EA81 Propeller Bolt pattern / size?

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Jersey Bob

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Sep 10, 2021
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Hello all, Greetings from Mays Landing, Southern Jersey. I have an EA81 powered Airboat that's got a 2 blade 60" Wood Prop. I drove 7 hours round trip to purchase a 3 blade Composite ground adjustable propeller because the dimensions provided by the seller were the same ... so I thought. The 6 holes align perfectly However, the Bolt holes on the "New" prop are smaller than mine. My Bolts are M10 x 1.0 thread - Is this normal for the EA81 mounting hub? Question 2, Can the "New" prop hub be Drilled bigger so my bolts go through it? Question 3, can I change the mounting hub on my engine to match the "New" prop? I'm a bit lost and seek your Great Knowledge. Thanks in Advance, Bob
 

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Turd Ferguson

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Prop hubs and props for airplanes usually follow SAE AS 127 standards for mounting. If you measure the diameter of the bolt circle on your prop hub, you can figure out what size it is. For example, if the bolt circle is 4-3/8" it's SAE 1; if the bolt circle is 4-3/4" it's SAE 2.
The bolt hole size for SAE 1 and SAE 2 flanges is 3/8" or .375" which is slightly smaller (~1/64") than 10mm which is why you're having that problem. Perhaps someone drilled out the bolt holes in the hub to match bolts they had on hand? I can tell you how they got those bolts through the wooden prop. They just drilled the holes out to a larger size.
 

rv7charlie

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That's really a question for the prop maker. If you were talking about a regular wood prop, 'upsizing' the holes wouldn't be an issue as long as they were drilled 'true' in centering on the original hole location and perpendicular to the plane of the flange.

If the prop in question has an aluminum flange, then only the mfgr (or a qualified engineer) could tell you whether there will be enough 'meat' left after drilling the holes larger. Probably won't be a big deal, but probably isn't really a word you want to go with when dealing with propellers, even on a ground/water-bound vehicle.
 

Jersey Bob

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Sep 10, 2021
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Thank you both for your input. This is a much smaller bolt pattern which I measured at 3" however, I believe it is the 75mm (2.95") "Rotax" pattern. I am still trying to determine if I can just replace the Aluminum mounting hub.
 
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