Propeller Bolts

Tbrown · Post by **Tbrown** » Thu Mar 18, 2004 1:18 am

Can anyone tell me the proper bolt size for my propeller bolts? My mechanic was doing the annual inspection and told me I have the wrong bolts on my prop. The reason is the bolts do not come all the way out of the nut on the back. He said there should be at least 2-3 threads showing to be correct. I have a McCauley prop (DM7651).I called Univair and tried to order the correct bolts. They told me it took AN6H-37A bolts, which are 3/8" X 3 7/8" long. This is the size bolt I have in my plane. In order for the bolts to stick out 2-3 threads, the bolt would need to be approximately 4 1/8" long. My mechanic looked in the Cessna parts book and it called for an AN6H-45 bolt. But the grip length on this bolt is 4 1/16" long. The grip length on the AN6H-37 is approximately 3 5/16" long. When I put the prop and the spinner plates and the washer thickness together, the maximum grip length for my prop is approximately 3 7/16" before the bolt would bottom out. The other problem is the thread length, which is 1/2" on both bolts and that is the length of the threads on the nut the bolts screw into. In other words, the thread length would need to be at least 3/4" to stick out of the nut 3-4 threads. My mechanic called Cessna and after doing some checking they referred us to the Cessna 170 Association. They told us that no one at the factory was born when the plane was built! I did a search of your site for the topic and did not find anything addressing my problem. I hope you can make sense of this, it is hard to verbalize without looking at pictures. Thanks in advance for your help.

My plane is a 1952 170B with a C-145-2 engine.
Troy

Post by **GAHorn** » Thu Mar 18, 2004 4:20 am

Troy, the prop bolts do not have to protrude 2-3 threads when their heads are safetied. It sounds as if you have the correct bolts. (AN6H-37 or -37A)
You didn't mention your serial number or which spinner you have, but if you have a '52 B-model, it's likely prior to 25373 which is the begining serial for the AN6H-45 bolts. If your serial is post 25373 (or if your earlier airplane has adopted the later airplane's spinner) and your spinner is the original "dome" style and not some after-market, then the IPC called for bolts AN56-H-45 (or AN6-H-45A) should work fine. The threaded end only need be just slightly protruding beyond the nut, almost appearing flush.
To further complicate matters, some aircraft have been modified with O-300-C or D engines which use a different prop-bolt/nut combination entirely. (They use a nylok nut and non-drilled bolts.)
And you thought this should be simple, 'eh?

zero.one.victor · Post by **zero.one.victor** » Thu Mar 18, 2004 7:52 am

Troy,I had the prop off my 170 yesterday & happened to measure the bolts. They are about 4-1/8" long.I have the C-145-2 engine,the Mac 1A170DM prop,and the bullet spinner assembly. Sounds like maybe your bolts are a little on the short side.
Looked in my Spruce catalog. They show two different prop bolts which seem to be a match for mine. The AN76-41 (3-1/2" grip,4-11/64" length) aand the AN6H-41A (4-1/8" length). Under the AN76 listing,they say that if the AN76 is out of stock,they will substitute the AN6H. Looks like the main difference is that the AN76 bolt heads are drilled 3 ways for safety wire,the AN6H bolt heads just one way. The AN6H-41A bolts at $3.35 each are also about half the price of the AN76's. Guess which one I'd buy?
My airplane seems to have 4 of one and 4 of the other,judging by the bolt heads. Guess a previous owner couldn't decide which to get so he went half and half!

Eric

Tbrown · Post by **Tbrown** » Thu Mar 18, 2004 2:00 pm

Thanks George and Eric,
There is a special place in heaven for guys like you. George you were right, I spent 3 1/2 hours talking about these bolts yesterday morning! I never new something so simple could be so complicated! Here I am STILL talking about them! I agree with you about the safety wire but then one mechanic said something about the stress points of where the bolt stopped in the nut. (You see how this is going?) There are 8 bolts aren't there??

Eric, I think I'm going to try a little longer bolt as you indicated, just to find some middle ground here. Yesterday on the bench I put the bolt with the washer through the spinner plates and the prop. There were only about 4 threads left before the end of the grip would be past the last spinner plate which to me would then be close to bottoming out. Sounds like you have the same set up as I do, so I'll order some -41A's at the $3.35 price (after all they both have a tensile strength of 125,000 PSI). My only dilemma now is whether to use Cadnium Plated or Stainless Steel (just kidding

)

My SN 20981 and I do have the bullet spinner.

Thanks again guys,
Troy

zero.one.victor · Post by **zero.one.victor** » Thu Mar 18, 2004 6:10 pm

There's a special place in heaven for George alright,but it's a little downstairs from where the rest of us are gonna be! And his landlord is gonna look like this---

Eric

funseventy · Post by **funseventy** » Thu Mar 18, 2004 8:50 pm

I don't care who you are, That's just FUNNY!

Post by **GAHorn** » Fri Mar 19, 2004 2:44 am

You know, Troy, it's an interesting thing when you get a mechanic who is intent on performing meticulous maintenance, ...when he suggests using a longer bolt because he has a theory about thread engagement.... and then you point out to him that the factory IPC specifies a particular bolt.
Try asking him what his basis of approval is to subtitute a different bolt than that specified, and watch his response.

(I'm curious,...what good are threads stuck out in the air beyond any fastner's reach? It's only the threads that are engaged that perform any work.)
A good technique is to magnaflux bolts that are installed in critical locations, such as wing attach, stabilizer attach, prop attach, etc., prior to installation. Some airframe mfr's require it, or issue special pn's to otherwise ordinary AN/MS bolts that have been NDT tested for those applications.

FredM · Post by **FredM** » Sat Mar 20, 2004 5:59 am

Your mechanic is correct in noting that it is standard practice to have 2-3
threads protruding (minimum of one thread) when using bolts to join parts
together. However he is incorrect in assuming you have the wrong bolts because they don't extend 2-3 threads. Aircraft manufacturers deviate from what is standard practice on a regular basis. They are allowed to because they have engineers that can back their design up when it comes time to explain things to the FAA. They usually have a very good reason for doing so. In this case I would guess that it is because the part in question is a rotating component. If you have too many threads protruding
the engine could more easily snag loose objects while it is running such as the scarf of a person getting too close while the engine is running. My guess is that it was this safety issue that led to the change. If you are interested in the source of the 2-3 thread rule it is in the AC 43.13 book acceptable methods techniques, and practices. page 7-5 para 7-37 also the basis of approval for installing a longer bolt is there also.
" bolts of slightly greater grip length may be used, provided washers are placed under the nut or bolthead. The maximum combined height of washers that should be used is 1/8 inch. This limits the use of washers necessary to compensate for grip, up to the next standard grip size."
"All bolt installations which involve self locking or plain nuts should have at least one thread of the bolt protruding."
I do not recommend going to the longer bolt. My plane had the longer bolt installed when purchased it. I noticed that when installed and properly torqued the bolt head was not quite tight against the prop hub. In other words the bolt shank will bottom out on the nuts just before the bolt heads touch the prop. the installation looks normal but prop will be slightly loose even with the bolts tight. to correct this you have to install washers and there went your extra thread protrusion you wanted. you are back where you started.
Sorry If I am starting to sound like Gahorn. I should have just said " If it aint broke don't fix it"

FredM · Post by **FredM** » Sat Mar 20, 2004 6:11 am

Oh, I forgot your question about the purpose of the extra threads. I guess the reasoning goes like this. If the threads of a bolt attaching some part to an aircraft were to fail ( strip) as long as the nut is not stripped and there are a few extra threads protruding that were not subjected to the forces that stripped the other threads there is still some hope that the part may stay attached to the airplane. A very small safety margin that would not exist without the extra threads.

Post by **GAHorn** » Sat Mar 20, 2004 5:11 pm

I was beginning to think you were sounding pretty good there, Fred!

(Except that second msg about stripped threads/protruding threads. That's got to be pure speculation, 'eh?)
I believe the real reason for protruding threads is the same as for "witness" holes..... they're merely a convenient method to determine proper/full engagement in instances where excess length is not critical.
A stripped fastener is no longer capable of meeting requirements regardless of it's length. (Proper torque stretches a fastener within it's elasticity/yield. A stripped thread is damaging to both bolt and nut. It's important to use proper torque and excess fastener length does not obviate that requirement. Designers do not add bolt length for any of us who use only our impressions as to what constitutes proper torque.)

Tbrown · Post by **Tbrown** » Sun Mar 21, 2004 5:25 am

George and Fred.

One thing ya'll do is make one think. That's the great thing about the 170 Association and this web site. I don't think you could own one of these fine aircraft without it. Cessna sure hasn't been much help on this matter. As a "Newbie" to general aviation I've got a steep learning curve.

I tend to agree with you. I've already ordered the bolts, however, it will be interesting to see how they compare. As for my mechanic he is open to what I find here. I want a thorough mechanic as long as he comes with common sense. I'm sure he is referring to AC43.13.

George, you bring up a good point about the magnaflux and NDT testing. From talking to Univair last week, he gave me an AN number for the bolts in his bolt kit. I plan to talk to them again to make sure. I thought the extra cost was for the insurance companies and the lawyers.

Thanks again for the insight.

Troy

zero.one.victor · Post by **zero.one.victor** » Sun Mar 21, 2004 4:03 pm

Looked at the nose of my ragwing at the airport yesterday. The -41 bolys I have stick out about 2 threads worth behind the flane. I just looked in my 1948 IPC,it sez the correct bolts are AN6H-45,this is for the original skullcap spinner. Doesn't seem like the bolt length would need to different for the skullcap & the bullet spinners.It also seems like bolts longer than the -41 size would be too much,but who am I to say?

Eric

Post by **GAHorn** » Sun Mar 21, 2004 9:02 pm

There were also several designs of nuts into which the prop bolts screw. My prop flange uses Nylok type nuts, while some designs used T-nut styles. The longer bolt is req'd for the Nyloks, but would protrude through the T-nuts.