Where do torque specs for a specific component come from?

Question

I am asking about the manufacturer specific torque specs for a given year, make, model, component/part and bolt/stud (not asking about bolt grade and size standard specs).

Random Example: 2003, Ford Powerstroke (6.0 liter PSD), Injector Hold Down Bolts are 24 foot lbs in the service manual.

Maybe the question could be more simply stated as: Where do service manual publishers get their info? In the example, does Ford specifically make public the original torque spec used for each and every part? Does this information have a centralized source?

This question is somewhat related to data source ownership and legality of republishing said torque specs found in a given service manual. However, legal consequences are NOT the question itself. I'm simply looking for an explanation of the flow of information from the manufacturing line to the service manuals.

Answer 1

Reputable sources (e.g. Haynes, Chilton's, Alldata, etc) are paying a licensing fee to republish OEM information, or at the very least asking permission.

Haynes is probably the only "paper" DIY repair manual source left, and typically they only publish information on older popular vehicles, where the income stream from the OEM manual is little or no interest to the OEM publisher.

While proprietary, the information is not that closely protected, especially on vehilces that are no longer produced by the OEM.

About 99% of automotive fasteners are NOT critically torqued, so any discussion about bolt grade is moot - as you suggested in your question.

The "original" information is derived from a collaboration between the OEM engineers, and the OE component manufacturers. So an injector hold-down torque is determined by the application, the materials, and the ultiate goal. Maybe that bolt could be a lot tighter, but not without cracking or distortion of the injector flange, crushing a seal, etc.

(For instance, Subaru knock sensor uses an M8 bolt, but is ultra-sensitive to the distortion created by overtightening. Final torque is a small fraction of what seems appropriate.)

The engineering information is addded to the OEM "repair manual" to aid technicians in replacing components. Often there are general torque rules, which apply when no torque is specifically mentioned. Specialized torque values are provided when the general rules don't apply (knock sensor, oil pan, transmission pan, timing cover, water pump, valve cover) and the desired torque is typically much lower than a blind assumption based on the general OEM rules or bolt size.

The information flows from the OEM design engineers TO the manufacturing line AND the "repair manual" ... "electronic database". In fact, part of the "design" of the new production vehicle would be developing the tooling to tighten injector hold-down bolts to the correct values. I can't speculate on the exact timing of the information flow, but I am certain the information is not "centralized" with the possible exception of OEM manufacturing enginnering oversight.

Answer 2

There are many methods of tightening threaded fasteners. The most common being torque control tightening.

Torque specifications are determined by the fastener being used. After engineers determine the size and strength for the application, whether the fastener is dry or lubricated, they can calculate the torque specification.

T = K x U x D x P

T = torque
K = standard representing 1.33
U = coefficient of friction
D = diameter of the fastener
P = necessary preload

This does not apply to special fasteners, like stretch bolts which have special instructions to achieve proper torque.

Sources:
How to Determine Torque Specs
Fastener and Screw Torque Calculator
Methods of Tightening Threaded Fasteners

Answer 3

If there are doubts about the aftermarket publishers recycling figures, here is a late 70s Datsun factory service manual:

and Haynes..

You can pretty much find every drawing in the Haynes book in the FSM.

Re torque specs Haynes took the liberty of rounding ranges to a single number, eg 93-107 ft-lb in the FSM is 100 in the Haynes book.

I did at a quick look find one significant torque discrepancy (almost a factor of 2). I'd guess that's because Haynes covers a span of near 10 years while the FSM I have is for a specific year. You can see if the specs get changed over the years that's hard for the condensed book to capture. (It could also be a typo of course)