Vibration cracks on inner coastal mounts 180hp Lyc.

[minton]

I would like to begin by stating that proper verbage is essential in getting important information spread throughout the industry.

Thus, Intercoastal = stringer assy. Stringer and Bracket-engine mount attachment (ref. 9/1/1956 170B parts manual figure 20)

This consists of two parts (stringer and bracket) There seem to be a couple of issues out there that impact the integrity of this assy. Prop balance, engine model/design/weight and outside forces (prop strikes) as previously mentioned. Of late the C-172/175 have been addressed through some Cessna service bulletins and AD's. The poor old 170 has not. I have encountered several cracked stringers in the past. I have yet to see a cracked bracket. They are not easily detected as they occur under/behind the bracket-engine mount attachment p/n 0411281-6. They seem to be closely related to the Lycoming 0-360 conversion but could also be linked to the Continental 0-360. But since few of those are out there we probably haven't yet detected those problems. The stock C-145/0-300 is not immune as prop balance/indexing and prop strikes are still issues. I'm not buying just the engine vibration issue in creating those cracks. Extra engine/prop weight, prop imbalance and prop strikes surely could be factors. I guess the bottom line is look the area over very carefully at each annual. Remember these old work horses are 50+ years old and your logbooks probably don't reflect the true picture of the aircrafts past history. And of course (IF) a service bulletin or AD is ever generated lets hope Cessna picks up the tab. It could be a very expensive fix.

[GAHorn]

Good words, indeed. (And another suspected-cause is aircraft that have been operated on floats without the windshield V-brace, ....the V-brace adds strength to the engine-mounting by tying the forward/firewall area to the overhead/upper doorposts area.)