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Loud crack heard then engine stopped.

What could it be?

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Craig Spence09/04/2016 19:35:21
1170 forum posts
615 photos

Hi all,

I run both my SC52fs today in my Mossie, had a tank through both on the ground & then in the air.

When I restarted them for another flight I heard a loud crack & the STB engine stopped running?, on inspection I found that I turned with next to no resistance!.

I thought the prop nut had come loose but all good there, so what is it? = broken rod?, crank?.

I will be taking it apart later but I'm really looking for ideas & why it happened. The engine was running fine then crack!.

Feedback greatly appreciated & I'll keep you updated when I strip it.



flight109/04/2016 19:47:07
734 forum posts
37 photos

Valve spring/ lets hope the valve ok will need to take the cam cover off and check .

Peter Beeney09/04/2016 20:16:58
1593 forum posts
59 photos

The SC 52 two stroke has been known to occasionally break the odd crankpin or three; could the four stroke be something of a copy cat? A a quick glimpse in the crankcase will reveal all. Unfortunately, I’m told that a new crankshaft may be fairly expensive, too.

Good Luck


Craig Spence09/04/2016 20:29:49
1170 forum posts
615 photos

Thanks gents,

I'll look in crankcase first & cross my fingers!, then the rocker cover, I'm hoping this doesn't turn out to be a big job.

It's definatley not a copy, I find it strange though as although old they haven't done a great deal of work. I'll keep you posted.


Former Member09/04/2016 20:53:11
3577 forum posts

[This posting has been removed]

Craig Spence09/04/2016 22:27:24
1170 forum posts
615 photos


I have discovered the problems!, I have to say I am not happy but delighted at the same time (explanation after). So, the valves and springs are fine, crank pin has sheared and piston rod has punched its way through the crank case!!, the piston head is stuck and it has damage to the lower half.

I have attached some photos, someone please explain to me how this has happened because for the life of me I have no idea!!!, the highly annoying thing is that this engines was from a twin!, so I can't really get a new one!!, ill need a secondhand one thats run in quite a bit!!. Or Ill have to spend ages bedding it in.

The massive plus side is, if that had gone in the air, likelihood is I would have probably lost the plane.

Nice hole!



Sheared pin!


Stuck piston head!



Help and explanations very welcome,



J D 809/04/2016 22:43:26
1497 forum posts
84 photos

Has the crank pin seized in the connecting rod or is it free?

Dave Hopkin09/04/2016 22:44:12
3672 forum posts
294 photos

One would assume that the crank pin sheared on the power stroke, so the con rod would have just continued down and smacked the crank case wall punching that quite neat hole


Happened to me in Rover 2000 years ago on the motorway at 75ish - very loud bang, a smokescreen worthy of a good sized destroyer, dashboard like Blackpool illuminations and a quite dramatic dead stick situation....

Edited By Dave Hopkin on 09/04/2016 22:44:28

Stevem3akm09/04/2016 22:48:05
52 forum posts

Hi, currently 4/5 engines for sale on eBay. Regards Steve

J D 809/04/2016 22:56:27
1497 forum posts
84 photos

Had it happen with a little OS 15 when flying,aircraft was flat out in a dive at the time and it let go with a hell of a bang, spinner and prop coming off. Crank pin had seized in the big end, the twisting action causing the pin to shear.It was possible to deduct thisis what happened by looking closely at the remains.

Edited By john davies 8 on 09/04/2016 22:58:02

Craig Spence09/04/2016 23:34:33
1170 forum posts
615 photos

Thanks lads,

I have stripped it completely & no obvious signs of failure apart from as said by Dave, it's sheared on the power stroke & continued down. However I can't explain the damage on the lower end of the piston head?, perhaps myself trying a restart!?.

Oh well, I think it goes to show that electric starters may be the cause!, I bet if I used the chicken stick all the time like I used to I wouldent have had this happen. Fuel locks etc..

Something to bear in mind I think. I personally do not think that is normal wear & tear!.

The search begins,

Thanks for the help.

Martin Harris09/04/2016 23:50:17
9351 forum posts
252 photos

All too common, I'm afraid. As well as seeing several instances on "clones", I saw/heard it happen on a Saito 150 a year or two ago so it isn't confined to budget engines. Whatever the cause, the Saito importers didn't take any responsibility for the failure although no user fault was identified.

If the big end hasn't seized, perhaps pre-ignition caused by a lean mixture or carbon build-up has lead to "hammering" or it was just simple material failure?

I wonder if corrosion might start a crack?  The crank seems to be showing slight rusting in your pictures...

I think your piston damage would be due to tangling with the thrashing conrod and the distortion has caused it to jam in the bore.

Edited By Martin Harris on 10/04/2016

Dave Hopkin10/04/2016 00:38:48
3672 forum posts
294 photos

Can't really make it out properly in the pic - but it looks like the crank pin has sheared almost cleanly apart from wheat looks like a sloping fracture at the bottom of the crankshaft.. I wonder if that clean break might indicate a twisting fracture caused by the pin seizing up in the con rod?

Craig Spence10/04/2016 00:48:43
1170 forum posts
615 photos

Nothing seized Dave, it just sheared, striped the lot & all smooth running no indication of faults just a material failure I think.

Gutted!, lol.

Martin Whybrow10/04/2016 01:08:49
884 forum posts
33 photos

Are the crank pins plated (chromed) on these engines?

John Olsen 110/04/2016 03:00:49
446 forum posts
23 photos

Just for interest, take a close look at the actual break. Quite often you will find that part of the break is smooth, perhaps with a faint pattern like tree growth rings, while the other part is rough, looking almost crystalline. What happens is that a tiny crack starts, perhaps from a scratch. With each reversal of load, the crack grows a little. With a crankpin, the load reverses every cycle. Eventually the crack is big enough that the rest of the metal fails abruptly, giving the rough break. When the crankpin on my AM15 failed many years ago, that was exactly what the break looked like. It does not help that there is a stress concentration in the corner, and it is kind of hard to design a crankshaft that does not have such a stress concentration, although there is usually a radius there to help with that. But to some degree, if it was strong enough to never break, it would be too heavy. For a given loading, there is a critical crack length, beyond which the crack propagates through the material at the speed of sound, so the final failure is very sudden. Also corrosion will both help such a crack get started, and help it to grow once it has started.

On the other hand, if the engine was too lean and did a backfire, it might have managed to put enough stress on the pun to just bust it outright.

A crack growing is not exactly the same as fatigue, although the effect is similar. (eg failure after some time in operation.) But metals that are prone to fatigue will fail even without an initiating scratch. The repeated cycles of loading are able to alter the crystal structure so that eventually the piece fails under a much lower load than it could withstand when new.


Former Member10/04/2016 07:44:24

[This posting has been removed]

Former Member10/04/2016 08:21:55

[This posting has been removed]

Peter Miller10/04/2016 08:23:29
11117 forum posts
1309 photos
10 articles

Back in 1966 I had a Fox 36 in a Rat Racer. IT sheared the crank pin just like that. The only salvageable part were the backplate and cylinder head.

However back in those days Duke Fox promised that no matter how badly damaged an engine was Fox would repair it for $5. Yes, five dollars.

I was in the RAF in German and we had an American PX on our station so I got a $5 dollar money order and sent it off. True to Dukes word the engine came back, brand new with just the original head and backplate.

Duke made this promise because as a kid he had damaged an engine and it had cost more than a new engine to get it repaired

Wish companies offered that same deal now. Not even Fox do now.

cymaz10/04/2016 08:26:54
9258 forum posts
1196 photos

I had an SC four stroke pin break. Luckily no other damage.

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