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Old 08-27-2014, 09:26 AM
BarracudaHockey is offline
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United States, FL, Jacksonville
Joined Dec 2007
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I've seen hand drilled holes that are putting a load on the bolt cause it too
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Old 08-27-2014, 10:51 AM
JoeAirPort is offline
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Milwaukee, WI
Joined Jan 2006
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That's what I'd bet on. Side load. Even with a prop jig on a drill press I have yet to have a prop that didn't need the holes opened up. Without the 2nd step of opening the holes, the holes always drag on the bolts and that is side load. It has to pass my test of freely moving up and down the bolts partially threaded into the engine's hub. Back plate has to pass that test too. Never broke one bolt in my rc life. 5 different brands. China brands too.

With wood props, the hubs compress a lot. They need constant re-tightening. And not over tightening. I prefer CF props with CF hubs like Mej or CF with hard wood hubs like the Biela. Laminated wood don't compress much either. The beech wood are horrible for compressing but I have not broken bolts with those either.

So I think bolts break on loose props or props that are tight with side load.
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Last edited by JoeAirPort; 08-27-2014 at 11:03 AM.
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Old 08-27-2014, 11:36 AM
rooman is online now
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They're get'n lower mate.....
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United States, GA, Savannah
Joined Nov 2006
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What method are you using to drill the prop ? do you start with the engine side first and drill half thru then turn the prop over and drill from the front side of the prop and drill all the way thru ?
All this using an indexed jig ?
It is not recomended to drill a prop in one shot as the web of the drill will cuase it to wander, this may be a side loading issue from missaligned drilled holes ?
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Old 08-27-2014, 06:09 PM
N726AC is offline
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United States, FL, North Fort Myers
Joined Sep 2006
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Hey OP, how about an update. Did ya find anything?
I had a DA 50 shear the bolts. After I tightened the bolts on a new prop they promptly failed. The new prop was thinner than the old prop and yup, the bolt had bottomed out give the feeling of being snug. I won't do that again !
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Old 08-28-2014, 07:57 PM
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Hammond, Indiana
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If you want to really understand why loose bolts fail...start reading this:


http://www.boltscience.com/pages/basics1.htm
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Old 08-28-2014, 08:59 PM
David Childress is offline
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Joined Jul 2014
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I've owned 4 DlE 55s, 1 Dle 30 and broke 1 bolt. I drill from eng side 1 shot. I keep 3 sets of bolts, 1 below and 1 above. I think 45,50 &55 mm L and 5 mm D( not sure without checking. Different props have different thicknesses. Always check depth and leave slack. If it is close, after tightening, you might be bottoming. Good luck.
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Old 09-19-2014, 07:15 AM
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United States, AZ, Vail
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The key is never let the bolts get loose. Check bolt tightness every 5th flight. Once they are tight for several checks they will not loosen.
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Old 08-25-2019, 10:58 AM
alqwad is offline
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U see what had happened wuz...
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United States, NM, Alamogordo
Joined Jan 2008
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All good inputs. Here’s a warm fuzzy tweak. I always drill the prop for appropriate size then chase with next largest bit. Since GS props have middle bore, no problem with centering up or wobbling. The extra room prevents bolt side load when the prop is compressed. I had a zdz80 RV with tiny boots and was breaking them often. An old timer gave me the tip and never a problem again. I do it every time from 20cc to 170cc. Problem solved!!

By the way, I’ve never had a problem with a carbon prop doing this because the hubs are so dense/stout. I guess I had already figured out the problem by the time I upgraded to $$ carbon. Still chase the holes with a larger bit though either way.
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