Thorp Air Command -

Supporting Owners, Builders and Pilots of the Thorp T-18 and its variants.
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 Post subject: O-290 questions
PostPosted: Fri Mar 24, 2023 8:50 pm 

Joined: Sun May 15, 2022 8:28 pm
Posts: 29
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.


Steve H
 Post subject: Re: O-290 questions
PostPosted: Sat Mar 25, 2023 8:15 pm 

Joined: Mon Jul 04, 2022 9:45 am
Posts: 7
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

 Post subject: Re: O-290 questions
PostPosted: Thu Mar 30, 2023 7:00 pm 
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Joined: Thu Oct 22, 2009 2:24 pm
Posts: 498
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Location: Yelm, WA USA
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

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

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