The International Cessna^® 170 Association

The 170B parts book calls for NAS145 series bolts to secure part of the tail assembly to the fuselage. I think it is the vertical stab but do not have the book with me at this moment. The question is: Is there a suitable AN bolt substitute for the NAS145? Thanks. Steve.

I do not think you will find an equivalent bolt.
Tensile strength is 160-180ksi.
Small diameter head with an internal allen wrench.

The NAS bolts are available at http://univairparts.com

If this is the bolt you are talking about, it is unique. Del Lehman replaced mine a year ago because some previous mechanic had installed a hex head AN bolt. The head of an AN bolt was engaging the bracket which created a serious danger of cracking. The washer underneath the bolt head is also chamfered to match the contour of the bend in the bracket.

Steve, our resident Russian Espionage Agent (KG who owns a B), .... has explained exactly WHY it's important to use the correct hardware, especially in this application.

Ordinary AN hardware does not accomodate the L-bracket, which has a slight radius that the NAS internal-wrenching bolt-and-washer must match. (And the internal-wrenching NAS bolt-head will allow it to be properly-t orqued without damaging the adjacent structure of the bracket and vert. stab.)

I've seen several 170 with the INCORRECT (AN) hardware in this location. Two of them suffered cracks. Guess how much that costs to repair...IF...IF you can find the brackets?

Guess how discomforting it might be should you be practicing crosswind landings or spins and the vertical stabilizer come off the airplane? (Pretty large side-=forces are generated on this surface.)

Marstal,

I replaced my dorsal fin bolts (Fig. 13-7 and 13-5) on my 1955 170B several years back. The bolts shown in Georges pictures are MS 20005-11 and MS 20005-6. The washers are MX 20002C5. I ordered them from "Genuine Aircraft Hardware Co. http://www.gen-aircraft-hardware.com. Phone number 888-247-2738. If I remember correctly Cleo Bickford gave me this info. Please cross check that the the part numbers are correct when you order them as I only penciled the new part numbers in my parts manual Hopefully some one has a cross reference manual to check my old notes.

Also, these need to be carefully torqued; so, get your mechanic involved

Here's a pic of the illustration in their catalog Here's a chart to the lengths of the bolts:

Note the radius under the head of the bolt in the illustration above. The correct washer must also be used, with its chamfered hole facing the bolt head.

Many thanks to everyone who replied with info.

Reeve airmotive (907) 272-8522 in ANC has one length and bonaero.com (209) 754-5175 has the other at substanially less $$$$

I have the tail off and new bolts, the post says the proper torque is important. What ft or inch pounds might that be? I don’t have a service manual. I might be able to just reference the torque for a standard 5/16 AN bolt or I could ask around. Any ideas?

Dooley, As an A&P I'm sure you're familiar with FAR 43.13(a) which says you've got to have appropriate technical manuals. Download your Pre '63 100 Series Service Manual here. Page 1-8 there has a torque chart that matches AC43.13-1B Table 7-1 for a 5/16-24 bolt: 100-140 in/lbs. The bolts there have to be tightened in a certain sequence. Del has a good post here regarding installing the vertical fin. (I’d nitpick with two small details in that post. I think he was looking at the “Alternate Values” column in the Cessna Service Manual when he wrote 100-150 pound-inches. Secondly, according to AC43.13-1B paragraph 7-45, in addition to the special countersunk washer under the head, one should “use a special heat-treated washer (NAS 143) under the nut” rather than the common AN 960-516 washer. NAS143-5 is now superseded to MS20002-5). The L-19 manual has similar instructions:

Thank you so much! I especially like the service manual download. Clear concise and prompt. I have the washers so no more hurdles. You did me a great service. Remember when Service Manuals were only available from Mfgs. and were very expensive. I do.