Craig Spence Posted April 9, 2016 Share Posted April 9, 2016 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. Cheers, Craig. Quote Link to comment Share on other sites More sharing options...
flight1 Posted April 9, 2016 Share Posted April 9, 2016 Valve spring/ lets hope the valve ok will need to take the cam cover off and check . Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted April 9, 2016 Share Posted April 9, 2016 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 PB Quote Link to comment Share on other sites More sharing options...
Craig Spence Posted April 9, 2016 Author Share Posted April 9, 2016 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. Cheers. Quote Link to comment Share on other sites More sharing options...
Former Member Posted April 9, 2016 Share Posted April 9, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Craig Spence Posted April 9, 2016 Author Share Posted April 9, 2016 Gents, 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, Cheers, Craig. Quote Link to comment Share on other sites More sharing options...
J D 8 Posted April 9, 2016 Share Posted April 9, 2016 Has the crank pin seized in the connecting rod or is it free? Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted April 9, 2016 Share Posted April 9, 2016 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 Quote Link to comment Share on other sites More sharing options...
Stevem3akm Posted April 9, 2016 Share Posted April 9, 2016 Hi, currently 4/5 engines for sale on eBay. Regards Steve Quote Link to comment Share on other sites More sharing options...
J D 8 Posted April 9, 2016 Share Posted April 9, 2016 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 Quote Link to comment Share on other sites More sharing options...
Craig Spence Posted April 9, 2016 Author Share Posted April 9, 2016 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. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted April 9, 2016 Share Posted April 9, 2016 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 Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted April 10, 2016 Share Posted April 10, 2016 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? Quote Link to comment Share on other sites More sharing options...
Craig Spence Posted April 10, 2016 Author Share Posted April 10, 2016 Nothing seized Dave, it just sheared, striped the lot & all smooth running no indication of faults just a material failure I think. Gutted!, lol. Quote Link to comment Share on other sites More sharing options...
Martin Whybrow Posted April 10, 2016 Share Posted April 10, 2016 Are the crank pins plated (chromed) on these engines? Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted April 10, 2016 Share Posted April 10, 2016 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. John Quote Link to comment Share on other sites More sharing options...
Former Member Posted April 10, 2016 Share Posted April 10, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Former Member Posted April 10, 2016 Share Posted April 10, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Peter Miller Posted April 10, 2016 Share Posted April 10, 2016 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. Quote Link to comment Share on other sites More sharing options...
cymaz Posted April 10, 2016 Share Posted April 10, 2016 I had an SC four stroke pin break. Luckily no other damage. Quote Link to comment Share on other sites More sharing options...
Former Member Posted April 10, 2016 Share Posted April 10, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Peter Miller Posted April 10, 2016 Share Posted April 10, 2016 Some where OK on lower nitro and some neded a lot more. the 35s were always fine on low nitro but the 36s needed 15%. Our club used an awful lot of Foxes back in the 70s because I could get them at trade price from John Haytree. Quote Link to comment Share on other sites More sharing options...
Former Member Posted April 10, 2016 Share Posted April 10, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Cuban8 Posted April 10, 2016 Share Posted April 10, 2016 Happened to my ASP 120FS in a YT Spitfire that was subsequently damaged beyond repair in the D/S landing in the rough. I did rebuild the engine and it now powers a H9 P47, but the cost of spares is very high in comparison to a new engine (over fifty quid for just the crankshaft and connecting rod) - but as the motor only had a couple of hours running from new I had to swallow the cost. Couldn't claim on the JE 2 year warranty as I bought the engine with the original model and for one reason or another, didn't get around to building it for three years. Tough luck, but something to be aware of if you buy an engine and then lay it away, or just use it as a space sample during a lengthy build. Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted April 10, 2016 Share Posted April 10, 2016 The last time I saw a neat hole punched in a crankcase like that it was on a BSA A7 bike engine - both BSAs and Triumph twins had a habit of the conrods letting go if you tuned them and revved them a bit hard Was the piston damage caused by it's abrupt stop when the skirt hit the backplate/crankweb? Quote Link to comment Share on other sites More sharing options...
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