Fasteners & Torque Chart for Nuts & Bolts

[Randy Webb]

Let's see if I can load this chart.....

[911pcars]

I wonder why the chart describes the various grade bolts as "iron"?

BTW, these are generic specs for generic applications. Always use the factory torque specs.

Sherwood

[Randy Webb]

Dunno -- agree re the generic nature of these. It's handy to put in the toolbox _underneath_ the Tech Spec book.

[IROC]

These torques are in the correct ballpark, so with the lack of any other info, they could be used. Bear in mind that the above torques will result in a preload that achieves around 80-90% of the yield strength (pretty high), so if you're torquing valve cover nuts you might want to back off somewhat. :>)

Also bear in mind that using a torque wrench to preload a fastener is such a crap-shoot that you really don't know what kind of preload your getting...

Mike

[911pcars]

"Also bear in mind that using a torque wrench to preload a fastener is such a crap-shoot that you really don't know what kind of preload your getting..."

Mike,
Not sure I understand your statement. Care to elaborate? What's the alternate tool (e.g. valve cover nut @ 6 ftlbs, spark plug @ 15 ftlbs, lug nut @ 95 ft.lbs)?

Sherwood

[Jim Sims]

"Also bear in mind that using a torque wrench to preload a fastener is such a crap-shoot that you really don't know what kind of preload your getting..."

If the joint was properly designed the range of possible preload using the expected tightening methods (like a torque wrench) was taken into account. All tightening methods produce tension variability even using an extensometer or other "stretch" measuring methods.

[Randy Webb]

Carroll Shelby discusses preload in his fastener book -- apparently a fairly high degree of preload is good.

[IROC]

The whole object of applying a torque to a fastener is to induce a preload or clamping force. Heck, that's what fasteners are used for most of the time. Anyway, there are several methods of inducing this preload and one of them is torquing. Unfortunately there are so many variables associated with torquing that it is a very unreliable method of accurately preloading a bolt. Applying a given torque to a given fastener will induce a theoretical preload. The problem is in the real world the accuracy of using this method is usually +/- 25%.

If you really want to accurately preload a bolt, you can't use a torque wrench. In my industry it is not uncommon to use the change in length of the bolt during torquing to achieve the desired preload (using the extensometer that Jim mentions to achieve a +/- 5% or so variability). Of course this is not the kind of thing you want to do in your garage.

My only point was torquing is fraught with inaccuracies. Jim's point is also well taken that the joint should be properly designed (and analyzed) to take into account the torquing method.

As for the "high degree of preload", that is a neat thing. Basically, if a fastener is properly preloaded and it doesn't fail during torquing then chances are it will never fail - no matter how much external load you apply to it.

I rambled on this...sorry about that...

Mike

[Wil Ferch]

Randy Webb....I *think* you mean Carrol Smith...

---Wil Ferch

[Randy Webb]

Thx Will, I keep making that mistake....

IROC,
Yes, torque values are not very accurate. It is useful for "mid-range" accuracy. For things like rod bolts, fastener length or an analog is often used. At the other extreme, people often torque things that are non-critical. If a torque spec is given by PAG I use it. If not, I think about how important the bolt is and either use the chart above or just use my brass wrist (I upgraded from iron years ago). I hope to have time someday to post some exceprts from Smith's book...

Another issue is the common misuse of torque wrenches ... such as not having oil on the threads when using them.

[911pcars]

Minimizing the value of a torque wrench is doing a disservice to many relative neophytes and newbies on this forum. How does one factor in a lack of experience in tightening certain fasterners or the size and length of the ratchet/breaker bar/pipe extension? A flick of a manly wrist with a long handle Snap-On 3/8" fine-tooth flex-head ratchet is hardly the same as someone elses attempt with their tool (whatever that is).

I think we've established that a torque wrench is not the most accurate method of tightening a bolt, and there are several variables associated with torquing a threaded fastener. However, by paying attention to parting surfaces, cleanliness and the correct lube and spec, it's close enough that manufacturers recommend a torque spec in their published manuals for their trained technicians who are, in general, more experienced than 80-95% of the DIYers on a typical internet forum. Do people realize repair manuals are designed for dealership technicians and not the general public? If torque were not that important for experienced techs, why would they include them? If only to prevent a lot of stripped threads, adhering to specified torque values is a useful practice until one has enough muscle memory to know when something is close to tight enough.

In most DIY manuals I've read, the material is directly derived from the manufacturer's repair manuals. No where have I seen an example where a substitute tightening technique (by feel) is recommended over using a proper torque wrench (the exception are spark plugs where X turns past washer contact substitutes for 15-20 ftlbs - so maybe we can assume torque up to that point is not all that critical in this case).

It was my turn to ramble,
Sherwood