The International Cessna^® 170 Association

Hi all,

Took my spinner off today to retime my magnetos, and my mechanic informed me my prop bolts may be the wrong type and to do some research.

It looks like the p/n on the head of the bolt is NAS 7506, whereas in the parts catalog it reads AN6-H-45

Are these bolts acceptable? It's on my 56' 170b O300-A

Just a quick look at things, it looks like at least a few bolts have had the bolt heads drilled out for the lockwire, interesting in that that’s old school but not really done to bolts that have reduced heads. Generally drilling holes in bolts sets up stress risers, maybe you drill a full size head of a generic low grade steel bolt in a situation where the bolt was not subjected to high vibrations or load, but we are taking old old school. I’m not saying it can’t be done but generally its not done.
Take a wire brush and remove the primer to get a better look at the bolt head markings so we know exactly what you have. Likely the bolt has sufficient strength, that should be confirmed but more importantly with bolt substitutions, the grip length and thread length have to be correct. A quick check would be to look behind the prop flange and ensure that bolt has fully gripped the prop flange bushing. What cannot be seen is whether the bolt is thread bound, likely not but in such a situation the last thread bound in the fastener can also set up a stress riser. Its not a bad idea to remove a prop periodically and check for corrosion and also check the backing plate which often has cracks in it often with high time engines.

I believe those are NAS1306 bolts and if so, are NOT appropriate for propellers because they are SHEAR bolts… not to be used in Tension. (edited for error)

Reece,
Are those bolts engine, airframe, or propeller parts? The Continental IPC lists p/n 24771 in fig. 4-26 as a 3/8 X 4" long bolt. The illustration seems to indicate that is for a wood prop. McCauley also lists prop bolts in their MacCauley Propeller Systems Owner/Operator Information Manual. We talked about that in this thread. That Manual actually shows two entries for the McCauley 1A170/DM propeller but the bottom entry is what they specify for the C-170. I'd really be interested in knowing what the difference is between the McCauley bolts and a regular AN6H-45 bolt, the p/n in the C-170B IPC.

mit wrote:http://www.itmexicali.edu.mx/jljv/mecat ... GE%203.pdf

That’s a great resource, thanks for posting it.

Aircraft Spruce used to publish in their old catalog an excellent description of aircraft hardware. Their “new” method (online catalog) of presenting their products is not nearly as informative. Their new method describes NAS 1306 as “shear” bolts.

The Cessna IPC makes assumptions (along with some occasional errors) that no alterations or modifications have occurred since the IPC was published, regardless of engine, prop, or other vendor updates/changes. So it is hazardous to declare things “correct” or “incorrect” in any particular instance.

The McCauley prop bolt kits are very expensive (for what they are) and less costly alternatives are available but it’s important to use hardware approved for the service. I’m pretty certain that “Shear” bolts are not appropriate for prop installations. Jim made an excellent observation on the drilled “thin” head NAS bolts in the original post, and I think it would be best to replace those Reecewallace.

Thanks Tim, flipping through the material sure brought back memories. Lots of this stuff we learned as we went. I think Douglas aircraft used every technique in their aircraft.

Jim,
Given the U.S. security concerns outlined on the first page of that manual you, as a Canadian, need to immediately delete that download! Quite ironic that the download comes from a Mexican website.

n2582d wrote:Jim,
Given the U.S. security concerns outlined on the first page of that manual you, as a Canadian, need to immediately delete that download! Quite ironic that the download comes from a Mexican website.

…AND…. we can only use that data for MILITARY purposes.

Never got to work on MISSILEs, everyone’s lucky there but gosh I wish I could have.
Miles was probably lucky to have tinkered with a few.