The International Cessna 170 Association

I have a 1952 Cessna 170B that I bought out of a garage. I just noticed that the prop bolts do not reach the back of the prophub. There are two or 3 threads that I would think should be used. The previous owner was going to tow banners and put a DM7652 prop on this aircraft. There is a backplate for a spinner and a front plate. Where can I look for guidance on the proper bolt lengths, torque and spinner assemblies?

The Cessna 170 IPC (Illustrated Parts Catalog) would be a start.

But whatever bolts are used, they should utilize all the threads of the female fastener, whatever setup is utilized.

How can I tell if this prop is legit to be on this airplane? I don't see where this was ever put on the airplane in the log. Don't we have a list of props somewhere?

You didn't state which prop is presently installed but.... ALL approved equipment has it's basis in the TCDS (Type certificate data sheet) which is found at the home page or the FAA website.
Generally it's got to be either McCauley 1A170 DM or MDM , or McCauley 1C172 EM series, or a Sensenich M74DR series.

DM7652 is the only number I can find without pulling it. I was hoping that would tell us but I guess not. We are going to have to pull it for the short bolts anyway.

j3pup wrote:DM7652 is the only number I can find without pulling it. I was hoping that would tell us but I guess not. We are going to have to pull it for the short bolts anyway.

That number is a McCauly prop number. It is the original model prop. I would suggest, if you dont know the history on this prop then I would take it in and have it overhauled. Generally nothing goes wrong with these props when they are cared for but if you don't know the history it could have cracks in it by the hub. Also pay close attention to the hub area where the spinner is. Make sure there are no wear marks from the spinner on the prop back(The side of the prop that is facing forward.The face of the prop, the flat side, is toward the cockpit) If there is then I would have it overhauled. You need to inspect the spinner backplate anyway if you have the bullet spinner. They are notorious for cracking between prop bolt holes. Good thing is when you get the prop back you get a zero time prop with a new logbook. V/R Doug

Unless you have serious reason to question the airworthiness of your prop ...I would NOT take it in for overhaul.

Prop shops are notorious for taking a belt-sander to them and removing every little scratch and imperfection...and THEN measuring them to see if they are too short/too thin/too etc etc...

Then you have a condemned prop, when previously you only had one that needed dressing, painting, and balancing.

It looks like it just came out of an overhaul. I am told it is a climb prop, that the guy was going to use to pull banners but never got the chance. Their is an oddity to the number stamped on the prop. on the number DM7652 it looks like the 2 was stamped twice over a 4. Is this what they do when they repitch them?

j3pup,
Yes, they DO overstamp the pitch number when they repitch a FP prop. At least some prop shops do, I have two props in the hangar that are overstamped.

I ran into the exact same problem during my last annual. The prop bolts showed about 1/2 or 1 full thread from reaching the end of the crank flange. My IPC (A model) shows it should use an AN6H-37. These WERE the bolts that were installed. I am running a McCaulley DM7653-1A170 prop with a cone style spinner (only uses the backplate, not the one in front of the prop). I went to a specialty fastener guy in Long Beach, CA and we tried to up to an AN6-41 but the threads bottomed out before the head of the bolt hit the prop. This left me trying to find an AN76-41 (can't find the receipt but I'm pretty sure it's a -41). The "76" series has a little more thread for the same size bolt and is meant for prop applications....other than that, there is no difference in the bolts. The guy in Long Beach was able to find me some somewhere in Texas I think but they are hard to come by and pretty expensive.... about 18 bucks per if I remember right. Anyway, look in your spruce catalog (it lists 76 series bolts but they can't get them) and check the lengths vs. the AN6 bolts, you'll find its just barely enough to get you what you need. If you need someone to find them for you, he's a little tough to deal with but contact Mr. Mitch at Deering Industries, Long Beach, CA. Hope this helps.

BTW, I'm surprised not to hear more problems like this with IA's not catching you without 3 threads past the crank flange on these bolts.

Aren't prop bolts special? Seems to me that they had more threads on the shank than standard AN bolts of the same length.

LBPilot82 wrote:...and we tried to up to an AN6-41 but the threads bottomed out before the head of the bolt hit the prop. ....

It is permissible to use washers beneath the bolt-head to take up the extra length provided not more than 1/8" of washer-combinations are used.

Seems to me I should try the AN6-41, because I have a front and back plate for my spinner. Hopefully that and maybe a washer or two will work.

Thanks,

Randy

It seems to me that one should run the bolts into the prop flange, before trying to install the prop, to see how many threads protrude through the flange before the threads bottom out. Checking the thread protrusion after prop installaton would preclude allowing the bolts to bottom out before proper torque was attained.

I won't be changing the bolts myself, but I would like to get the numbers ready for my AP friend. Where do I find the torque values for my prop?