 inch by 3 inch square tube section cut to the width of the rear frame (in front of the holes I'll need for the bumper mounts. It will fit between the "C" shape of the frame and be plenty sturdy....I think.

Generally, when I work on suspension components I use Grade 8 bolts. Any reason why I shouldn't use Grade 8 hardware to replace the rivets on the leaf spring shackle mounts once I cut them out to remove that crossmember?

[leegreen]

Those OEM riveted joints are better than you think.
-no paint between pieces reducing shear strength
-hot set rivets get pretty tight when they shrink
-rivets fill the entire hole giving good shear strength

In comparison bolts
-have a cross section area reduced by thread depth
-are weakened at thread root
-often do not fill the hole

Rivets will often win in shear strength despite being lower grade metal. And the rivets I've drilled out on frames have not bee the softest to drill.

I'd personally use grade 8 fine thread self locking fasteners for those shackle mounts, if possible get unthreaded shoulder on the bolts to suit metal thickness and drill a closely matched hole size. it costs more than grade 5, but not a lot in the grand scheme of rebuilding a truck.

[TX3100Guy]

Originally Posted by leegreen

Those OEM riveted joints are better than you think.

Rivets will often win in shear strength despite being lower grade metal. And the rivets I've drilled out on frames have not bee the softest to drill.

One of the reasons I was reluctant to cut out the rivets with my angle grinder and remove the crossmember is that I cut several of rivet out on the battery tray. While access was limited, they didn't come out easily.

PS. I could always weld the new crossmember into place.

[fauXGT]

While my situation is a bit different that yours being I have C4 suspension on a stock frame, I opted to remove the spare tire crossmember to fit a tank there. Early 50's Chevy sedan tank for a side fill. I then made the re-enforced back piece so it didn't turn into a Pinto timebomb, but the frame was also a bit tweaked when removing the old cross piece. Used a hydralic spreader and then built the extra support. Just measure before cutting out the old to get the proper dimensions back. Like I saw in my circumstance.... cutting out the old can show some un-sprung issues.

[_Ogre]

Imo, grade 8 bolts should never be used for critical components like suspension or steering. I'd rather have a critical bolt bend and give vs break. Just the opinion of a retired structural civil engineer

[TX3100Guy]

Originally Posted by _Ogre

Imo, grade 8 bolts should never be used for critical components like suspension or steering. I'd rather have a critical bolt bend and give vs break. Just the opinion of a retired structural civil engineer

That is good input. So better to use a Grade 5 or stainless steel bolt?

[leegreen]

well I am not an engineer, but my understanding is:

gr8 is stronger than gr5 in both clamping force and shear force

gr5 will bend further than gr8 before failure but gr8 takes more force to bend to failure than gr5

specs I looked at support that

Maybe if the application involved repetitive flexing of the bolt lightly loaded gr5 is better? But I think for the shackle mounts in question we have already failed if the bolt is flexing. One reason I prefer gr8 here is that the fine thread gets you tighter and that metal-metal pressure resists shear.

But I am not an engineer.........

[jwhotrod]

grade 8 bolts are brittle, not good for shock loads especially side loads in suspension, etc. i have seen grade 8 bolts fail in coilover mounts. grade 5 bolts take the shock loads much better due to the fact they are more ductile. as to stainless steel bolts, the hardware store stainless steel bolts are softer than grade 2. which we call butter bolts. you would be much better to go to ARP for bolts they generally are 185,000 psi structural bolts that are not only stronger but also ductile to take the shock loads. their stainless steel is also 185,000 psi. good luck!

[Accelo]

Structural civil engineers live and die by using the correct fasteners.
I would consider following his advice.
Cheers.

[leegreen]

Have a look at what hardware ships with replacement spring hangers, it is 8 or 10.8. Same for replacement control arm bolts.

Breaking suddenly at a much higher load is not the same as weaker.

If in doubt use bigger bolts. Shear loads need well drilled holes so all fasteners share the load

[TX3100Guy]

Well......the continuing saga of installing a gas tank under the bed continues.... I found several tanks that are 38 inches wide with 3 inch per side brackets for mounting on the underside of the frame. Part of the tank protrudes upwards and part of the tank sits lower than the lower frame section. All of them would require removal of the spare tire cross member, which I've resigned myself to.

Today to get a handle on how they would fit and look relative to the quick change differential I have, I made a cardboard template of the tank and its mounting brackets and discovered a "gotcha" I wasn't considering. While the height and width of the tank is acceptable, the side brackets interfere with the leaf spring shackle mounts. Once I remove the spare tire crossmember, I still will have bolts (see previous discussion on bolt grades) holding the shackle mounts.

Many of you that have done a bed mounted tank also appear to have ditched the leaf springs in favor of coil overs. That modification is not in my plans or consideration. My only option would be to trim the side brackets to allow the tank to be positioned between the shackle mounts. The mounts are approximately 5 inches in width and the tank side brackets are 10 inches in width, so a sizable portion of them would not be able to be used to mount the tank to the underside of the frame.

I'm thinking I need to locate a different tank or mounting option.