Spring Tightening Reference Question

[Wildergarten]

Or "Springing into ACTION," sort of...

On page 4 of my TIGHTENING REFERENCE section of the 1967 Factory Manual under SPRINGS, item: "Spring Shackle Bolt Nuts" it shows two different specified torque values of 45-50 ft-lbs and 35-40 ft-lbs for two successive thread sizes, BOTH listed as 7/16-20. This makes no sense to me, particularly the thread on the front BOTTOM shackle bolt of my '68 W200 is 1/2-20 for which there is no specification but is usually on the order of 85 ft-lbs for unplated Grade 5 and 90 ft-lbs for unplated Grade 8. I'm assuming Grade 5.

There is no specification for the front TOP Shackle Bolt which is is 5/8-18. My Chilton's Manual for 1967-88 shows a spec for an "Eye bolt" with no indication of size having a torque of 150-175ft-lbs. Maybe that's it. /sarcasm The Factory Service Manual does show a number for a "Spring Bolt" that is 5/8-18 having a torque of 150-175 ft-lbs, so I have to assume it PERHAPS applies to the top front shackle bolt as that number corresponds to the usual for an unplated Grade 5 5/8-18.

The rear bolts on the front springs are 1/2-20, and those look to be Grade 5, which is commonly 85 ft-lbs for unplated threads but, again, there is no torque specification for 1/2-20 spring bolts.

The rear shackle bolts on this truck are ALL 3/4, but there is no specification for a shackle bolt of that size AT ALL. The ones I have are plated Grade 8. An online reference shows the torque for a plated Grade 8 3/4-16 bolt at 315 ft-lbs, which I doubt I could even attain. To be read as 'as tight as you can get 'em,' 'add blue LocTite,' or, 'I just use an impact driver.' I get it, but I don't think that's reasonable approach in a Factory Service Manual. I know that these trucks are meant to be forgiving of shade-tree mechanics, but...

Anybody who has read the Machinery's Handbook would know how serious a matter fastener torque is by which to derive the correct clamping force. It is not merely a matter of bolt diameter, as bolts are in effect linear springs. The shorter the spring the faster the clamping force develops, the more serious is the torque specification, and more concentrated is the stress in the fastener. The front shackle spring bolts are shoulder bolts, meaning the actual threaded section is rather short, thus making the torque more critical (bolts holding springs being a rather critical item). So from what I see, the actual specs are typical of bolts of those diameters, so the real issue is the missing numbers for common and critcal bearing elements. It is astonishing to me that, with the amount of liability involved in critical vehicle fasteners, these specifications are not taken more seriously.

Bizarre. Waaay back when, when I was in my 20s I knew the torque numbers for just about anything up to 1/2". So this whole exercise has been a long journey down Faulty Memory Lane. Worse, there are numbers there for the U-bolts by size, but the ones on my 1980 open-knuckle front Dana 60 include two plated studs (128 ft-lbs) into the differential case paired with an unplated U-bolt (170 ft-lbs). Expecing the clamping force on the spring to be the same with that setup is beyond unlikely. Any experienced recommendations would be appreciated.

[martincom]

I don't know if this will help you, but here is the tightening reference from the 69-70 service manual:

[Wildergarten]

I don't know if this will help you, but here is the tightening reference from the 69-70 service manual:

tightening reference.jpg

I'll give it a closer look, but its detail appears vastly better. Thanks