¼ coarse thread vs. AN-4 bolts on an engine

704CH · #1 06-12-2012, 03:28 PM

Hey guys, sorry if this is an easy search, but I found nothing.

The question is regarding engine halves and oil sump bolts. Van's has you change out some of the bolts with AN-4 bolts for example on the mixture control bracket for the IO-360 horizontal aspirated engines..

The question is, can AN-4 bolts be substituted for the Coarse thread bolts that came with my Aerosport engine? Of course assuming I use proper length.

This might be a simple question, but I don't want to do anything wrong, and it seems easier to use the AN-4 bolts I already have vs order some longer engine specific bolts for areas that I want to add clamps etc and need a longer bolt.

Thanks

mannanj · #2 06-12-2012, 03:40 PM

You can sub some of the 1/4" x 20 bolts with AN-4 bolts of the proper length, but you should reduce the torque somewhat. I'm not sure of the ratio. Someone jump in here.

Think of the threads as an inclined plane. The AN bolts have a shallower incline therefore requiring less torque to produce the same clamping force.

Hope this helps.

az_gila · #3 06-12-2012, 03:51 PM

Generically, the coarse thread bolts with a deeper thread than the AN bolts are stronger in tension...

I wouldn't substitute at random myself...

mannanj · #4 06-12-2012, 04:20 PM

Quote:

Originally Posted by az_gila

Generically, the coarse thread bolts with a deeper thread than the AN bolts are stronger in tension...

I wouldn't substitute at random myself...

Wouldn't that depend on whether it' a grade 3,5, or 8 1/4"x20 bolt, Gil?

I'm not suggesting random subs either, but it can be done.

az_gila · #5 06-12-2012, 04:43 PM

Quote:

Originally Posted by mannanj

Wouldn't that depend on whether it' a grade 3,5, or 8 1/4"x20 bolt, Gil?

I'm not suggesting random subs either, but it can be done.

Yes, if the materials are different.

But, for the same strength material, the deeper threads essentially make it harder to pull off the nut....

AN bolts are 125,000 psi min tensile strength

Grade 5 bolts (under 1 inch) are 120,000 psi

Grade 8 bolts are 150,000 psi

GAHco · #6 06-12-2012, 05:12 PM

A) The Course thread will hold better than a fine thread if the threads are in tapped aluminum.

B) It will be even better if there is a helicoil installed for the threads to go into.

C) For even more strength a fine thread with the proper material hex nut.
It will be able to develop more tension than the same diameter course thread bolt.

D) A Keensert a (Threaded Solid Insert) properly installed in good material is just as good as any nut.

Lars · #7 06-12-2012, 05:46 PM

Hmmm... all things being equal, a fine thread bolt of the same material is always stronger in tension than a coarse thread bolt. More tensile stress area. The threads in a tapped hole will be more difficult to strip if they are coarse, but the fastener will be weaker.

From Shigley and Mischke, 1989: "A great many tensile tests of threaded rods have shown that an unthreaded rod having a diameter equal to the mean of the pitch diameter and minor diameter will have the same tensile strength as the threaded rod. The area of this unthreaded rod is called the tensile-stress area At of the threaded rod..."

The minor diameter of a given fine thread is always greater than that of the corresponding coarse thread, so for a given nominal diameter, bigger (finer) is better.

Since we are talking about case bolts, which usually (though not everywhere) have a nut on them, a fine thread bolt wins.

In the case of tapped holes the above is irrelevant since you have to use bolts with the same thread as the tapped hole.

rvbuilder2002 · #8 06-12-2012, 05:58 PM

The bolts related to the question posted by the O.P. (oil sump bolts for mounting engine control brackets) are installed only at locations where a nut is used (a fine thread AN bolt could not be threaded into the course thread case holes).

I am pretty sure the bolts being replaced are grade 5, which makes the replacement bolts actually slightly superior, tensile strength wise.

mannanj · #9 06-12-2012, 05:59 PM

Tom:

What is the torque ratio between nf bolts and nc bolts of the same diameter to produce the same clamping pressure.

I know there's a table somewhere, just can't come up with it right now.

GAHco · #10 06-13-2012, 07:26 PM

Quote:

Originally Posted by rvbuilder2002

The bolts related to the question posted by the O.P. (oil sump bolts for mounting engine control brackets) are installed only at locations where a nut is used (a fine thread AN bolt could not be threaded into the course thread case holes).

I am pretty sure the bolts being replaced are grade 5, which makes the replacement bolts actually slightly superior, tensile strength wise.

AN Bolts are 125 Ksi min tensile strength, if I remember correctly grade 5 is min 105 Ksi. With the exception of physical configuration either one should be adequate.