In my time playing with engines, one thing never fails to amaze me: the sheer durability of these pieces of machinery. I've seen an engine with a hole burned in one piston, a missing valve on another cylinder, and a horribly corroded spark plug on another. Not only did it run, it even idled fairly smoothly. Based on tortured troopers like this one, I started to believe that engines really were, more or less, bulletproof.
My reality check came by way of a tired valve spring.
Read the full article on Hagerty.com: https://www.hagerty.com/media/maintenance-and-tech/the-fragility-of-the-durable-and-other-things-tha...
Got evidence of indestructibility sitting on my desk right now; an 8 mm hex head bolt about 5/8" long that bounced around on top of an intake valve (or valves) in the head of a Toyota 7M-GE engine for at least 80,000 miles by my reckoning. Engine ran fine and bolt was apparently too big to drop into combustion chamber (which would have changed things). It was in there long enough to round the hex head into a near-circle and wear all the threads off. Only discovered after head was removed for a blown head gasket (typical for that engine).
I have a nice stainless one-piece intake valve from a 351C 4V that's bent on my desk from kissing the piston top when the engine only -slightly- over-revved the build sheet max rpm. Lesson learned.... rev-limiter.
Took my 10 year old son to the drag races and we watched a front engine Hemi dragster just explode at the starting line. After watching the crew clean up the debris, we followed them to the pit area and my son asked the crew chief for a piece of the engine as memorabilia, and without missing a beat, he said," This is not a good time right now."
Bought a badly abused Honda Super 90 for restoration. Before tearing it down, I wanted to get it running. After an hour's work, it started on the second kick and ran like a watch. Only a frozen clutch kept me from taking a ride. Removed the side cover to replace the clutch and a piece of aluminum fell out. The case was full of them. Someone replaced a blown piston and never split the case to remove the pieces. After a teardown, everything as to spec and only cost a gasket set. Another example of Honda indistructability.
Redirected the oil feed to the top end on my Honda Ascot based roadracer. Made the oil orifice too large. Pressure drop at the big end cost me my crank and a Carillo rod$$. Little things really do matter.
My "oops" moment came after I decided the slightly modify the intake manifold on my own 383 Chevy engine by carefully cutting a receiver groove for an 1/8" thick, 1" high stainless steel plenum divider. The manifold was a Weiand Stealth intended for use with a Holley carburetor, but my shop could build a Quadrajet that metered fuel better and idle cleaner (gotta love California smog laws!) and if you know Q-jets, you know the primaries like a crisp vacuum signal from the engine. So, crafty me put the thing together and drove away happy, for about three weeks. Suddenly, the engine developed a slight miss. A compression test showed low cylinder pressure on number eight, so the coolant was drained and the carb came off in preparation for intake and head removal. As I removed the carburetor, I discovered the reason for the miss. My special divider was no longer in place. The majority of it was lodged under the number eight intake valve, keeping it from closing. A forensic investigation revealed that the characteristics of the cam coupled with that "improved" vacuum signal caused the non-rigid piece of stainless to flex back and forth until it failed, and away it went. A 1/4" thick piece of 6061 aluminum was welded in and all was well. Lesson learned: Do not try to reinvent the wheel, try to make it rounder. And look for a manifold that works with a Rochester.
I disagree with you diagnosis. Problem is obvious from looking at the pictures. Rear drum brake. You no doubt over rev'd the engine trying to slow it down due to the fact that the drum brakes on those bikes dated back to the first Honda ever imported to the US. Yes you could buy a set of brake shoes for a 1960 Honda 50 and they had the same part number. My memory tells me the brakes were good for about two stops with new shoes and then they were done. Zero stops if wet. Going down three gears all at once hoping to make the next corner is the culprit. I am also sure that you offended the entire valve train by disconnecting the automatic decompression release cable that was activated by the kick starter. The normally activated exhaust valve got good and pissed off and talked the adjacent intake valve into deserting ship.
One reason flatheads are so durable, no valves above pistons. One time tho', I had put old used but refinished valves in our Model T, on a long trip far from home one valve broke off, the head went thru' a piston, we drove home 200 miles on 3 cylinders anyway, I put in all new SS valves after that fiasco, live and learn.
I was racing a 1969 VW fastback in SCCA. I had a built-up engine with Gene Berg remote oil filter bypass plate over the oil pump to route the oil to a remote filter. It had a small ball check valve the let excess pressure bleed back into the pump. I thought that was silly, so I blocked it. Cranking the engine for the first time, I heard a weird pop. When I went to look, the oil filter has split and spit oil everywhere. Thinking that it must have been a faulty oil filter, I cleaned it all up and got a free replacement. They had never seen it happen either.
Put in the new one in and heard the same pop when I cranked it again. Since it was in a Type III pancake engine, I had to pull the engine to get past the fan shroud to get to the oil pump. Set the ball check valve back in and no future problems.
Failed to adequately tighten an exhaust valve retaining nut after a valve adjustment session an a 1997 Kawasaki Concours. The nut came loose and went through the timing chain while I was checking carb sync at high rpm. The chain skipped a tooth, resulting in eight bent intake valves. I count myself very lucky that it didn't happen on the highway. The valves will make good coat hooks.
Hmmm... Son, what we have heeeerea is a failure to COMMUNICATE...
Apparently the valve and the piston were having some sort of argument; possibly involving use of Cilantro or Parsley in some Eastern European recipe. The discussion got heated, involving rantings regarding great Grandma's use of non-Hungarian Paprika, in the chicken Paprikash, (which led to yet another outbreak of hostilities in parts of the Holy Roman Empire, back in the day) and thus the assault and battery seen here, twice. Of, course it could just be a butterfly sneezed in some small Australian town and the Cosmos, thusly out of balance, had to find a suitable act to balance the universe to avoid the Chaos and this was the answer. Twice. (It could be the butterfly had Covid) What... No, I'm just the observer here, I'm not making this s#$% up. 🙂
I suppose it's even possible the valve spring pressure was a bit low and the intake valve bounced, when it landed on the seat and after it, and the pistons, valiant attempt to enhance our knowledge of impossible physics; reality just snuck up and clobbered both of them into submission. Twice.
Proper seat and open pressure aside, I think my observations are far more likely and more interesting than the more terrestrial and mundane ideas put forwarded by the author and the other contributors. 🙂
You might look into a salvage cylinder head. Even after freshening the used head, it would probably be cheaper than trying to fix your original mangled head. They made a lot of those XR 250s so it shouldn't be very difficult to find one. Good luck.