Something I have come across is that over the years people cook the engine and some numb nuts machines the head which has bowed from the overheating and surprise not they lift up in the centre and with overhead valve engines you do not come across the warning sign something is not right the cam shaft does not turn freely like for instance in an L series engine.

Where it becomes an issue is the chamber sizes now vary with the end chambers being smaller and more likely to detonate simply because they machined a head instead of getting it straightened and then skimming it.

You need to cc the chambers and even a syringe from the vet will give you an idea if that is the problem,The correct method involves a burrette a plexi glass plate to seal the head gasket surface with a light smear of grease to make it seal and some kero to fill it and measure the volume.

Unfortunately whilst I have a burrette etc to do this job most do not so you have to ask around to see who can measure it accurately that along with detonation it causes are the most likely culprits that will need investigating I bet the issue started with the flat tops and increase in compression that pushed the end cylinder over the top compression wise so that cylinder alone detonated much harder to hear than all doing it together.

I had a closer look at the rings and the broken ring has a concave wear patters on the axial face of the ring and you can clearly see the sides are shiny (high spots) compared to the center were ist not being touching the piston as is darker (carbon)

The good rings are not as severly worn as broken one but show slight concave wear.

Piston grooves dont show any wear though.

Not sire what would cause wear as such besides being hammered by detonation? Rings rotating to much or to fast? Never realy heard of this or why it would happen.



Rings maybe have maximum of 10-15000km so the wear seems quite excessive

Also confirmed rings gap and its 0.42 mm so seems plenty of anything on the high side

That sounds about right, I erred on the larger end of spec just to be safe.

I wish I could offer definitive advice; I'm with some of the other guys on detonation, my experience with it has always been melted pistons and busted ring lands.

Note I also use a ring spreader tool to install them.

Its been a while but from memory it was 0.35 mm

Do you remember what gap you set the rings at?

OK, I'm not trying to start a pi$$ing contest here, I've spent 40+ years working on cars for a living and I have seen a lot, not everything, no-one could, that said this is based on my experience. Knock, ping, rattle under acceleration, funny noise, pre-ignition, detonation all cause engine damage. Over time you should see erosion of pistons or combustion chamber, broken spark plug porcelain, or a combination of the above. Detonation does not just cause a broken ring without other damage being obvious. Like I stated before maybe you need to take a good look at everything and not be quick to blame it on one thing, and it might be a good idea to get someone else to look at it also, most times two or more pairs of eyes are better than one. IF you still come to the same conclusion here are a few things to consider, richen up the carbs to see if it goes away, a "wet" mixture is less prone to pre-ignition because it's cooler. Try a step or two cooler spark plug, incorrect spark plug heat range can and usually will cause problems if your not sure what to use ask around to see what others are using, and finally look for vacuum leaks, they will cause a lean condition in 1 or 2 cylinders. Again, GOOD LUCK.

Im kind of running out of ideas.

Pistons and all are fine rings go in all the way when fully compressed.

I find it odd that only cylinder 4 did it this time.

Forst time it broke all 4 only top rings which make me think its detonation when this happened i had a weber down draught carburetor on it (34ADM)

Now i have cbr1100 38mm mikuni bike carbs and i have only broken number 4.

But I am starting to wajder if itsnrunning to lean. I will obviosly have to hook up a wide band to verify.

Also there is no damage to pistons at all and all look very good woth no signs of detonation.

it's hard to hear detonation, so make some detonation cans

btw the first image that comes up there is a broken piston land

a12dato120y I'm running 38 degrees of total timing but I use 100 octane fuel.

Is this caused by detonation?

Yes