Choosing A Different Thread Pitch

[Solidworks-Mike]

So I've been having trouble finding the exact thread pitch Subaru uses for its bolts. For example they use M12- 1.25 and M14-1.5 for lost of the hardware that gets reused. All I'm able to find though is M14-2.0 and M12-1.5 pitch flange bolts online. Even non-flange bolts don't seem to come in the pitch that Subaru uses.

Anyone see any issue using the alternate finer thread pitches?

[Bob_n_Cincy]

So I've been having trouble finding the exact thread pitch Subaru uses for its bolts. For example they use M12- 1.25 and M14-1.5 for lost of the hardware that gets reused. All I'm able to find though is M14-2.0 and M12-1.5 pitch flange bolts online. Even non-flange bolts don't seem to come in the pitch that Subaru uses.

Anyone see any issue using the alternate finer thread pitches?

This is what I bought from Belmetric for the front end. (2 cars)

Products
------------------------------------------------------
12 x WFHV16 - Hardened Flat Washer (WFHV16) = $6.12
4 x BH16X1.5X40 - Hex bolt 10.9 (BH16X1.5X40) = $18.24
12 x NNH16X1.5 - Class 10 Nylock (NNH16X1.5) = $15.36
12 x NNH14X1.5 - Class 10 Nylock (NNH14X1.5) = $16.32
4 x BH14X1.5X60 - Hex bolt 10.9 (BH14X1.5X60) = $18.96
4 x BH16X1.5X65 - Hex bolt 10.9 (BH16X1.5X65) = $19.52
------------------------------------------------------
Sub-Total: $94.52
Free Shipping Options (Free Shipping): $0.00
Total: $94.52

I bought most my other hardware from McMaster Carr.
Bob

[Evan78]

Funny, I just read an article on fasteners in the May 2016 issue of Grassroots Motorsports. Here's what they said on thread pitch:


There are several reasons why fine-threaded fasteners are stronger than coarse, not the least of which is the larger minor thread diameter (better shear strength) and more threads (improved tension load).

Fine threads also have less of a tendency to loosen because the thread incline is less steep. While fine threads are more easily tapped into hard materials and thin-walled tubes, coarse threads are better suited to softer materials, like aluminum and cast iron.

[Solidworks-Mike]

Funny, I just read an article on fasteners in the May 2016 issue of Grassroots Motorsports. Here's what they said on thread pitch:

That quote pretty much says there is no downside to using the finer thread bolts, especially since they are all through-bolted applications (not being threaded into anything but the locknut).

[Mechie3]

In metric, the 1.5 and 2.0 refer to the mm spacing between threads. Standard refers to the number of threads per inch. The systems are reversed in terms of lower/higher number and whether it is finer/coarser.

Standard: 20 is finer than 18
Metric: 1.5 is finer than 2.0

[Evan78]

That quote pretty much says there is no downside to using the finer thread bolts, especially since they are all through-bolted applications (not being threaded into anything but the locknut).That was my thinking. If you really want to dive into it, you can order a copy of Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook (http://www.amazon.com/Fasteners-Plumbing-Handbook-Motorbooks-Workshop/dp/0879384069?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage).

[wleehendrick]

Is there a formula, or rule of thumb, for a changing a fastener's torque spec if using a different thread pitch?

[Mechie3]

http://www.engineersedge.com/calculators/torque_calc.htm

[RM1SepEx]

Is there a formula, or rule of thumb, for a changing a fastener's torque spec if using a different thread pitch?

It's not related to thread pitch, it's related to bolt strength. You want to preload a bolt so that under load it doesn't stretch and become loose, when it does the load factors change are likely to cause failure.

[wleehendrick]

It's not related to thread pitch, it's related to bolt strength. You want to preload a bolt so that under load it doesn't stretch and become loose, when it does the load factors change are likely to cause failure.

Right, but I thought I heard once that since finer threads introduce more friction, that additional torque when tightening is needed to get the same preload... or is this negligible? I threw it out there since if the effect is significant, if making the switch to finer threads, some amount more torque may be required?

[STiPWRD]

Right, but I thought I heard once that since finer threads introduce more friction, that additional torque when tightening is needed to get the same preload... or is this negligible? I threw it out there since if the effect is significant, if making the switch to finer threads, some amount more torque may be required?
Torque spec is independent of thread pitch. The formula is:

T = P*C*D

where T is torque (in-lbs), P is preload (lbs) or force that is created in the bolt from torqueing it, C is the coefficient of friction at the threads, and D is the nominal diameter of the bolt (in). C can vary quite a bit depending on how clean or lubricated the threads are. If the threads are ultra clean, friction coefficient can be as high as 0.35 and if they are lubricated, friction can be as low as 0.1-0.15. A good average assumption for C is 0.2

[wleehendrick]

Torque spec is independent of thread pitch. The formula is:

Thanks for the formula... that makes sense to me as an engineer, though I deal with objects with far less mass (photons and electrons:p)

I don't recall where I heard/read that pitch can effect torque, but it seems likely that thread condition, or lubrication impact C much more so than pitch.