Fastner Torque settings for Differnt Metals?

[John L.]

This is a general question regarding the correct torque specificaiton for fastners. From the chart found at this website it give the various torque settings for particular size and grade of fastners. Does consideration need to be given as to the metal that the fastener is being screwed into...such as aluminum, or steel plating, or lets say a cast piece like the third arm bracket on a C-1. My thought - and from past careless experience - is it has been easy to strip the threads on a cast piece rather than steel.

Am I making a big deal over a little variable??

http://www.engineershandbook.com/Tables/torque.htm

Thanks,
John Lanning

[Bill H.]

Re: Fastner Torque settings for Differnt Metals?

The torque values will be the same.
But with anything that has been apart before, you can't be sure whoever put it back together used the proper torque. I've seen so many people who love to overtighten everything. Its hard to tell if the threads have been stressed.

What does make a difference is wet/dry. There are different torque values for oiled bolts, ARP oil and antisieze.

[Duke W.]

Re: Fastner Torque settings for Differnt Metals?

I'm not so sure. I believe the typical torque specs based on bolt grade, shank size, and thread pitch are for a bolt that is theaded to a same grade nut. When the bolt threads into cast iron or cast aluminum, the manufacturer's torque specification should be followed, particularly when threaded into aluminum.

I'll give you and example. The 1975 Cosworth Vega Shop Manual Supplement specifies 25 lb-ft for the oil pan to block bolts which are 5/16-18 Grade 5. An old reference card I have specfies 20 lb-ft (I notice the referenced link says 19.) for a 5/16" thread, but does not distinguish between coarse and fine threads. I recall that fine thread bolts typically have a slightly higher "standard torque" than a course thread of the same shank size.

Many a CV owner has suffered oil leaks due to over-crushed gaskets and stripped threads in the block.

Further inspection of the 1974 Vega Shop Manual, which is the base manual for the CV specifies 15 lb-ft for the same bolt, oil pan, and block, and so does the Chevrolet Power Manual. All other 5/16-18 bolts that thread into aluminum including the coolant and water pumps bolts are specified at 15 lb-ft.

It took me a couple of pan gasket jobs to figure this out.

I'm sure anyone who has ever worked on aluminum engines has stripped a thread, so it's best to have a good idea of what torque values should be and question any that appear to be higher than your own experience dictates or what the shop manual specifies.

Duke

[John L.]

Re: Fastner Torque settings for Differnt Metals?

Thank you Duke and Bill for your insights...seems I should consider the material that will be bolted into when lookiing at the proper torque - which may mean the research and asking questions.
As always much appreciated,
John