Thorp Air Command - T18.net

The O-290 I plan to buy from the chapter was, nearly as we can tell, assembled/overhauled and never run.
1. First question: is there an easy way to tell what compression pistons are in it? We think it may have higher compression pistons, but not sure. All I can tell from the boroscope is that they are flat top.
2. Am I seeing a GPU crank? Is that the reinforcing collar that we are supposed to add for GPU conversions?
I have a crank from El Reno still in the box and its flange this thicker. Does anyone know the difference in flange thickness between a regular flight crank and a GPU crank?
Even if that is a GPU crank in the engine, is it worth pulling everything apart to swap it, or just fly it with the GPU crank and be that much closer to having a running engine?
And for GPU cranks, can you still use a 4" prop extension, or is it best to bolt the prop right to the crank?

I have attached photos of the engine on the stand and the other crank.

I know, lotsa question, but thank you for any help.

Yes, the crank on the stand does have the reinforcement plates (collar), and I assume it only fits the G crank. The G flange is much thinner. I have a set that is un-installed. I don't think they are available any longer. I have 4 gpu engines (1 converted, 2 conversions started) and would like to get at least 1 more set of the plates.

I don't know about the piston and extension questions.

The gpu cam has narrower lobes...less surface area...and wears quicker than the D cam, reducing the hours till overhaul.

There are good articles available concerning the gpu conversion. I think John Thorp wrote one, but I could be wrong.

I'm sure some with more information will chime in here before long.

Good luck!

Steve H

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Steve H
N80LC S-18
SN 38
2OK7

I have a O290-G engine on my Thorp with a 4" extension, its been on the airplane for 53 years and >2000 hours with no problems. I have D2 cylinders and pistons with a 7.5:1 compression, my current cam is I believe from a O235 part no. 75706

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

N6CM has a 0290G with a little over 1500 hours. She has been running great for last 20 years, but is starting to get a little wheezy and grumpy at speed. We bought a run-out IO-320 and are in process of engineer overhaul and change given difficulty of getting 0-290 rings and other parts.

We have the O-290 torn down to where we have the jugs and pistons removed. Looking inside, the cam lobes all look great and we are finding assembly lube everywhere, so no corrosion issues. Some of the assy lube has gotten gooey, so we'll be cleaning all that out.
Everything was looking very good, to where I was thinking finish cleaning everything up and put it back together, and then I noticed that the case halves seemed to have been assembled with grey Permatex. Where it squeezed out, it is still pliable in some places, and hardened and brittle in others. So now I'm thinking we need to split the case to clean it all out and reassemble it correctly. Just dang.

The other thing is that the oil pump has an aluminum gear and a steel gear.
I need to re-read the SBs to see if this is an issue for me.

According to my engine log book, there is an oil pump AD 96-09-10 dated 7/15/1996. Compliance for my engine was to install gear kit 05K19423S. I have an O290-G and it was done during a prop strike inspection.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

We'll definitely be pulling the case apart to clean out the Permatex and reseal the case halfs.
We removed the pistons. On one of them the rings moved freely, and on the others the assembly lube (I'm assuming that's what it was) had accumulated and gummed things up pretty good. Soaking in carb cleaner seemed to take care of it, but I'll be checking them some more tomorrow morning. Very good advice from Bob about not trusting an engine that has sat for a while. That definitely would not have been pretty.

All of the rebuild info I have references the Lycoming 78027 rod bolts, which are the one-use "stretch" bolts.
But the engine I have on the stand has con rod bolts with castle nuts and cotter keys. Are these also one-use? Or can I re-use them?
I get that the torque procedure on the stretch bolts works, but I do like the security of nuts secured with a cotter key.
Thoughts? Comments?

And am I correct in thinking the fuel is all gravity feed? Is there any reason to have a fuel pump on my engine?

I have the engine completely apart. Good thing we did, if nothing else but to get the Permatex goo out of the inside of the case.
While I've got it apart, I'll balance the rods and pistons.
Crank bearing journals are all spot on the Lyc specs.

The engine mount biscuits have been on back order from Spruce for months. If anyone has any, let me know; I'm interested.

I also have one drive lug missing from my crank. Again, these are out of stock at Spruce, and everywhere else unless I want to pay some exorbitant price.

Anyone have a 5/8" lug, or a set of lugs?