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Pistons

A

AirAg

Well-known member
A friend of the family has a GPU 220 apart and I looked at the pistons today. I have not seen any for a/c before so I have a question. These pistons are dome shaped in a way. So without a picture, are flat top pistons completely flat or do they a slight dome shaped? Trying to decide what these are. Brent
 
Stock GPU pistons have a large dish in them. The ones in my dad's motor are from an aircraft 0470 and have a small dome. yielding over 10:1 compression (180psi cranking comp.).

Larry
 
The story with this boat is he was running it (using lots of oil), and one day the motor locked up on him. After investigation, Blake found the cylinder to be cracked and a ring was frose on the piston in that cylinder. So, he sent them away and got one new cylinder and the others reworked. Ran it for approximately 10 hours and the motor still using lots of oil, and he is hearing a ping sound. Decides to pull the jugs again and found a bad piston again. Not sure what was wrong. Now my question, since he has 10:1 compression, would the gas he is using cause this problem? Or does anyone know another reason? He is using 93 octane. If I hear more of a story, I will let you know, but this is all I know right now. And Dean, this is Blakes boat, so if you know more, feel free to post. Thanks, Brent
 
10:1 is a lot in an aircooled motor. He should probably be using av gas or race fuel to keep it from detonating
 
See, he bought the boat used and just had motor work done. And now the motor is back apart and the pistons have a dome shape. From my memory, it looked as if the center of the piston face, they were approx. 1/8" in a dome shape. That is why I asked. Dont know where the pistons came from or the compression ratio...
 
Air Ag, look inside the piston and there should be a part number. If you give us the part number, we can tell you what engine that piston was approved for.
 
Sounds like he may have the aircraft 470 pistons usually the top of the dome piston needs to be machined down about the size of a half dollar.
 
Hey yall, I posted a new thread and replied to this post, so I will try it again. So if this is a repeat, I apologize.

The piston off the "dome" piston was 535133.

Mine, stock 4 cylinder GPU, was 632250.

So I am looking if anyone would know the compression ratio of the dome style piston. Thanks, Brent
 
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