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Engine frustration

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MikeS17/03/2015 14:00:36
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818 forum posts
240 photos
Where do I start. I have three ASP four stokes two of which have lived in my H9 Piper Pawnee.

First fs was a 70 which ran perfectly for for many flights and then gave me starting problems. On removing the glow plug metal fragments where found. On inspection the piston, ring and liner where scratched Badly. Second was a 91 which before living in the Pawnee was on my tutor.

Today the 91's behaviour echoed that of the 70. Removed the glow plug to see metal fragments.

Fuel is filtered when filling and on the way in to the carb.

I cannot work out why this has happened twice.

Mike
Mowerman17/03/2015 14:20:23
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1536 forum posts
105 photos

Could your flying site be sandy or very dusty causing grit to be picked up during take-off or ground running prior to flight?

Also how are your models stored, could debris be getting into the carb ?

Edited By Mowerman on 17/03/2015 14:22:44

MikeS17/03/2015 14:27:39
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818 forum posts
240 photos

A couple of good points, our flying site is grass and I would not call it dusty but this was my first thought. All models are stored in my attic which is not dusty.

Mike

MikeS17/03/2015 15:37:15
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818 forum posts
240 photos

Taken the rear cover off and found lots of metal fragments. It seems the piston has sheared off the top of the conrod. In actual fact it is the piston pin that has sheared.

Mike

Edited By MikeS on 17/03/2015 15:57:54

Mowerman17/03/2015 16:38:08
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1536 forum posts
105 photos

Are the engines mounted inverted ? If so a hydraulic lock due to over fueling during start-up could be the cause.

With inverted engines, (at the risk of telling you the obvious) always turn them over by hand a couple of times before connecting glow power and electric starter,

MikeS17/03/2015 16:46:36
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818 forum posts
240 photos

Not this one. It is on its side like the 70 and yep I also turn over by hand just in case but good advice.

Mike

ROY DAVIES 117/03/2015 16:48:26
139 forum posts

Looks like a case of 'Hydrogen embrittlement' Have a look under that heading. It's a well known cause of metal failure in the metal machining processing industry. Very small items like gudgeon pins (wrist pins) and the like - small screws are a common example being made in large no's. Due to their size (thickness) it doesn't take much to get the heat and or chemical treatment a little off standard and a poor batch will be produced. Quality control should sort them out but if there is variance in the batch it could easily be missed.

Lets hope Mike your spares will be of a better quality.

Jon - Laser Engines19/03/2015 15:48:47
4774 forum posts
179 photos

I find it unlikely that two different engines of different sizes and ages would have the same mechanical problem. I would be looking at the model to see if overheating has played a part or there is something else a miss given that at least one of the engines ran wall in another model

Some photos of the damaged parts would be interesting, and what fuel are you running?

Edited By Jon Harper on 19/03/2015 15:49:36

ROY DAVIES 119/03/2015 16:05:47
139 forum posts

It's quite common to have 'common' parts in some makers engines even crank cases! Of course it could be the fuel but that would only contribute to a weakness inherent in the motors to snap a gudgeon pin. Crankpins have snapped on some makes in the past so why not other parts too. As I mentioned if over hardened some parts would have the strength of a raw carrot as they say, A big prop' and a rapid stop would be enough to shear a 'pin.

If both motors have been used and stored in similar conditions it is quite possible that a similar snag could occur on both motors. It just needs a quiet bit of detective work to sort it out. Metal fatigue has often reared it's ugly head in 'similar part' situations many times in the past.

MikeS19/03/2015 16:36:15
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818 forum posts
240 photos

I did think that two engines failing could be down to the model or my after care. The 70 didn't fail to a broken gudgeon pin, however I'm not sure what caused it.

An email from just engines said this is uncommon for a pin to sheer unless the pin was hard as suggested above or the engine seized. In the 91 the piston, piston ring and liner are undamaged or discolored. On inspection of the engine the piston was free to move.

Mike

Edited By MikeS on 19/03/2015 16:36:53

Stevem3akm19/03/2015 20:00:19
52 forum posts

Hi, I would suggest checking the crankshaft bearings. I have had engines where the bearing cage has slowly failed giving similar symptoms ie blown plugs, metal fragments, scoring of piston/liner etc. and a plug per flight. Hope this helps. Steve

Jon - Laser Engines19/03/2015 20:05:10
4774 forum posts
179 photos

Pre ignition could kill off a gudgeon pin, especially if the hardening was a bit wonky. It is very unusual though, but as you say it sounds like the issue with the 70 was not the same as the issue with the 91.

Pre ignition could be caused by the wrong fuel/plug or a faulty plug. Overheating can cause it, in correct prop or excessive rpm and the most likely candidate is too lean on the fuel. Again, its hard to tell if this is the case or is the part was faulty

MikeS20/03/2015 10:03:57
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818 forum posts
240 photos
I use model technics contest 10 in my engines and plugs in the fs are os f plugs.
Jon - Laser Engines20/03/2015 10:11:59
4774 forum posts
179 photos

That all sounds pretty reasonable to me. Personally I don't use more than 5% nitro as its not needed (apart from my model cars which use 16%) but 10% is hardly going to make any odds.

MikeS21/03/2015 16:24:21
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818 forum posts
240 photos

I was chatting to my daughter this morning about the 91. She was with me at the club when I was flying the Pawnee. She reminded that the I had suffered a dead stick. This seems to be the moment the engine failed. I can remember I was climbing vertical at almost full throttle and went to idle to flip the nose down. On opening the throttle the engine had cut.

I didn't try to restart the engine until the next time at the club where the damaged was found.

Here are some photos of the 91 piston and conrod wrist pin.

dsc_0174.jpg

dsc_0175.jpg

Engine Doctor21/03/2015 16:54:10
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2271 forum posts
25 photos

That does look like a brittle gudgeon pin ,or has the gudgeon pin siezed in the piston and con rod ?Hopefully a new piston ,pin and conrod will sort it if they are a reasonable price.

ROY DAVIES 121/03/2015 17:40:33
139 forum posts

With reference to Stevem3akm.

'A worn main bearing' - either one or both will put tremendous side loads fore and aft mainly but precessing as well on the conrod to piston connection and oscillating it at quite a high frequency too. Of course if the fit of the remains of the pin is still a good fit in the piston the gudgeon pin must be the weak link that has failed. Was the engine heavily propped on the set up that failed. Small engines can and do 'nip up' and then let carry on running or shearing something almost unnoticed.

ROY DAVIES 121/03/2015 17:47:32
139 forum posts

Looking at your pic's it appears that the pin may have snapped on one end between the rod and piston bearing. Is it the same on the other piston bearing. The pin looks like it has 'necked'- stretched before it broke. Are there any hairline cracks visible on the pin? You may need a magnifier for this.

MikeS22/03/2015 14:10:22
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818 forum posts
240 photos

The pin has not seized in the conrod but the remains of the pin have in the piston. I have not checked the bearings yet but I did notice that when turning the main shaft the rear bearing inner race was occasionally slipping and not turning. I thought this was odd.

Mike

Jon - Laser Engines22/03/2015 21:57:20
4774 forum posts
179 photos

Most likely the rear bearing is full of debris from the failure

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