I recently mounted my Jab 3300 on my CH750. The mount uses 4 AN-6 bolts and 2 AN-4 bolts to secure it to the firewall. AN-363 (metal locknuts) are used. It's been my experience that metal locknuts require a little more "oomph" to wrench than do the usual fiber locknuts. Does this affect the torque values? I've looked at the FAA torque tables and they don't mention AN 363 nuts (or I missed it! LOL!). Also, different values are given for AN-364 and AN-365 nuts used in tension and shear. Looks like an engine mount is in both tension and shear?

Anyone know the correction or actual torque ranges for AN-4 and AN-6 bolts with AN-363 metal locknuts?

John

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I own torque wrenches and find in my mind they are a lot to do about  nothing, but there has to be a standard to be guided by. Eaa has some videos on this subject perhaps a message to them would give you an answer. For years I have used the practice of a 90* turn past the first resistance using a normal length wrench handle. As I interpret that the lock nuts are to replace lock washers, I could always observe when the lock washers were  flat, tight enough!!!!  Then again how do you know the torque wrench is calibrated. Check the EAA Buiders Hints videos and the values in the Zenith builders guide.

KenB

I think I missed the point. I would add the torque to turn the locknuts to the torque values Sorry

KenB

If you have a beam type torque wrench, ck the torque required to turn the nut BEFORE it bottoms out on the washer. You then add this torque to the listed value for your final torque.
If torque values were not important there would not be a table of values for them listed in the FAA maintnance manual. Yes they are important as they control the stretch of the fastener under load and prevent the attached nuts from backing off due to vibration and other causes. ARP fasteners has made an exhaustive study of torque and torque values, while primarily based upon race cars the results are the same. They provide a special torque lube for fasteners that allow you to create the correct specified torque each and every time as long as you are using a calibrated torque wrench. Remember, you will not be able to pull over and stop for repairs at 8,000 feet if something fails or falls off. The torque value is listed for the fastener based upon size. Hope this is some help.
I have to assemble critical high pressure extrusion dies for my job.  We always torque the bolts to a specific value, and it makes a difference.  Designers use these values to determine how many fasteners, how far apart, etc.  The one thing that is new for me is the "drag torque", you get from locknuts.  The construction standards say to add that on to the bolt torque when you are doing the bolt torque.  With a beam wrench it's easy.  I don't have one, so I turn down my "clicker" wrench as far as possible, and turn it up until I can turn the bolt loosely w/ no click. (so only the drag for the nut shows).  Then I add this value to the bolt torque value to tighten.  The construction standards show most of the raw bolt torque values.

Thanks to all for the good info!

Caleb replied this a.m. that for the metal 363 nuts, use the 365 torque tables (as found in AC43.13-1B) for the bolts. He did not specifically mention, but as pointed out by others above, one would determine the drag torque of the nut and add that to the value.

Thanks again,

John

Good question, normally in a situation more critical they will use a wet torque for say a crankcase, conrods, props this provides a more consistant torque or if they want to be more precise again they will have you measure bolt stretch some engine makers make you do this with conrods.  I've never adjusted a torque value due to the nut used though, although you certainly won' t do any harm by going say an extra 5% on standard AN values if your concerned.  Just use a bit of common sense and be consistant e.g. think about what your clamping, sometimes recomended torques are crap I find myself going a little higher in lots of senario's especially with smaller fasteners.  Most people won't torque the majority of AN bolts installed on an aircraft unless specified by the manufacturer and I've seen plenty of people do up engine mounts without torquing  them and they don't fall out although I'm not suggesting it though.

It seems my concern about the metal locknuts was "much ado about nothing"! It seemed to me that there was an awful lot of drag when using the metal locknuts, so that's why I was concerned about the torque values ... I think the engine mount is probably one of the places where proper torque is a good thing!

However, I found that although initially the drag is high, once you get the nut going on the threads, the drag drops off considerably. With AN 6 bolts, the drag torque on the metal nut was only about 2 ft lbs. - Sooo, looks to me like just torquing to mid range gets you safely within specs.

Thanks,

John

Hi John;

What about castle nuts and cotter pin. Might be a worry free alternative.

 

KenB.

I think using a castle nut on a motor mount would cause a lot of worry! LOL! I would think you need that mount defintely torqued-down so there is no possibility of motion where it meets the firewall which would result in fretting. I don't think castle nuts are intended to be used where there is a significant element of tension in the bolt - they are more or less just retainers for a bolt in shear on a rotating part like a control linkage.

John

Just FYI there is such a thing as a castelated lock nut.  I used theme where I really really didn't want to have to worry about whether or not I cotter pinned.  i.e. control surfaces.  Not that I would ever intentionally leave out a pin and count on it, but it just might save my bacon some day...  for an -3 they are about a2 bucks a piece...  Not cheap, but ehh...

Torque wrenches are a great tool for getting consistent values but accurate calibration should not be taken for granted.

When I bolt things together my real peace of mind is Loct-Tite. Get the red stuff. The blue stuff doesn't hold like the red. Be careful not to get carried away on the smaller fasteners like AN-3 caps and nuts as the threads will often be damaged during dis-assembly.

Speaking of dis-assembly, warm up the nut to about 150 Farenheit with small propane torch on nuts to get easy release if necessary.

Brian

 

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