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Nuts on carbs and fuel servos

vic syracuse

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While doing a prebuy on Friday on an RV-8 I noticed a nut missing from the aft fuel servo mounting stud. This is not the first one of these I have discovered missing on an airplane. I know standard practice has been to use a nut, a lock washer, and washer, but for many years now I have been using AN363 all metal stop nuts with a washer in these locations and I have never seen one come loose. You might consider it on your airplane.





Vic
 
There was an RV-6A that had engine stop running a number of years back just short of KEMT airport. There were no injuries but the airplane was totaled.

The FI servo was loose.
 
Mine are installed "standard practice" but I've always kind of wondered about them and check them regularly. Nothing loose so far...

If you have a flat washer, a lock washer, and a nut aren't you essentially just locking the nut to the flat washer? I'm guessing this is why you brought it up. I wonder why this method is "standard practice" here.
 
Mine are installed "standard practice" but I've always kind of wondered about them and check them regularly. Nothing loose so far...

If you have a flat washer, a lock washer, and a nut aren't you essentially just locking the nut to the flat washer? I'm guessing this is why you brought it up. I wonder why this method is "standard practice" here.

I've always thought the same thing about the lockwasher on a flat washer...didn't make any sense. Also, it seemed silly that the airbox is safetied to the carb or servo but the carb or servo is not safetied to the engine. Always seemed bassackwards if anything.
 
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Lock Washers

act as springs, so to speak. That is why we throw away the internally toothed lock washers after one use. There is not enough "spring" left in them to allow the plain nut to maintain torque if they are used again.
I agree with Vic that a metal self locking nut is probably better practice. I will admit to being concerned that a really tight self locker will unscrew the stud when you take it off... but that is why blue locktite was invented for the studs.
 
I know, we've always heard that the lock nuts will pull the stud out, but I've never seen that happen yet (I know I haven't seen everything yet) and I decided a long time ago that the stud could be taken out of the nut in a vise if the stud came out and then reinserted back into the engine. Haven't had to do that yet.

You are absolutely correct on the internal tooth lock washers. Always throw them out when you remove them and use a new one.

Vic
 
Though I have never found any loose if they have been torqued properly with new new lock washers at installation, piggybacking with a Palnut (google it) is another option for safetying critical course thread nuts.
 
I wonder if some of these loose nuts are due to torqueing them dry as opposed to with a lubricant as specified by Lycoming. I installed mine on the carb per plans but also keep an eye on them. So far they have remained properly torqued.

Chris
 
Loose Nuts

I think this is mostly either under torqueing and/or re using lock washers. Remember the connecting rod bolts on Lycoming's are not safetied.
 
I use p/n STD 1763 palnuts to lock the plain nuts as per the Lycoming O-360 parts manual.. Got mine from Spruce. Hope that helps
 
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I think this is mostly either under torqueing and/or re using lock washers. Remember the connecting rod bolts on Lycoming's are not safetied.


I think you are correct. Don't forget we have many first-time builders that are now first-time airplane mechanics once they get their airplanes flying. Most are very careful to reassemble something exactly the way it came apart, not realizing that some parts such as internal tooth lock washers, are not supposed to be reused.

Sometimes it is good to take a look at something and see if it can be made "maintenance proof." In this case, there have been a lot of great suggestions (locktite, palnuts, double nuts, AN 363 metal locking nuts, etc). About the only ones that are "reusable" would be the double nuts, AN 363's, and perhaps a palnut.
First timers in the field don't always have loctite and new lock washers available.
Those who work in shops or on a lot of airplanes certainly do, but for the majority of the new owners the carb may come off once to change a jet, and it is here I think that the reuse of the lockwasher sets the table for a future loose/lost nut.

Just a suggestion. :)

Vic
 
Palnuts

.....

Sometimes it is good to take a look at something and see if it can be made "maintenance proof." In this case, there have been a lot of great suggestions (locktite, palnuts, double nuts, AN 363 metal locking nuts, etc). About the only ones that are "reusable" would be the double nuts, AN 363's, and perhaps a palnut.
.....
Vic

I had to look that one up since I haven't used many palnuts...:)

According to the FAA bible AC43.13 they are not reusable.

https://tinyurl.com/l3gsxl6

So if you are using them, buy a bunch extra as spares.
 
That's what I did. Double nut acts as a lock nut

Larry

I have never seen anything documented in the FARS, AC43, hardware manuals, or airframe manuals that state this. Not saying it doesn't make sense, but a "safety" device, devices, or techniques are well covered and this isn't one of them, or at least one that I can find.
If someone can dig up a reference please share.

Certainly not trying to be argumentative, but every time I think there is a logical solution to a perceived problem, there is something that I was not aware of that dashes it on the rocks.

A nut and lock washer has been the standard for eon's in this application for every type of carb, throttle body, injector, etc....
I believe, per Vic's original post, the problem isn't the hardware. It is the hardware not being properly installed and checked.
 
....

A nut and lock washer has been the standard for eon's in this application for every type of carb, throttle body, injector, etc....
....

That's what I thought too, but a check of my O-360 (ND and WD) manuals show palnuts called out not lock washers, with the older ND manual calling out a STD-36 rather than the STD-1763 mentioned. The Lycoming superseding parts list says a STD-36 => STD-475.

I guess there are several Lycoming 5/16 palnut versions around. :)

Update, the Lycoming supersede list has STD-1763 => STD-475, so STD-475 is the current part number for all.

They are also pretty cheap -

http://www.aircraft-specialties.com/lock-washer-3125-lock-int-teeth-sl-std-475/


SECOND UPDATE

My mistake, I didn't look at the STD-475 picture - it's a star washer - so Lycoming seems to have removed the palnuts from their updated use for carb attachment.

My mistake, lock washers are the correct part, but haven't been used for eons...:)
 
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I have never seen anything documented in the FARS, AC43, hardware manuals, or airframe manuals that state this. Not saying it doesn't make sense, but a "safety" device, devices, or techniques are well covered and this isn't one of them, or at least one that I can find.
If someone can dig up a reference please share.

Certainly not trying to be argumentative, but every time I think there is a logical solution to a perceived problem, there is something that I was not aware of that dashes it on the rocks.

A nut and lock washer has been the standard for eon's in this application for every type of carb, throttle body, injector, etc....
I believe, per Vic's original post, the problem isn't the hardware. It is the hardware not being properly installed and checked.

Sorry. I didn't mean to imply that it was an aviation standard and that others should use it. I was just throwing out another alternative for others to consider. Clearly, the accepted standard isn't working or being followed as inspectors are seeing this somewhat regularly.

It was my opinion that double nuts would work better than a star washer so I used it. Double nuts as a loosening prevention method goes back over a century, before we had lock washers. I thought I mentioned, but apparently didn't, that their use requires proper technique to be effective so this isn't the fix to the problem out there, which I would agree is an "installation issue."

http://www.boltscience.com/pages/twonuts.htm
 
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Sorry. I didn't mean to imply that it was an aviation standard and that others should use it. I was just throwing out another alternative for others to consider. Clearly, the accepted standard isn't working or being followed as inspectors are seeing this somewhat regularly.

It was my opinion that double nuts would work better than a star washer so I used it. Double nuts as a loosening prevention method goes back over a century, before we had lock washers. I thought I mentioned, but apparently didn't, that their use requires proper technique to be effective so this isn't the fix to the problem out there, which I would agree is an "installation issue."

http://www.boltscience.com/pages/twonuts.htm

Great reference. Thank you.
 
Double Nut

Some certified Pitts models use double nuts on the outboard end of flying wires, Plain nuts are also used as locking devices(jam nuts) on opposite end of flying wires and on various control rod end bearings.
I go back quite a while with both Continental and Lycoming powered aircraft. I have a distinct recollection of pal nuts on Continental but not on Lycoming. The big issue with double nut is whether the stud is long enough to adequately engage the second nut.
Of course none of this is really relevant on EAB, any reasonable locking method is acceptable. For example on the exhaust I use split stainless lock washers with no flat washer.
 
This is very timely. Just yesterday, I called Airflow Performance to see what kind of bolt they prefer a builder to use when attaching my RV-8 snorkel to the throttle body of my FM-150. They didn't use anything that special due to the fact that the FM-150 is tapped for 1/4-20 bolts and the bolt can be torqued to spec without a nut on it. I went one step further and used a drilled-head hex head bolt,
MS20074-04-06. It's a 20 thread per inch "engine" bolt instead of the usual 28 thread per inch found on that size bolt. I was then able to safety wire the bolts.
 
I just mounted my carb a few days ago and was not happy with how easily the nuts would come off using the standard star lock washer. I ended up going to Lowes and getting some grade 8 split type lock washers, these give noticeably more resistance when removing and the spring tension remains even after you begin to loosen. If you think there is any negatives from using this type of lock washer, please voice your opinion.
 
I have yet to have anything come loose using split grade 8 washers with a thin backing washer. I buy them by the pound at Tractor Supply. Wouldn't hurt to throw blue loctite on and maybe even a palnut.
 
Carb nut caution..

It has been in earlier forums, and at least several RV's I know of, including mine, have had the carburetor attach nuts back off (even with lock washers) due to the engine torque and a very tight FAB snorkel set-up. If your installation is "new", make sure your flex joint at the cowl inlet is not too tight. The engine torque will turn the snorkel assembly into a breaker bar and loosen the carb nuts, resulting in leaning and backfire/idle problems initially, worse if left unchecked. I routinely reach in the inlet and look for play in the FAB. I also ran some safety wire around my carb studs to prevent the nuts from fully backing off in the event they loosen. The carb bowl screws/nuts are equally as vulnerable, even with the bent tab safety's.
 
Unless I'm missing something the current Lyc parts catalogue from their website lists a plain washer (STD-35), plain nut (STD-1410) and palnut locknut (STD-1763) for the O-320 B & D.

Our engine had the lock-washer and plain nut configuration which seems to be what all new Lyc's leave the factory with. Do you reckon I could find a reference for that?

Installed a new carb on our O-320 with new lock washers and the original single plain nuts and never felt comfortable. Way too much rattling going on there to suit my liking.

Swapped out the plain nuts for AN363's and immediately felt a sense of ease.

Also, where does it say you can't reuse all metal self locking nuts? I presume the prevailing torques mentioned in AC 43.13.1B, Section 4, Para 7-64, Sub Para F and Table 7-2 relate to all styles of self-locking nuts?

If not, what would the argument be for reusing anchor nuts?
 
AN363 course thread

Anybody know where to get a all metal self locking 5/16x18 nut?

I’ve come to much the same conclusion as this discussion (ie lock washers don’t do much when installed per standard practice) but can’t find a source of all metal lock nuts (preferably stainless) that will fit the studs (carb, starter, governor, etc) on a Lyc A1A.
 
Anybody know where to get a all metal self locking 5/16x18 nut?

I’ve come to much the same conclusion as this discussion (ie lock washers don’t do much when installed per standard practice) but can’t find a source of all metal lock nuts (preferably stainless) that will fit the studs (carb, starter, governor, etc) on a Lyc A1A.

https://www.mcmaster.com/

Search for "flex top locknut"
 
AN363

I read this post a while back and had the question in my mind but too lazy to respond I guess. How do you use AN363 nuts which are fine thread on a Lycoming stud which is coarse thread ? Almost all Lycoming engine hardware is coarse thread unlike Continental’s which use fine thread.

Also no need to use Loctite on Lycoming engine studs that go into blind holes like exhaust, inlet etc. If you look closely you’ll notice Lycoming part # studs are manufactured with a slightly different thread pitch on one side that makes for an interference fit as it goes into the engine. The end that is flat should be the one left exposed. I’ve seen them installed backwards and then the securing nut is difficult to turn due to the interference fit. I don’t think Loctite works well in a high heat environment anyway.
 
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. . . are not safetied. nor have lock washers!

I think this is mostly either under torqueing and/or re using lock washers. Remember the connecting rod bolts on Lycoming's are not safetied.

+1 I have actually said this!! Bolts/nuts are used to stretch the bolt to maintain sufficient clamping force in all conditions that the nut will not come loose. It is called abutment.
 
Connecting Rod Nuts

+2 on connecting rods as well. On the higher horsepower Lycoming Engines that utilize tongue and grove rods ( ie angle valve motors ) you torque the rod bolt by stretching it to a very specific length, 2.255”-2.256”. This provide the tension that keeps the rod nut tight indefinitely or until a catastrophic failure !!
Back to Vic’s OP the nuts lost tension and came loose. Keep in mind your compressing a paper gasket as well so periodic checking would be prudent.

Another thought old Lycoming engines utilized cotter pins to safety the rod bolts with castle nuts. I’m thinking back in the 60’s-70’s. Guess they didn’t have confidence in the bolts metallurgy/ manufacturing to hold the stretch.
 
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Anybody know where to get a all metal self locking 5/16x18 nut?

I’ve come to much the same conclusion as this discussion (ie lock washers don’t do much when installed per standard practice) but can’t find a source of all metal lock nuts (preferably stainless) that will fit the studs (carb, starter, governor, etc) on a Lyc A1A.

I ordered mine from B&B Aircraft Supplies. AN363-518 or MS21045-518.

http://www.bandbaircraftsupplies.com/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=19964&category_id=225&vmcchk=1&option=com_virtuemart&Itemid=73
 
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AN363-518 source

I tried calling BandB, but the gal who answered the phone didn't know what I was talking about, and their website doesn't turn up any coarse pitched locking nuts on search, or under "nuts" in the menu.

Any other sources?
 
Nord Locks

I used Nord Lock washers on my carb mounting studs. No flat washers. They go on with normal torque but require much higher torque to crack loose. Cheap and reusable.
 
AN363 stop nuts for Lycoming engines

All of my usual sources (Spruce, etc.) carry 363's in fine pitch, but my LycClone has 5/16x18tpi studs. I finally found some high strength stainless nuts on McMaster, but for future reference???
 
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