Thorp Air Command - T18.net

On the pole for which engine, I voted for O-320.
I bought a T-18 with an O-290G. It's converted to O-290D2, but not really.
It has an O-320 crank, updraft sump and MA4SPA. It has an O-320 gearcase as well.
But it has solid tappets. Also, the "conical mount" isn't. The mount ears are unmodified.
So I've been trying to learn about O-290 conversions. Bruce F turned me on to the 161 page study by Don Pridham.
http://www.7ts0.com/manuals/lycoming/29 ... 0-290D.pdf
which is not very technical. It's more of a survey of practices from several of the builders of that period.
Rooting around on the net, I've found that the O-290 case can be bored to accept narrow deck O-320/O-360 cylinders, and perhaps an O-360 crank might fit with some internal clearancing.
So will this make the "Lightest O-360" ??
Also, could anyone direct me to more info on O-290G conversions? I'm finding it a pretty interesting topic. No doubt fueled by the fact that the one in my ship seems to run very well.

I have a GPU case #2455-20 I weighed at 43LBs.
I have a O320 case L4939-39 that weighs 33LBs

I, Also, have a O290GPU with an O320 crank, prop hub & pan that I weighed at 269LBs. NO exhaust,alt,carb,mags or oil.

The weights vary for each model. But, the weights that I remember most common are:

O290=269lbs
O320=290lbs
O360=320lbs

Bob MO

Thanks Bob;
I can see how the 'trimmed' O-290 case is sporting some extra material that the O-320 case is not. Not withstanding the nuts, Dipstick tube & oil pressure regulator, 10 pounds is a lot.
Also, I think I see some refinement in the O-320 casting, particularly around the lifters? Is that the oil for Hydraulic lifters?
My eldest says: "They don't make 'em like they used to, and it's a darned good thing".
So I could save some weight by using an Original Aircraft O-290D2, or keep the same weight by upgrading to an O-320 and add 10 - 20 HP?
The engine is 267 TT since conversion, but it's also about 40 years olde. I hope the 9 years' nap hasn't hurt the engine, and many years of trouble free service are ahead.
But now that I have a Lycoming powered ship, I'm naturally curious.

At 2090hrs, my O-320 was deamed airworthy and then pulled from the Cessna that it was in and replaced with a 360. It was built in1968, and was untouched. Still had the disposable Slicks in the block, and The intake hoses were cracked. It was drained and cleaned and parked on top of an old tire then wrapped with plastic. It sat on this tire for ten years, I just happened to call the Gentleman that had it in regards to a 290 he had for sale,he asked me if I would be interested in a320 with logbooks for the same money as the 290,that question was easily answered as he was asking 3000.00 for the 290. I bought the 320, it all checked out, compression was mid 70s, carb needed some TLC. With the plugs out, clean oil,LPS#2 in the cilinders it would push 40lbs oil pressure with the wife turning the blade. It ran well,and still does, it now has 2300 Hrs on it. I have replaced the front crankshaft seal as it got hard with age, looks like I will replace the pushrod seals they weep a bit they must be hard also. These are a few things to look at when they sit for some years dormant, put new paper gaskets in up at the heads on the intake also for good measure. This engine sat in a hanger in Laughlin Nevada for the ten years, pretty hot and dry air there. It came out of Colorado, if it was involved in a lot of hi altitude flight it may have very seldom ever made 150 HP just speculating. Thes are a few areas for you to take a look at if she sat for a number of years, hope this helps......Dan

By the way, the 290 is a good power plant , Ira has one and it does very well on very little fuel. dean has one that we are dialing in, all indicators point to this one being a strong engine also.Never underestimate a Thorp with a 290, The Thorp does very well with this powerhouse and they can stay in the air for a considerable amount of time......I was just going to say that this is just my observation, but I think I will say that these are facts. Dan

Them there gaskets and seals need no explaining after all that time. Some may be OK and others will weep. I would be concerned with the cylinders and rust/pitting ? If you don't want to pull them for inspection, you can have them bore scoped.

I bought a 1958 Fiat four door sedan that had been sitting in a driveway for many years. This was right after I got married in 1974 and needed a second car to drive back and forth to College at Cal State LA. It had 4 flat tires, brakes were shot (hydraulics) and the gas tank was full of rust ! I bought the car for $10.00. Had my buddy bring out his air tank and pumped up the tires and towed it to his house. I spent the next month, flushing and purging the brake system and the fuel system. The engine had not been started in years. Flushed the engine with kerosene and put in "new oil." A new $10.00 battery, some 40 cent a gallon gas, pulled the "manual starter" and with a few turns the engine came to life. For the next 2 years I drove that car to and from College and only put gas in it. Never changed the oil (I think I put a quart or 2 in it ?) With the parts I put in it, I think I had maybe $100 ( that included $20.00 for 4 used tires !) in it. I sold it to the same buddy that helped me fill the flat tires to for $60.00.

RB

I went back this AM with a larger digital scale and got totally different results.

O320 = 51.4 lbs
0290g= 57 lbs

Five pounds seems more realistic.

Bob MO

This old 320 still has the factory jugs, if they had been replaced with Nitride cylinders at some point in time I would have been very concerned as they are moisture magnets and will fall prey to a drop of moisture. The bore scope showed the old bugers were spotless as I expected....Dan