Jet-nut life / reuse

[Dave Harmison]

How many times can you reuse a jet-nut? I had a couple on my CV joints tear out their threads when torquing after half-shaft removal and reinstall.

[Robins Ken]

How many times can you reuse a jet-nut? I had a couple on my CV joints tear out their threads when torquing after half-shaft removal and reinstall.

Hi wear area Dave. Are they all M8 and not mixed with 5/16 bolts. Or the opposite. Also, snug is good. No more.
KR.

[Lotus7]

Judgement call on your part.
In my old aerospace life, jet nuts were never reused, but money wasn't the issue of course, safety was.
They don't lose all their anti-rotation characteristics after a single use, but slack off some each use.
You need to gauge whether or not they have sufficient resistance to turning for the application.
YMMV as always.

[DaveW]

An issue I've seen with Jet-nuts is that they wear the threads down on whatever they are used on because these nuts are harder than most bolts. The softer the bolt, the faster that happens. In the extreme, the bolt threads wear so much that a) the anti-loosening function is lost, and/or b) the bolt threads get so worn that they will no longer be able to take the required torque.

[Bob Wright]

FWIW, I re-use jet nuts as long as there is resistance when tightening. Once they spin on freely I throw away both the jet nut and the bolt. IIRC, the jet nut cuts into the threads of the bolt and that's part of the safety aspect.

As a note, while using jet nuts with CV joints- it's important to know whether you have metric or SAE bolts; they are very similar. The easiest thing to do is replace with known AN bolts, then there is no question. My car has nothing but AN bolts and jet nuts on it.

[Garey Guzman]

How many times can you reuse a jet-nut? I had a couple on my CV joints tear out their threads when torquing after half-shaft removal and reinstall.

I have always been a frugal guy but I like to keep in mind what the failure would be. If it's something I check a lot and I would catch before any big issue, no problem, keep using. If it's something I don't check much, have to take apart for some repair, and expect not to increase checks in this area, I will replace whatever is necessary with the aerospace mentality you mentioned.

[Lotus7]

Side note: A very clever friend of mine saves his worn out jet nuts to use on Hewland gearbox case flanges; the lack of anti-rotation 'grip' helps stop the studs from winding out with the nut :-)

[924RACR]

An issue I've seen with Jet-nuts is that they wear the threads down on whatever they are used on because these nuts are harder than most bolts. The softer the bolt, the faster that happens. In the extreme, the bolt threads wear so much that a) the anti-loosening function is lost, and/or b) the bolt threads get so worn that they will no longer be able to take the required torque.

+1 to this; after stumbling across this on my car, I started getting smart about lifeing my key AN bolts as well.

[Dave Harmison]

Thanks for replies. Double checked, bolts are definitely 5/16 UNF. Used a non jet net as a go / no-go gage and threads on bolt engage well, no slop and look good under magnification.

Lesson, save myself the aggravation of having to pull a side cover and just throw jet nuts away every so often.

[Steve Demeter]

I saw an episode of Air Disasters on smithsoniam channel. It was A P-51 that lost an elevator trim tab at the Reno Air Races. Crashed and killed 11 people. Long story short: the nuts on the bolts holding the tab to the hinge were self locking nuts that had been reused. One or more came loose subjecting the bolt (s) to fatigue. And off it went. They said that FAA requires only 1 use of them. YMMV

[DanW]

I saw an episode of Air Disasters on smithsoniam channel. It was A P-51 that lost an elevator trim tab at the Reno Air Races. Crashed and killed 11 people. Long story short: the nuts on the bolts holding the tab to the hinge were self locking nuts that had been reused. One or more came loose subjecting the bolt (s) to fatigue. And off it went. They said that FAA requires only 1 use of them. YMMV

True. A&P mechanics call them "One Time Nuts". They will replace both bolt and nut. I usually get three uses out of the nuts and bolts, then replace with new.

[chrisw52]

True. A&P mechanics call them "One Time Nuts". They will replace both bolt and nut. I usually get three uses out of the nuts and bolts, then replace with new.

while building my RF98 I went through three nut & bolt kits trying to figure out how to put the car together after I bought it in pieces

I think the key takeaway I got from this is now I use old bolts when setting up the car. Once I have all the geometry where I want it, I used new nuts & bolts on the final install.

[Ed Womer]

I have used grade 5 fine thread bolts on my FV for 30 plus years and until the nut has been used as in off and on so much you can turn it with your fingers, I will re-use them. Now you should always be looking at the fasteners and if they look worn you should replace them.

If you only use fasteners for one use that is your choice, but that doesn't mean you have to do that. I have never had a nylon lock nut back off even if it wasn't tight.

Ed

[DaveW]

Don't have an excessive # of threads exposed after a locknut is tight. The more threads the nut has to rotate past before it's tight, the faster the locking capability of the locknut will wear off.

Also, make sure the 1st few threads don't have rough or sharp edges. Nylocs in particular can lose their locking capability threading past rough edges.

Having said all this, the most important/effective way to have nuts/screws not unintentionally loosen is to have them tightened to a high enough tension that they don't ever lose their preload during operation. Locknuts are usually just a safety factor to assure the nut doesn't just fall off once it loosens. A fresh jetnut/screw can help keep things tight, but that is not a cure-all if the assembly is not tight enough to start with.

[emackey]

What is the recommended CV torque spec?

[DaveW]

What is the recommended CV torque spec?

On my Lobro (VW) CV's (Zetec/Staffs) I use 18 lb-ft w/ 5/16 bolts and jetnuts. BTW, I've used the same jetnuts and bolts many times over - never had one come loose.

You do have to be careful to not get excess sealant between the flanges because it will stress-relax and loosen the assembly, possibly causing things to come loose or fatigue.

[John LaRue]

On my Lobro (VW) CV's I use 18 lb-ft.

I torque to the same figure. I now use an allen cap bolt with Nord-Loc's at both the head and behind the drive flange against a fresh jet-nut. These still routinely need to be snugged. I have tried multiple approaches to keep the fasteners in this application tight (including the typical AN or NAS bolt) and nothing has been particularly successful. DW, what are your thoughts?

[DaveW]

I torque to the same figure. I now use an allen cap bolt with Nord-Loc's at both the head and behind the drive flange against a fresh jet-nut. These still routinely need to be snugged. I have tried multiple approaches to keep the fasteners in this application tight (including the typical AN or NAS bolt) and nothing has been particularly successful. DW, what are your thoughts?

Are you using Lobro CV's? They are all steel. If you're using tripod CV's they are aluminum, IIRC, and could yield.

I assume you shift as recommended , i.e., clutchless. The peak alternating (impact) torque on the CV's may be just enough to make them loosen. I perform every shift using the clutch - that may cushion dog engagement just enough to keep the CV's from loosening. Other than that, not much I can think of. You might try harder (grade 8) bolts and higher nut torque to see if that helps.

[aims20]

[...] I now use an allen cap bolt with Nord-Loc's at both the head and behind the drive flange against a fresh jet-nut. These still routinely need to be snugged. I have tried multiple approaches to keep the fasteners in this application tight (including the typical AN or NAS bolt) and nothing has been particularly successful. [...]

I just wanted to throw a couple more things into the discussion:

1) Ensure that all CV Grease is totally cleaned PRIOR to the installation of a new, dry (unlubricated) nut/bolt/nord-lock washer, etc. Any debris or CV grease will degrade any locking features in the joint.

2) Ensure that the surfaces between the drive flange and the CV housing are clean and free of debris prior to tightening.

3) Ensure that the bolt is long enough to completely engage full threads against the jet-nut or lock nut - we had a situation once where the mechanic claimed the bolt was long enough because he saw threads protruding through the nut but the taper at the end of the bolt was actually continuing into whatever jet nut or locknut we were using at the time and reducing the locking on all 6 bolts on that particular CV joint.

4) I'm pretty sure that most Citations/FCs extend the exhaust past the CV, but on some F1600 cars or short exhaust setups, check that your exhaust pipe is free of leaks and that the exit is pointing away from the CV - baking the fasteners in the 200F to 400+F mixed/downstream exhaust flow WILL heat up the nut before the bolt on exposure alone, expand it, and lose some pre-tension or locking friction. If you can't re-direct the flow, you can potentially look for exhaust nuts and 400 series stainless nuts... but any 18-8, 300 series stainless, or alloy steels have a coefficient of thermal expansion that nearly twice as large.

[DaveW]

I just wanted to throw a couple more things into the discussion:

1) Ensure that all CV Grease is totally cleaned PRIOR to the installation of a new, dry (unlubricated) nut/bolt/nord-lock washer, etc. Any debris or CV grease will degrade any locking features in the joint.

2) Ensure that the surfaces between the drive flange and the CV housing are clean and free of debris prior to tightening...

3 &4 above are really good points.

However, dry fasteners (1) will exacerbate the installation and removal wear on the bolt and jetnut, shortening their useful life significantly. And (2) I use RTV in the joints between the 2 flanges and the CV to keep from spraying CV grease all over nearby stuff. Neither have caused me issues. YMMV.

I do agree that all of your points make good sense if one is having issues keeping things tight.

[tstrong]

Are you using Lobro CV's? They are all steel. If you're using tripod CV's they are aluminum, IIRC, and could yield.

I assume you shift as recommended , i.e., clutchless. The peak alternating (impact) torque on the CV's may be just enough to make them loosen. I perform every shift using the clutch - that may cushion dog engagement just enough to keep the CV's from loosening. Other than that, not much I can think of. You might try harder (grade 8) bolts and higher nut torque to see if that helps.

I have had the similar issues with an5 bolts and jet nuts torqued to 18 lb-ft. I believe this provides marginal clamping force to prevent motion between cv and flange thus resulting in lossening. All this gets worse if there is any contamination between mating surfaces. I have upped the torque to 20ish lb-ft and seen some improvement but you are very close to pulling the threads off the AN bolt which as you mention is much softer than the nuts. The nuts were designed for use on the NAS 6000 series (160-180ksi) bolts. Going to 160-180ksi bolts and upping torque to around 24 lb-ft has completely ended the issue. As you said grade 8 or socket head Allen’s could allow the extra torque as well just without the QC and grip length control of aircraft spec bolts.

[stonebridge20]

Lobro CV's

Socket heads and Jetnuts.

Mating surfaces spotless.

25lbs on my RF81 CV's or they loosen up in one session.

[DanW]

Lobro CV's

Socket heads and Jetnuts.

Mating surfaces spotless.

25lbs on my RF81 CV's or they loosen up in one session.

Mike,

Do you use the Lobro tab washers on your CV joints? The tab washer help keep the bolts from loosening under cyclic load.
https://www.bughaus.com/CV_Joint_Tor..._113501329.htm

[stonebridge20]

Mike,

Do you use the Lobro tab washers on your CV joints? The tab washer help keep the bolts from loosening under cyclic load.
https://www.bughaus.com/CV_Joint_Tor..._113501329.htm

Dan, I don't but at 25lbs I don't have a problem. I've got a bunch of customer FF's set up at 25lbs and they never have issues.

[Bobsy SR 6]

I also changed to Allen bolts and jet nuts for CV joint attachment after constant loosening using AN bolts and nylock nuts. This resolved the loosening problems. I torqued to 18-20 ft/lbs. I’m in the process of repowering to a BDG / FT200 set up. I will be upping the torque to 24-25ft/lbs and hopefully won’t have any problems with loosening. The FT200 is using the same Lobro CV’s as my old MK set up. I did replace them though with new ones.
Great thread with excellent information. Thanks guys!

Tom

[DaveW]

Dan, I don't but at 25lbs I don't have a problem. I've got a bunch of customer FF's set up at 25lbs and they never have issues.

So, to summarize, as always, it comes down to 3 things: a) clamping force, b) coefficient of friction, and what's needed in terms of c) peak torque or tension force to be resisted.

Increase a and/or b, or reduce c, to solve the loosening issue.

And, as I've said many times, the above is what's most important. Locknuts of any type are there to prevent the nut from rotating or falling off if the joint loosens. They WILL NOT prevent the joint from loosening if a, b, and c above are not in the proper ratio needed to keep things tight!

[rdracr]

Jet Nuts are a "Deformed Thread" type causing the thread wear on the bolt first then the nut thread. The softer the bolt, the faster the wear. The nuts are 180KSI material (Really Hard). at some time the nuts thread upsets will wear as well. If you are going to replaced the fastener pair, replace the bolt first always remembering that not replacing both means whatever broke will be more expensive.

[Jerry Kehoe]

All the info is interesting regarding the nuts and bolts but one thing that seems to be true is anything powered by a 4 cylinder engine seems to vibrate the crap out of anything and loosen up nearly any nuts or bolts regardless if it's an FA or FF or my 56 TR3! Also interesting is anything on my V12 has never loosened up in many years! I remember Jennings telling me the 4 cylinder motors were harder on his dyno than anything else so perhaps there is another component to the issue of this!

[DanW]

So, to summarize, as always, it comes down to 3 things: a) clamping force, b) coefficient of friction, and what's needed in terms of c) peak torque to be resisted.

Increase a and/or b, or reduce c, to solve the loosening issue.

Have a look at the holes on a used cv joint cover. The metal has become thinner at the hole.

The cv joint boot covers are made of dead soft mild steel so they can be formed to shape in a press. The steel under the bolt heads is so soft it moves from the cyclic loads of the bolts. The washers spread the clamping force across the metal and reduce the chance of coming loose. I've been torquing the bolts to 18-20 ft/lbs, but glad to know I can go to 25.

[stonebridge20]

I'll add that I also do not put jetnuts on anything less than AN and I also put a smudge of Copaslip in every jetnut I use.

[CTR28]

I love irony. I just received an email from Pegasus. Under their Trending Now banner is jet nuts. Don't get up, I'll show myself out.

[CTR28]

I love irony. I just received an email from Pegasus. Under their Trending Now banner is jet nuts. Don't get up, I'll show myself out.

[stonebridge20]

I love irony. I just received an email from Pegasus. Under their Trending Now banner is jet nuts. Don't get up, I'll show myself out.

I chuckled when I opened up that same email today.

[mark elder]

The problem we are all trying to solve is the CV bolts coming loose. Yes its a 4 cylinder and the CV cover is soft tinplate that compresses with time. AN bolts are softish (vs jetnuts).

You cannot win this all the time because of the metal of the CV cover. My solution is to use a 'decent' bolt and a decent flanged nut, torque it up and then add a jetnut as a locknut (so 2 nuts jammed together).

It does not stop things loosening over time but it does stop them loosen much. It can only loosen the amount of the CV cover compression. It certainly stops the nuts winding 3 turns out on all 6 bolts (with all that brings....). I then check tightness with a spanner on the head of the bolt; I do this every second race.

Just my thoughts.

[924RACR]

A little curious to see all this; I think I finally understand that you guys are using through-bolts with nuts.

Both my racecars have the same inner VW Lobro joint, but are both still using M8 Allen head bolts only for install - thread into flange, no locknuts. The 924 came that way of course from the factory, the P2 has a Quaife diff built the same as the 924.

On the 924, we have had problems with the bolts backing out (and subsequently some snapping). Have even had this on street 924s. For the racecar, for quite some time we cross-drilled and safety-wired the bolts; a real PITA for install/removal (considering there's 4 CVs, so 24 bolts to wire if you pull the driveshafts). More recently, we got Stage 8 locking bolts, but I'm not sure they're that much of an improvement - just easier to install. May be not as strong as the original bolts, as I have had a few snap.

On the P2 - never been a problem! Torque 'em to roughly 20 ft lbs, by hand, but I guess the weak little 1.0L torque is nothing close enough to work them loose ever, not compared to the mighty 2.0L in the 924 (making 2/3rds the hp but heaps more torque at 1/3 the rpms).

I should add that on the P2, I just use the original allen bolts without any washers, just the aero CV boot; on the 924, we do still use the original Lobro 2-bolt washers across the stock non-aero corrugated CV boots.

[stvsxm]

this is an interesting discussion with a lot of good and valuable anecdotal advice but in the end, the answer to this question and all others like it are in the A+P handbook of standards . This is the book that the FAA defines airframe hardware usage in the aviation industry. these standards and practices have evolved thru strict materials testing and a trillion hours of real world data collecttion . buy the book, its cheap and know the answers for real.

as for this question... jet nuts are single use hardware. it memory serves , they lose 80 % of their holding capacity after a single installation. if an licensed A+P guy re-uses a jet nut and the inspector catches it , he is in serious trouble. so that begs the question here.... you guys ( and myself) go to great pains to use aviation hardware because you want to do things the right way.... but now you are saying... ' yeah... but i can just ignore how to use it properly so save 60 cents " . why would you do that ? if you want to save the money why buy good hardware in the first place ? with all due respect.... here is the scenario.... every time to do a " nut and bolt " job you go through the same analysis.... " what does this nut and bolt have to do in this set of conditions ? "
so you pick a nut and bolt that when properly installed , creates the correct clamp load and has the correct retaining required by the application. " so here we are using jet nuts on CV joints on typical Grade 5 grade 5 bolts which are all AN bolts are. whats the torque ? whatever the A+P manual says it is which is the defined by how far you stretch the bolt in order to create the clamp load before you get near yield or its elastic limit. somebody else already did that math for you ... so whats the question ? if you changed those bolts to grade 8 then you need a different number because to stretch a grade 8 bolt takes a greater torque. the answers are all right there. and look... you aren't using gr8 and jet nuts to hold your cup holder in... you are using them in critical applications.... why re-invent the wheel ? do what the professionals in the aviation industry say to do in these sort of things and you can never be wrong. thats why they publish that book and those standards. someone here makes reference to that " aviation disasters " series... its worth watching. after a few of them , the root cause of the vast majority of them is very simple and so obvious that it is a real head slap moment.... and that root cause is self inflicted negligence. people saying to themselves... " oh... i can get away with that" . THIS question posted here is THAT moment. its a question that seems SO innocent... right up until it kills you. I used to have a sign in my shop as a reminder... it said " most amateur mistakes are make by professionals because they always theink they know what they can get away with..." don't be that professional...






'

[stonebridge20]

5/16" 24tpi cap head torque spec 25lbs

[DanW]

5/16" 24tpi cap head torque spec 25lbs

Thanks for clarifying.

Great for socket head cap screws with 180,000 PSI.

AN5 bolts are minimum tensile strength 125,000 PSI, so lower tension is specified.

[Charles Warner]

I'm surprised Steve J. left out (or I missed it) what he has been preaching for eons. To paraphrase . . . "It's the washers, stupid"

Most of us use standard AN washers and expect the CV joints to stay tight. Many moons ago Steve wrote a beautiful explanation of what occurred (I have it somewhere) and explained that the simple solution was to go to hardened washers. The AN washers give, especially when reused. Order some hardened washers and be done with it. After I switched I never had an issue whether using jet nuts or nylocks.

[DaveW]

I'm surprised Steve J. left out (or I missed it) what he has been preaching for eons. To paraphrase . . . "It's the washers, stupid"

Most of us use standard AN washers and expect the CV joints to stay tight. Many moons ago Steve wrote a beautiful explanation of what occurred (I have it somewhere) and explained that the simple solution was to go to hardened washers. The AN washers give, especially when reused. Order some hardened washers and be done with it. After I switched I never had an issue whether using jet nuts or nylocks.

I use hardened "high" lock washers, partly because they are hard, and partly because they raise the jet-nuts off the cover flange far enough to get a thin socket in there. McMaster-Carr link: https://www.mcmaster.com/92962A114

That may be part of the reason I don't have loosening issues.