The International Cessna 170 Association

I certainly understand as I'd wonder if I owned a '48 with squared end lip that is only 3/4", if it was original. But, as there is no evidence Cessna made the lips 3/4" on the squared of cowl and so I think it much more likely they were all 3" but some modified down to 3/4" to mimic the later cowl and achieve higher temps.

Of course having a '49 with a 2-1/2" lip myself, should I ever see another 2-1/2" lip on a '49, I may start thinking it's a movement only Cessna might have made.

Measured the lip on the '48 170, serial # 18118. Measured approx. 3/4 inch.

If we could be certain that no one had tampered with customizing 70 year old airplanes we could make definitive statements about such things. Logs don't tell the full story of all the mods these airplanes have undergone. Despite the errors inherent, the IPC is still the best resource for making such determinations, IMO.