The International Cessna 170 Association

The bolts holding my engine mount to the fuselage are not all the same. Some have castellated nuts and some don't. Additionally, the nuts are all on the inside of the fuselage. Is there a standard for direction of the bolt and the type of nut (metal locking, fiber locking or castellated)? One of the bolts is even too short so no threads protrude.

TW, welcome to the association and the forums. Please let us know what model, year you’re flying one more time! Do you have the IPC ?
Above the bolts on a “B”. General rule, nuts to the pilot but not always. Having the right bolts generally works best
Thks for joininh

You have a lot of questions to answer.

Direction of the bolt. As specified, as depicted or head towards direction of flight or direction of rotation are the rules that come to mind. So first a maintenance manual may specify the direction. Next a IPC or maintenance manual may depict the bolt in a direction though be careful IPCs are seldom approved sources. Last head in the direction of travel or rotation so in theory if the nut came of or the bolt sheared, the bolt or parts left of it may stay in place holding parts together.

More later as I have time.

Thank you, thank you for the picture from the parts manual. Never gave it a thought re looking at the manual.
By the way, my 170 is a 1952 170B N2674D

More. There is no difference in strength of a standard AN bolt whether it is drilled for a cotter pin or not. It is improper however to us a drilled bolt with a mechanical locking nut and it would be weaker if you did and the mechanical locking nut was over or outside the drilled hole. The castellated nut, there are several kinds, and cotter is just a method of locking the nut in place.

Mechanical locking nuts come in all steel pinching and fiber insert type. General rule is the all steel versions would be used anyplace the fiber type might be destroyed by heat. Generally you don't find fiber nuts firewall forward. The exception is of course if a fiber nut is specified forward of the firewall by an IPC Our C-145/0-300 have a few fiber nuts specified on the engine by the manufacturer and so while contrary to the general rule, could not be questions as being correct. Few would find it a problem replacing fiber locking nuts with metal type but not necessarily the other way around.