After 5 years of on-again, off-again working on this "project", my '66 CC/CS with 390/C6 is now titled and surprisingly is almost drivable!! I've done lots of stuff - power disc brake conversion, power steering conversion with orig steering wheel mods, instrument cluster rebuild with underdash wiring harness rewire, new windshield/gasket, C6 tranny swap, rear axle bearing/seals R&R, and lots and lots of other "stuff". Got to the point a couple of weeks ago where I said I think I actually need to see if any of this stuff works and drove it around the neighbor hood and nothing caught on fire nor not much fell off the truck! Dang, maybe I should title it!!

But I have an elusive problem I can't identify the cause and really could use some of this forum's collective wisdom to troubleshoot. There is a sag to the right: ~ 1" in the right rear and 5/8" in the right front. It actually was leaning when I got it off the transport truck over 5 years ago. Here's the relevant data and actions performed so far to figure this out: absolutely NO rust anywhere - none in the cab mounts, bed mounts, radiator supports, etc. NEVER been in a wreck and has all original sheetmetal - no visible wrinkles/creases anywhere underneath. First action was to replace the two rear leaf springs (5 leaf) and did so from an early 70's donor truck with identical leaf springs...............NO help - right rear still lower by 1" measured at the wheel lip (tire pressures equalized, same size tires and rims verified). So this weekend I put the original left rear spring which was riding 1" higher on the right rear, and nada - still about 1" lower. Many hours of labor wasted again.... As well, I used the '73 390 donor truck's coil springs during my conversion to power disc brakes, so again the chances of a weakened/sagging right front coil is very, very small.

I brought my truck to Tilley Brothers in Fuquay to do a front end alignment and also because they have a laser alignment frame machine. Guess what, their best front end guy could not find any thing out of spec with my frame and could not figure out why the sagging other than did I change the leaf and coil springs which I have already done.

Doing a search thru these forums I did see a few other instances of similar sag stories, and interestingly almost all were the same right side!?! Many of the responses were change the springs, but I've all but eliminated that as a factor. I have new nitrogen gas shocks all the way around as well. The one thing I haven't changed out is the floating rear shackle that attaches the rear spring to the shackle frame mount, but hard to see how even a worn bushing there could effect a full inch of drop, but as you can see I'm searching for straws now.

OK - I think I've conveyed all the pertinent facts. I really could use some help.

Thanks!!!
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Do you think the rear spring perches are bent or the axle tab is off by any chance? It sounds like the truck is loaded for when you accelerate with a open differential. If I remember correctly when you accelerate the rear right portion of the frame actually lifts in response to the tire spinning forward. The weight is then transfered throughout the rest of the frame to level it.

 

Ok, to start with where are you measuring from? The frame or fender openings,,,,, I would start with measuring from the frame in front and rear exact same spot,,, if the frame is lower, that eliminates the cab mounts and such, and you can start looking at spring perches and shackles and bushings.

 

You said you used the '73 donor coil springs.....is it possible that you switched sides with them during the install? I believe there is a right & a left on those. If you have them reversed, your truck could lean to one side....

 

My '66 F100 4X2 longbed is low in the right rear corner also (Also completely rust free, only 69K orig miles). My first test would be just to swap the rear springs side to side, that would eliminate the springs as the culprit if it still sags on the right. Since my motor isn't back in yet I haven't got that far. My front is level, it just seems to be the right rear. Interested to know your findings.

 

He said he already tried that in the original post and it made no difference....

 

Actually, he said he tried springs from a different truck, not the side to side swap.

 

Thanks guys for the inputs/suggestions.

Jhooch - when I got the truck with the orig leaf springs, the right rear was sagging 1". I then replaced both with a set of springs from a '73 donor with the exact same result - right side 1" sag. This past weekend I took the original LEFT rear spring and put it on the right side, with no change = still 1" sag on right.

Pickupman - I'm using the fender to level garage floor as the measurement points. I also measured a spot from frame to axle bumper pad and the right side was lower than the left side as well (though at that spot I believe it was 1/2" delta).

'37 Ford guy - dang, good question/point on the front springs. I thought they were the same but in doing a number of forum searches over the past few days I see some models did have different diameter springs for the left and right sides... And when I had the axles/king pins rebuilt I did take the springs off and didn't mark them. So, a 50% probability I have them switched. And I can confirm that the ladder frame system on these trucks do have opposite lever effects (eg., I raised the right front a bit and noticed the left rear lowered a bit). I don't know if I switched them nor if I did if it would make this much difference - - I am not very hopeful this is the issue since with the factory installed springs there was the 1" right rear sag. I will check though!

airharley - I do not see anything bent or misaligned on the axle nor spring mounts and neither did the alignment guy. As well, as a very pleasant surprise I found out this truck has a factory Dana 60 3.54 Posi in it

 

Not only are all the coil springs RIGHT & LEFT, but the wire diameter and the corresponding load rate is different.

In looking thru 1964/72 & 1973/79 parts catalogs, the right side wire diameter is thicker than the left.

For example: 1972 F350 / 135" wheelbase / 300 I-6 or 360/390 / SRW / GVW's of 6600 thru 8800 lbs.

Right coil spring 1725 load rate, wire diameter: .780"

Left coil spring 1670 load rate, wire diameter: .735"

 

How do you know the springs were "factory" installed when you had them out? Perhaps a PO got them reversed before you had your chance to.

Measure the diameters...only way to know.

 

Well, measured the '73 coil springs I have currently installed and sure enough, I put them on backwards: my RF coil spring is ~ .660 and the LF coil spring is ~ .690. Note that I got different readings on every coil!!? Are these the 1725lb rated springs?

I also measured my original '66 springs and the RF coil is .697 and the LF is .674 (again, variances). Interestingly the '66 springs are larger diameter than the '73's (and that donor was a 390CI truck). I guess the Camper Special Heavy Duty suspension was just that!

OK - so I believe my next step is to swap the springs (not sure if I want the older but thicker springs in there?). I still don't think this will equalize the rear by a full inch. What about replacing that right rear shackle - could a slightly worn bushing in there make a noticeable difference?

Thanks again,
dagger

 

Congrats on locating at least part of the problem.....I think you should probably put the coils in their proper places and measure again before replacing the rear shackle. It seems to me the bushing would have to be a lot more than "slightly worn" to make a substantial difference in the ride height but I could be wrong...

I used '78 F150 I-beams, radius arms, etc when I converted my '66 to disc brakes. I used the 66 springs because I read somewhere that the later model springs might raise the ride height. Plus, I would have had to pay extra for them at the Pick-A-Part where I found the donor truck. Yours might be different since your 66 springs are thicker than your donor springs. While I was at it, I installed a sway bar from a 76 F250. It made a big difference in cornering.

 

Not sure if anyone has pointed this out yet. But maybe checks the body mounts, I know the ones in my 61 are pretty worn. Just a thought.

 

He addressed that in the original post...... "absolutely NO rust anywhere - none in the cab mounts, bed mounts, radiator supports, etc. NEVER been in a wreck and has all original sheetmetal - no visible wrinkles/creases anywhere underneath."

 

Thanks guys - will do the coil spring swap tomorrow. I'll also check the spring height compared to the original '66 coils - that could also make a height difference. The cab and body mounts look OK, but worn with no rust around them. Maybe something else to replace down the road a bit.

Changing coil springs is fun!!!
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