'71 f-100 spring question.

The Flex-O-Matic rear spring system was available on all '67-'72 Ford trucks. When the pickup box is empty, Flex-O-Matic provides a long, supple spring to cushion the ride. When a load is added, a compensating shackle pivots into action to shorten the spring's effective length. Result: a shorter, stiffer spring as loads get heavier.

Properly-designed helper springs are designed to remain on the truck. They don't come into play until a certain load limit is reached. All other times they're just along for the ride.

Sounds to me like maybe your rear springs are simply worn out. Adding helper springs to them would help, but you might be putting a BandAid on a broken arm. If you're going to be regulary hauling loads, I'd look into replacing the rear springs. If your truck will only see occasional light hauling, the helper springs might get you by.

Keith

Thanks Kieth. I'll only be hauling loades on occasion but I thought about lowering the truck later on so perhaps better springs would be in order. There's a flexomatic equipped camper special in the wrecking yard. Same spring but with an extra leaf. Instead of flipping the rear axle I'd like to cut the mounts off the axle housing and rewelding them on the bottom of the housing. That way I can adjust the pinion angle before I weld everything into place. What's your opinion on installing the optional fuel tank?. I have access to one at the wrecking yard. I'm also contemplating removing the in cab tank. Do you happen to know the capacity of the extra tank? I also thought about retrofitting a later model rear tank. Have you ever researched this swap?

When you purchase a rearend flip kit, that's essentially what you're doing anyway...that is, just relocating the spring perches to the bottom side of the housing. You don't actually 'flip' the rearend over. (I'm not sure why it's called that.) You can buy the kit at places like AIM (

) or DJM ( ). There's a tutorial with some good basic info on lowering at if you're interested. As for the rear tank, as a matter of fact that's exactly what I'm doing now to my '67 shortbox project. I'm removing the in-cab tank and installing the factory in-frame tank to use as a primary. However, make sure that the auxiliary tank you get is a factory unit which mounts inside the frame and not a dealer add-on tank which mounts outside the frame. The add-ons normally aren't set up for a sending unit, so they're not suitable for use as a primary fuel supply. The factory in-frame tank has a 25-gallon capacity and does have a sending unit. If the CS in the salvage yard does have the in-frame tank, be sure to get EVERYTHING associated with it: the tank, skidplate, filler hose, filler hose cover (in the bed), straps and hardware, hangers and the front side-to-side crossmember. The rubber filler hose (from the filler neck to the tank) is hard to find in good condition and nearly impossible to find NOS, so keep your fingers crossed that this one would be OK.

Keith

Great. Thank's for the info Kieth. I thought it would be odd to flip the whole rearend over. hehe. The aux tank I'm looking at is in a '72, I believe. There's actually two that I'm looking at. The '72 (?) has the filler nozzle a bit lower on the side of the bed and a corresponding smaller protector plate inside the bed. It has a skid plate but I didn't look close enough to see if it had a sender. They're both inside the frame and both have an inline switch by the left driver seatbelt. I didn't notice a switch for the fuel gauge however. It could have been missing. I would install one of these so I can monitor the fuel level in each tank if I decide to keep both or replace the in cab unit with a rear. How big is the rear tank you're installing and are you going to post the procedure on your website? Thank's for taking the time to answer my questions. Mark

Oh, BTW, I snagged a disk brake setup from a '77 as per your site's instructions. I can't wait to install it. I'm a mustang guy and am loving working on my truck. I want it to be a daily driver for parts runs, towing, etc. I bought it as a salvage from a wrecking yard for 1500.00 with a 390 and c-6 automatic. It's a long bed. I recently freashened up the 390 with new rings, bearings, 3 angle valve job, Edelbrock cam, intake and carb. Headers are next but I'm waiting to see if I'll get the aux tank because a duel exhaust would have to be routed around it. It's very clean and mostly rust free. There's a lot of nice rust free examples here in California. Eventually I plan on improving the "cool" factor of the truck but I don't want to make it so nice that I can't use it as an occasional hauler.

The '67-'69 factory auxiliary tanks had a filler neck above the beltline, and the '70-'72 tanks had the filler neck below the beltline. If the auxiliary tank has a small aluminum door on the outside of the bed as an access to the filler neck, it's not factory...it's a dealer add-on and shouldn't be used as a primary fuel source, but will still work fine for an auxiliary tank.

The factory in-frame tank like the one I'm using is 25-gallons. I've got a couple paged devoted to that fuel tank's installation right now. Take a look at the following links:

It doesn't have the door but I'd like to find one to make it look custom.

Just a note in passing through the thread. When you "flip" a rear end the pinion gear rides on the wrong side of the ring gear. They're not designed to work that way although they do work...until that one quick, heavy load that puts the whole deal in the crapper.

Fordification (Kieth) just said, "you don't actually flip the rear end, you just move the pads to the other side putting the rearend above the springs." .