I have a '65 172 and love the O-300. When I first got it I had
problems with plug fowling (despite leaning on the ground, etc.) and
had a valve stick HARD. The previous owners had fed it a steady diet
of 100LL. I started using mostly auto gas and the problems have
gradually gone away. The engine was rebuilt with new Millennium
cylinders and to new tolerances in 99 and had 1050 hours when I bought
the plane a year ago. One cylinder had been replaced at about 600
hours due to valve problems. I flew a little over 100 hours in the
first year and compressions were all in the 70s when I had the annual
done last month. They key, like with any engine is to fly it
frequently.



1. What kind of fuel burn do you consistently get with an O-300? Is
the generally reported 105 KIAS @ 8 GPH true?

I plan 9 GPH block to be on the safe side.


2. Does the autogas STC help reduce the problem with valve sticking?


ABSOLUTELY -- so does flying frequently and using Marvel's Mystery
Oil.


3.In general, has your experience led you to expect to have to do a
top overhaul about halfway through the TBO period? What are the
ballpark costs of such an operation.

1200 HRS SMOH -- Compressions in the 70s, engine runs great, uses very
little oil -- (about 1 Qt per 15-18 hrs.)


4. In your opinion, would it be worth it to pay the premium to buy an
new 172 equipped with the O-320E2D?

No, but they're great planes too. I bought a 172 with the Continental
specifically because they are such good bargains. But you have to be
careful to get the right one. The Millennium cylinders and regular
use since overhaul were factors I considered in purchasing this one.


5. Besides the valve guide and cylinder problems (often attributed to
running on 100 LL), are there any other major problems that would make
this an undesireable aircraft?

The older 172s have narrower gear, lighter weight and slightly
different handling due to cuffed wings on the later models, etc. All
this makes them a little more challenging in winds, but still a
wonderful airplane. Like others have said, carb ice is a definite
issue, be proactive about checking for it and using carb ht when
necessary. Remember to apply carb heat right before takeoff if you've
been idling a long time; even if you already checked carb heat earlier
during your run-up.

- Brett