Holed piston mystery!

Sorry to see your poor piston. Does the bike run an air filter? Could something have got sucked in?

Best regards, Al.

Ignition and/or fuel problem. From your description of how it failed either it suddenly got a weak mixture or the ignition slipped. Manual advance/retard? cable ok?   

To much advance will do it, or a lean mixture.
John
 

A hole like that is usually caused by pre-ignition.

Common causes of pre-ignition are a too-hot grade of spark plug, buildup of carbon, or the timing and weak mixture issues mentioned above.  Even running low on fuel can do the damage.

Photo is a little blurry here, but is the edge of the piston at the inlet valve cutout eroded?  That would show a history of detonation (pinking).

Is quite accurate; there is evidence of pre ignition. I also feel the collapse of the piston looks like a structural failure rather than a melt (crack lines)  My piston failures (mainly on two strokes) have always been a melt due to mixture issues.
Had there been any work carried out on the piston previously? Motor & plug looks to be in good order otherwise.
Dan, with regard to non stopping bikers, I got a lecture off the boys in blue when I stopped to help a biker with out fuel on the M50. The gist was unsafe, untrained and a distraction to other road users.  I now follow the code of our new generations.
cheers

Hi Dan,

   It looks like this was your first distance run judging by the lack of carbon on the crown. What piston is this as the valve cutaways look offset to each other and I can see the part number cast in the inlet valve one, perhaps a Harley Davidson ? 
   Looking at the hole and its surrounding material it appears to have been punched through by the state of the jagged edges, and not burnt through. I think this may have been caused by a combination of pre-ignition and poor quality piston material as the pinking is like constantly hitting the piston with a centre punch, allied with 300 psi of combustion explosive pressure. You say the ignition timing is correct, but what data are you using as high compression and modern fuels need different settings than were used only a few years ago.
   I think you will need a full strip down as you may find a few pieces of the crown in the sump. If you are using the earlier Alfin bi-metal cylinder head, the compression ratio should be a maximum  9 to 1 using premium petrol or 9.5 if you have the 1957 all aluminium type.

Hi Jon,

   Sorry if I have repeated your info as I was having tea and cake while typing so didn't see your post when I started mine !

Can a bike like that ever reliably run a 9.5:1 ratio with road fuel?

Detonation at road speeds, that you can't hear is (perhaps obviously!) the worst kind.

A too soft (hot) plug will run at a higher temperature than the motor needs and will pre-ignite mixture and do damage. The plug is not showing that but has been well contaminated with burnt oil from the hole so its not readable.

Running at 60MPH you must be in the realms of main jet influence. Maybe too small or as  suggested  by my good friend Martin, build up or partial obstruction on the jet from deposits due to modern fuel characteristics. May be worth dropping the bowl and taking a look.  Then I guess fuel flow restriction could be further investigated back to fuel line, tap and tank breather.....
J

Are a source of great envy! I run a 280 Main jet on my single. It's a touch rich but helps with starting as she needs plenty of fuel to get going. 

Be worth confirming your static settings before you pull the motor down.
To be continued no doubt...
Jon

Dan, You didn't tell us what sparkplug you are using.

The size of main jet only matters if you have the throttle FULLY open for several seconds (5-15 seconds). I doubt that many road ridden machines ever experience that. 

If you look at Amal carburation graphs for the effect of all the jet sizes, over the whole throttle range, the main jet effect is only there at wide openings. The pilot jet and airscrew has an effect up to 3/4 throttle opening, the needle and needle jet are the main players in poor carburation.

Just my opinion but any other views are equivalent.

The technical information on Green Sparkplug site says that a hotter running plug ends up damaging itself but not the engine. I holed a piston on my twin a few years ago after only about a mile doing about 70.  I put it down to running lean on one side, because my siamesed pipes were not equally clear flowing inside.  I never heard any pinking.  I still suspect a defective piston.
Does high speed pre ignition happen without any warning sound of pre ignition on hard acceleration beforehand?

"The technical information on Green Sparkplug site says that a hotter running plug ends up damaging itself but not the engine."

Then that technical information is wrong.

Detonation and pre ignition are actually different things!
Detonation or knocking is the audible one, usually,  but not only caused by over advanced ignition. Lean mixture, excessive top land clearance on the piston, wrong squish area can also be causes. Combustion is still controlled,  but cylinder pressure peaks will be very high, which can cause damage if it is continually occurring. 
Pre-ignition is where combustion occurs uncontrollably, usually triggered by something too hot in the combustion chamber (plug electrodes and exhaust valves are usually favourites). In effect the engine is acting like a diesel. It is usually inaudible. If you are very sensitive you might be lucky and feel the loss of power and be able to close the throttle before damage occurs, but most cases lead to failure.
Detonation can lead to pre ignition,  but it isn't the only cause.
Hope you get to the bottom of the cause.

Regards, George 
(Too many years spent in engine development!)

Detonation and pre-ignition are different things, but that piston looks like it's seen quite enough of both.

Hi Dan,

   Just thought I would offer another possibility on your piston failure. If your piston is from a water-cooled engine it may not be made to the same specification as an air-cooled one. The general temperature of a water-cooled engine will be controlled at probably 95 degrees centigrade, whereas your big bore Model 50 will run at a much higher and un-regulated temperature. Older racing pistons, Specialoid in particular, had support webs under the the crown to give extra resistance to the increased heat and pressure and were possibly made of a higher spec alloy.
   The ignition timing sounds too advanced at 40 degrees BTC, 36 might be better on 9.5 to 1 compression and general road use. I think your piston had started to collapse over some time and the incline was the last straw ?

I am with Richard on this one. Thought the piston crown looked too thin for an air cool motor.Also todays fuel reputidly burns faster and needs less advance.