torque limits for our engine bolts

[frank williams]

What is a good reference for torque limits for our steel 4-40, 5-40, 1/4-32, .... into our aluminum engine parts.
I've been using about 12 inch-pounds for 4-40's ...... too much too little ..... what about glow plugs.

[Tim Wescott]

I thought "aha -- I'll just send you to Machinery's Handbook, 'cuz it's what the guys on the metalworking newsgroup always do to _me_!"

Right there, on page 1475, there's a start of a whole section on torque in fasteners, with equations and relationships between torque and tension and pages and pages of material.  All of which leads me to repeat the advice I got from my across-the-street-neighbor about rebuilding my 1948 Chevy:

Just tighten until the bolt snaps, then back off half a turn.

The first place I'd look would be in the engine manufacturer's specifications.  If they don't have it, then you want it as tight as it needs to be and no tighter -- really reefing on a head or backplate bolt will just warp things up.  What that torque figure should be is probably a bone of contention even within the confines of the engine manufacturer's shop: I've heard too many such arguments between Really Good mechanical engineers at work to believe that there are definitive answers.

[Paul Walker]

It all depends on the material and geometry of the bolt and nut combination.

Because of "these" type of issues, I only use grade 8 4-40 bolts anymore. Get from a speciality store. 2-56 are not for any significant structural use so hobby shop types are OK. 6-32 is bugger than necessary for most model applications so once again HS types are OK.

Is there a specific application you are worried about?

[RandySmith]

What is a good reference for torque limits for our steel 4-40, 5-40, 1/4-32, .... into our aluminum engine parts.
I've been using about 12 inch-pounds for 4-40's ...... too much too little ..... what about glow plugs.

Hi Frank

If your using alloy 4-40 or 5-40  I use  12-15 inch pounds, they will safely take that and it is tight, using Stainless or cheap steel bolts you will have to use much less, Grade 8 alloys  are easy to get, The only place on the engines I use less is backplate bolts, because the are very short, I personally use 10 inch pounds on the short bolts and use blue locktight.  headers  I use 14 to 15 inch pounds. This assumes you are putting them into good quality cases, and not porous weaker ones, if so then back off to 10 and 12.

Randy

[BillLee]

TFAR

[frank williams]

Paul, Randy,Tim
Thanks for the input ..... I am more worried about the threads in the aluminum pulling out than the bolt strength.
12-15 in-lb seemed to be about right.  I'll stick with that.  Head bolts too?
Bill
TFAR? .... Tentative Findings and Recommendations ?
              Task Force for Administrative Reform ?

[Crist Rigotti]

That feels about right?

[BillLee]

TFAR is the assembly instructions for designs which use TLAR.

[steve bittner]

Frank, I always used the old german specs when tightening things. That is GOODENUFF. By the way grade 8 bolts have great tensil strength, but terrible shear strength.

[Tim Wescott]

Frank got me interested, so I did some web searches.  "Machine screw torque table" seemed to get the most hits.  I didn't see any grade 8 hardware listed below 1/4" screws -- dunno what that means.

http://www.federalscrewproducts.com/torque-chart.htm
http://www.boltdepot.com/fastener-information/Bolts/US-Recommended-Torque.aspx

This one amuses me because their "ideal method" pretty much matches my suggestion, only for real.  Just build a baker's dozen planes, test the first 12 to destruction, and then you'll know just what to do on the 13th:

http://www.greensladeandcompany.com/pdf/Torque-Tightening%20Inch%20Machine%20Screws.pdf

[frank williams]

Crist
I think you got it .....

Bill, All
I've always remembered the Nelson 15 Goodyear head you gave me at the Nats one year.  This was before the "Nelson" plugs came about.  This head was exactly like the stock head, except the threads were extremely tight.  You couldn't thread the plug in with you fingers, it had to be wrenched in.  Bingo ... and instant 500 rpm.  Applying that principle to stunt motors, or the idea that a tight plug was a good plug, has led recently to a couple of stripped heads.  I don't necessarily think that a leaky plug will ruin a stunt run, effort is better spent on the carburettion  end of the motor, but you never know.

[Tim Wescott]

Bill, All
I've always remembered the Nelson 15 Goodyear head you gave me at the Nats one year.  This was before the "Nelson" plugs came about.  This head was exactly like the stock head, except the threads were extremely tight.  You couldn't thread the plug in with you fingers, it had to be wrenched in.  Bingo ... and instant 500 rpm.  Applying that principle to stunt motors, or the idea that a tight plug was a good plug, has led recently to a couple of stripped heads.  I don't necessarily think that a leaky plug will ruin a stunt run, effort is better spent on the carburettion  end of the motor, but you never know.

The idea of the Nelson plug isn't that it should be tight, but that the threads shouldn't be part of the combustion chamber.  Making a conventional head with a glow plug hole that's "squeaky tight" would do, but it puts considerable demands for precision on the glow plug, and (I'll bet, at least) that it soon either wears out to normal (i.e. leaks to the gasket but not beyond), or galls up and you never get your glow plug out.

[Terry Caron]

It's well to note that in our applications equal torque is more important than exact torque.
As long as you have enough. 

Terry

[john e. holliday]

I remember years ago at the NATS in Olathe, 68 to be exact, a father son team were working on their planes.  The son was putting the planes together and just finger tightening the bolts on everything.  The father cinched them down by hand.   He said the son had too much torque and would snap the bolts off.  I always tighten most bolts down by feel.   The old phillips head things I would grab with pliers on the back plate and tighten them.   Now I use a small handled socket head driver to tighten my bolts on every thing.   I've seen bolts broken using torque wrenches.

[Tim Wescott]

On many big engines, the bolts are used once, then tossed. On several German Diesels, Deuetz for one, the bolts are installed
increments of degrees, not Newton meters/foot pounds. NEVER reuse a bolt if at all possible. They do get streched and the tensile strength is less, thus torquing to specs will be very inaccurate and can cause bolt failure always when you least need this to happen.
Compare the price of say, eight bolts, to the price on one engine or plane.

Full-size head bolts use "torque to yield" -- you actually tighten the bolt just enough that it almost, or does, permanently stretch a bit.

Apparently it's become very common to torque down to some light amount, then turn the bolt by some specified number of degrees after that.  I know this because one of my customers makes an electronic gizmo that helps you do just that.

[Paul Smith]

All that "Machinist's Handbook" stuff is based on metal-on-metal.

If wood is involved: pine, spruce, oak, balsa, Finnish plywood, etc, you still need some finesse.  Who uses a torque wrench to bolt an engine onto a model airplane?  In 52 years of modeling I've never seen it even once.  But, admittedly, I have never checked anybody basement to see how they bolt on engines.

I prefer Holochrome Allen head capscrews.  The world's third most expensive threaded fastener.

[Brett Buck]

On many big engines, the bolts are used once, then tossed. On several German Diesels, Deuetz for one, the bolts are installed
increments of degrees, not Newton meters/foot pounds. NEVER reuse a bolt if at all possible. They do get streched and the tensile strength is less, thus torquing to specs will be very inaccurate and can cause bolt failure always when you least need this to happen.

    This is true if you tighten it to the plastic region of the bolt. That should definitely never apply to bolting an engine to a wood motor mount.

    Brett

[Lauri Malila]

Exactly what Brett said, the more important reason to replace bolts regularly is that the allen key hole gets rounded. That is most annoying and allways happens when you least expect it. For same reason, I don't recommend trying to save grams by using domed head-allen bolts. L

[Tim Wescott]

For same reason, I don't recommend trying to save grams by using domed head-allen bolts. L

Those things are a menace!

[Lauri Malila]

Would Trx heads be better? I have not tried them. L

[Tim Wescott]

Would Trx heads be better? I have not tried them. L

I don't have any mileage with torx heads -- I use them when a product comes to me that has them.