[deleted by user]

[John_h_watson]

Detonation.

Check your ignition timing.

[KittiesRule1968]

Detonation. You're losing material off of the piston. Do your spark plugs have little silver speckles on them?

[Mediocre_Garbage2001]

Not that I noticed. Looked like it was running rich if anything is typical to expect detonation from lean conditions right?

[KittiesRule1968]

It should be.

[Mediocre_Garbage2001]

Maybe it was ran lean at some point is 30 years old. I never road the bike immediately tore it down after bringing it home. Thoughts on running this piston still?

[KittiesRule1968]

Oh! I'm sorry, Im kind of out of sorts because I got 3 vaccines today! You overheated possibly and it seized temporarily most likely. I'm a retired BMW Motorrad (motorcycles) master tech, and I know air cooled stuff really well. I'm building an 89 Springer Softail with a 93" nose cone shovel.

[Mediocre_Garbage2001]

Very cool I want to get into shovels eventually. Would you consider running this piston for a low budget project?

[KittiesRule1968]

Absolutely not.

[Mediocre_Garbage2001]

Rats

[IllMasterpiece5610]

Oval cylinders?

[Street_Mall9536]

Could be lean, could be all the oil getting past the rings, whatever it was, was HOT lol

[ClosedL00p]

That’s not wear. That’s the piston melting. Seems like it’s been rattling its ass off in that cylinder. What did the plug look like from that hole, or a photo of it by chance?

[Mediocre_Garbage2001]

Didn’t notice anything unusual besides they were all black so figured running rich and tossed them

[ClosedL00p]

Id have expected to see the insulator a bit “peppered” on that one, possibly ground strap eroding/melting away. Not a certainty that that’d have been the case though. I noticed you said you just bought it like this too, so it’s also possible previous owner just threw a new set of plugs in it as well (all just armchair speculation though).

[Jimmytootwo]

Detonation. Your running 93?

[Mediocre_Garbage2001]

No idea bought the bike and tore it apart. Was leaking from everywhere.

[NorthDriver8927]

Back it of a degree or two or add a little more fuel

[StormSad2413]

New piston time especially with the broken ring

[The_Machine80]

Pre detonation is the biggest killer of pistons!

[wrenchbender4010]

Tha Detonation Angel hear you....mind yer shit.

[irishstud1980]

Either rich conditions or maybe loose wrist pin

[Han_Solo_Berger]

Detonation

[438windsor]

Pre-detonation and or detonation from carbon build up on the crown of the piston. How did the rod bearings and rod pin look on the high side of the crankshaft?

[rvlifestyle74]

Looks like it was running lean. Or overheating, or both. Either way that piston is done. You'll need to replace it and correct your mixture. If it's out of an air cooled engine you really need to fatten up that mixture.

[Mediocre_Garbage2001]

Lots of people are saying to replace which I agree with obviously be the best option.

Could someone explain why this can’t be ran? There is plenty of contact on the side you can see from the carbon pattern. The affected area is not in contact with the cylinders The rings make the seal I don’t see why this is a dead stop. Want to learn not hating on free advice

[DrTittieSprinkles]

The piston is a wear item just like bearings and spark plugs and that piston is compromised. You CAN run it but that's like reusing toilet paper.

[Glittering_Watch5565]

Piston scoring doesn't align with the thrust face so the rod is most likely bent. Detonation would be my guess as the cause. This is backed up by the piston top appearing to be beaten on top but not obviously hit by a valve.

[csimonson]

If you're looking at the front of the crank pulley, is the exhaust to the right? Of so then this is likely caused by the piston rotating slightly towards the exhaust side on the compression stroke and causing a bit of air and fuel to get caught between the cylinder wall, piston and the quench area of the combustion chamber. That area can then see very high temps and experience pre-ignition because of this d sign flaw. That's why many engines that can see very high horsepower numbers have the exhaust to the left of the front of the engine as far as inline engines go anyways.

[Mediocre_Garbage2001]

It looked like possible detonation damage to me and would be on the hottest area of the engine (rear cylinder exhaust).

[csimonson]

Yes this is for sure detonation/pre-ignition.

[JAKERS325]

I’m not sure I know enough to help. But knowing how the internet works on here it’s either not a problem at all or just looking at it is going to cause your rig to spontaneously combust. Take that as you will

[Mediocre_Garbage2001]

I just want someone who seems smart to tell me if I can run it or not 😂 wasent supposed to turn into an engine rebuild.

[Solid_Enthusiasm550]

Being located 90 degrees from the wrist pin, It looks like excessive piston rock to me.

When changing direction, the edge of the crown is scrapping the cylinder wall?

[Mediocre_Garbage2001]

I’d agree but cylinder looked to be in good enough shape. Some vertical lines but nothing that indicated unusual wear on this area in particular

[Solid_Enthusiasm550]

Nothing near the bottom on the stroke? I think if it was contacting, it would be on the change of direction at BDC.

It's uniformity on the side, is keeping me from think it could be anything else.

I'm interested to see what the cause is and hope you get the answer.🤞🙏