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Old 08-25-2014, 02:59 PM
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Prop bolts breaking on DLE 55-RA

I have three different DLE-55 RA engines and none have over a gallon of fuel run through them.
Each have sheered the prop bolts once and one of them twice. I have checked and double checked my instillation and can find nothing wrong with my procedure. I'm 70 years old and I thought an excellent mechanic. I now wonder.


I'm gona back up the timing an see if this helps. I'm thinking maybe it may be trying to backfire.


Has anyone else out there had prop bolts sheer off on a DLE-55 ?, or am I just getting senile.
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Old 08-25-2014, 03:14 PM
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Are you re-tightening the prop bolts after the first flight? Wood prop hubs tend to compress and even if they are tight for the first flight, they need to be re-tightened after the first flight. Then again after 4 or 5 flights, etc.

I'm not actually familiar with the DLE-55, so my comment is more generic in nature.
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Old 08-25-2014, 03:22 PM
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A few things to look at....
The stock bolts supplied with the engine "could be better". A trip to Fastenal or Ace Hardware will yield some much better bolts.
Are you using a torque wrench to torque the bolts to the proper spec? Too loose or Too Tight can cause this issue.
Are you using a Carbon Spinner Backplate? There is a consensus that these can heat up via the prop hub and deform, thus loosening the prop bolts. There are many instances of this happening....but...in their defence....there are lots that it hasn't happened to also. I personally will not run a CF backplate on any spinner when installed on a gas engine.

CB
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Old 08-25-2014, 03:30 PM
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Yes I did re-tighten bolts


Yes it does have a carbon fiber back plate and I have heard about this problem. Will try aluminum before changing timing.
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Old 08-25-2014, 05:49 PM
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Can you slide the prop and back plate up and down the bolts when all bolts are only part way threaded in? It should slide with no resistance. If it resists the through holes are not drilled large enough or not in line with the engine prop hub holes. Open the holes up in the back plate and or prop hub until it slides freely. I do this and have never sheared a prop bolt in many engines many years. Side load could be doing this.
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Old 08-25-2014, 11:22 PM
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This is when the single bolt hubs work great!
I too have had multi bolt hubs sheer the bolts, on the ground and in the air. Not cool.
Single bolt hubs are plenty strong and if it comes loose they just slip but not come off, with self locking nuts of course.

Mark.
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Old 08-25-2014, 11:27 PM
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Also,
There doesn't seem to be any GOOD torque specs for the different size bolts for the props.
Carbon you can get tight, but the wood keeps crushing and the bolts go loose.

Mark.
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Old 08-25-2014, 11:37 PM
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I have run several 55ras with carbon and aluminum backplates and wood props plus there are several others at our field. the only time I have seen them shear is when there was a prop strike and the bolts were stressed. the supplied bolts are fine and can be tightened down. last, make sure that your prop jig aligns with the prop hub of the engines. all it takes is for one bolt to fail, and the rest will soon follow.

good luck....
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Old 08-26-2014, 12:57 AM
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heat and carbon spinner backplates, can be a problem, but not seen it on smaller engines, 100cc plus and generally only the bigger 170cc and above.
torque wrench - i love em. and use them all the time. but i am yet to find a 50/60cc prop that will handle one. again 100cc plus prop hubs are 'meaty' enough and they will take 40 in lbs for wood hubs. try that on a 50cc and it will crush the hub. all my 50/60cc class engines we just tighten by hand.
seeing as you are checking tightness after a few flights that sounds good. Side loading the bolts is not good so agree with the above, check you have free play in the bolt holes. Also over tightening can cause this, as you compress the hub so much its under great strain, once the engine is running the hub compresses further and the bolts become 'loose' and shear. you will be surprised how little pressure is required on 4 or 6 bolts to hold the prop.
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Old 08-26-2014, 11:16 AM
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Great post, Clive.
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Old 08-26-2014, 08:37 PM
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Thanks for all the input it is all good and I promise you I have checked all the answers closely. I have built all kinds of engines in my life and I don't mean shade tree work. I have had several world record engines in everything from Go Carts to Mod street drag engines but I'm honest, this little problem has got me puzzled. I have probably 20 gas engines running right now and I do all the work myself. These 55 RAs are the only ones I have ever seen do this.
The last one, this past Sunday just had the prop put on It (3 D hobbies 23x8), tightened and put on the runway .Ran up fine and smooth but when released about 20 ft in the air it just exploded. I have decided to try it with no spinner and the timing backed up about 2-3 deg.. If anyone sees any straws floating around, grab them for me, I AM grasping at straws.


Keep the info coming because it is normally some stupid something you either did or did not do properly. THANKS
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Old 08-26-2014, 10:24 PM
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Prop bolts break from only one thing, being loose! Are your bolts too long and bottoming in the hub before they clamp the prop tightly? What brand of wood prop are you using? Maybe it's very soft wood that compresses too much? Try a carbon fiber prop like a Biela or Mejzlik and tighten up the prop bolts, I'll bet you don't break any more bolts. Bolts must be in tension or they will break easily and quickly from fatigue stress, same as if you don't tighten the lug nuts on your car wheel.
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Old 08-27-2014, 07:27 AM
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I'm not sold on the "the only reason your prop bolts are breaking is because they are loose theory", however it "may" be the most common. You mention this squawk has occurred on 3 different DLE 55's, do you have a 4th DLE that is NOT shearing the prop bolts? If so what is different? I would ASSume it to be very rare that all 3 engines are running too much timing, if you had 1 engine with this squawk, id say go that direction, but.... Are you using the same drill jig on all 3 of these suspect engines? Do the fasteners slip into the drilled prop holes or are they tight? What fasteners are you replacing these with, ie local hardware store, DLE replacement etc etc? As far as fastener torque rating, IMO for a 55 size engine, use a Bondhus ball driver and tighten them tight as I can with that tool.....I have never had issues
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Old 08-27-2014, 07:51 AM
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Get some quality 12.9 bolts from McMaster Carr, you can get a box of 50 for what you'll pay for a few of them at Ace

I don't believe for a minute that a 50cc is having an issue with a heated up carbon back plate
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Old 08-27-2014, 08:57 AM
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i totally agree - the only time these bolts can shear is if they come loose. the only way they will shear is if a side load is applied. in order for that to happen the prop must rotate, that can only happen when they come loose.
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