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Old 08-28-2019, 03:30 PM
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Soft prop bolts

Just a heads up on kind of a scary incident recently. Bought a beautiful used giant scale plane from a really good guy. I went through it as anyone would, but failed to notice the prop bolts were obviously not the originals from DA.

As you can see in the pictures, no serrations on the head of the cap screw and no letters or numbers stamped as most graded bolts will have. To qualify this, I have built 7 giant scale prop planes and have never had a loosening of a quality prop bolt let alone a failure of one.

My belief is that 4 out of 6 5mm bolts stretched due to not being hardened bolts. The bolts failure was found by accident as I was taking the cowl off to do something totally unrelated. 3 of them came out in 2 pieces and 1 was able to be broken by hand after it came out.

The moral of the story is please make sure you have quality bolts if the origin of the plane or engine is not 100% known.
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Last edited by Colt; 08-29-2019 at 02:51 AM.
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Old 08-28-2019, 03:51 PM
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Wow, that's amazing and truly scary
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Old 08-28-2019, 04:26 PM
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I bought a repacement set of engine bolts from Tower for a dle 30 and these were soft as well. They round out easy.
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Old 08-28-2019, 04:31 PM
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I rounded out the ones on my DA 35 from taking them off to much, got the same hardness ones from Amazon cheaper than buying them from DA fyi. https://www.amazon.com/gp/aw/d/B07BS...b_b_asin_title
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Old 08-28-2019, 04:32 PM
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I have a box of 50 12.9 hardened bolts from McMaster Carr that if I have any doubts about a motors bolts I swap them out.
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Old 08-28-2019, 05:40 PM
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Quote:
Originally Posted by BarracudaHockey View Post
I have a box of 50 12.9 hardened bolts from McMaster Carr that if I have any doubts about a motors bolts I swap them out.
+1
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Old 08-28-2019, 06:43 PM
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I did replace the old ones with metric 12.9 rated bolts, which I found at our local Ace hardware. Supposedly the 12.9's are similar to the SAE grade 10.
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Old 08-31-2019, 07:41 PM
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Prop was slightly loose for a few flights I’m guessing, and the lateral side to side force stressed the bolts enough eventually to weaken.

Or they were over tightened to begin with as well.
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Old 08-31-2019, 09:57 PM
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What spinner backing plate did you have? This is more consistent with prop bolts that are not torqued properly or the backing plate (carbon) has an issue. Even a soft bolt torqued at 90 lbin should never stretch( to put it in perspective, you can hand tighten tighter). Years ago I had this exact same issue when I used the aeroworks carbon spinners. I talked at length with Tony at DA when he still worked there about this exact same problem.
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Old 09-01-2019, 05:14 PM
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Tru-Turn aluminum spinner and back plate with a Mejzlik carbon prop. Same setup on all my big stuff and as stated earlier, never an issue.

Not all hardware is created equal. Just please make sure your starting with good grade prop bolts to avoid eating the prop.
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Old 09-01-2019, 06:03 PM
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Quote:
Originally Posted by Colt View Post
Tru-Turn aluminum spinner and back plate with a Mejzlik carbon prop. Same setup on all my big stuff and as stated earlier, never an issue.

Not all hardware is created equal. Just please make sure your starting with good grade prop bolts to avoid eating the prop.
That is not consistent with a bolt issue. Go back and search for carbon back plate spinner issues and you will see a ton of post with pictures just like you posted. That was a loose prop bolt issue and the side load broke the bolts. That is why you had 4 break at the same time. It is impossible for 4 out of 6 bolts to break within a single flight from bad hardware. Look at head bolts on DA 150s. Aluminum bolts holding a head on with high cylinder pressures. Years ago there were a lot of threads talked about in great detail. It always came down to a spinner or torque issue. Even having a prop hub not drilled straight could cause an undertorque issue. That is one reason I hand thread the bolts on a few threads and slide the whole spinner,prop and washer assembly to make sure it is smooth.
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Old 09-01-2019, 06:16 PM
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A n attempt to a maiden flight on my 42% PAU Edge. I had to go to the local hardware store and buy small EZ Out and call DA for a prop and prop bolts. I failed to tighten the prop bolts on my DA150. Started the engine, ran it up to full throttle brought it back to idle and prepared to go fly when the engine quit. Went to restart; 3 broken bolts, 2 bent, and one that could be reused. One 32-10 menz prop over my shop door with a note "Don't forget to properly tighten prop bolts".
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Old 09-01-2019, 07:05 PM
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A n attempt to a maiden flight on my 42% PAU Edge. I had to go to the local hardware store and buy small EZ Out and call DA for a prop and prop bolts. I failed to tighten the prop bolts on my DA150. Started the engine, ran it up to full throttle brought it back to idle and prepared to go fly when the engine quit. Went to restart; 3 broken bolts, 2 bent, and one that could be reused. One 32-10 menz prop over my shop door with a note "Don't forget to properly tighten prop bolts".
Seen that too. Wood props really need proper torque as you found out the hard way. Me too. If they are too tight the wood compresses to much and then weakens. Not much time is needed before the bolts loosen up because the wood is weak and can't hold torque. I think your spot on and this to make sure bolts are properly torqued.
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Old 09-01-2019, 07:19 PM
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Thanks for all the input guys. I appreciate it!
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Old 09-02-2019, 06:46 AM
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Glad you found this on the ground and not in the air.
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