That looks familiar. We used to take boxes of spare parts to every race and of course the part you need is always the one part you don't have with you. That clip on is pretty bent. Don't remember doing that to a bike. How did it happen?
Agree that it's not actually the pin moving side to side and impacting the walls. That was before my first cup of coffee.But.....if you look closely, the pits appear cratered from being repeatedly struck with something small, the pits are not rounded in a way that you would expect if it was the pin and Mike said there weren't any witness marks on the pin. Granted the pin is harder than the wall and would wear less. The pits also get deeper as they get closer to the end of the stroke, so an undamaged pin running at right angles to the bore couldn't make pits that look tapered outward like that.
From what I'm seeing in the photos, the pin boss is eroded downward (below the pin), I'm pretty sure that when measured, the lower extent of that erosion on the piston will lineup with the lower edge of the pit in the wall at BDC and vice versa at the top. Unless I'm missing something, all that can tell us is that something was rattling around in there. Based on the depth of the pits, if either the oil ring or second ring played a part, then there would be damage evident on the rings.
When we build engines, we try our best to do it right and fail to check our own work. With a 2 stroke it's important to check compression and leakdown after the motor goes back together and before it's fitted in the frame. It's amazing how often there's a ding or scratch that we overlook that's just enough to cause a tiny air leak and that's often enough to melt it down.Thanks for advice on the RG. I must have been lucky as I've not seen detonation marks like that before. Sadly I can't check because the engine is already in pieces because one big end bearing had seized. Doublechecking looks like a very good idea.
Lots of ways to make an LC faster - depending on the rules. Forget about old TZ750 reeds and think about YZ85 reeds with single petals each side as an easy upgrade. Porting is also fairly easy to do but pay attention to squish and head shape. Programmable ignition will add low rpm advance and more punch out of corners. 26mm carbs are fine up to about 60 or so HP but pipes are the key to making the package work. 34mm are fine on a 250 or 350 TZ but the revs have to stay up and that will require a different set of gears.F/A ratio's are good at 12.5~13 :1, should make bike a lot easier to ride.
Are you doing a 250 or 350 LC (Elsie ) I don't know how much information is available for them today but someone has probably posted tuning info from the 1980's? I found the reed blocks need a lot of work, you can make them flow much more than a Boysen set and still have 'street' like throttle response. Bigger carbs work, 34mm makes bike VERY peaky but 30 or 32 gives wider spread of power. (I think they have 26mm?) Oh, don't bother to work any of the intake covered by the reed blocks. Cut out piston bridge and match cutout to port width. Decent set of expansion chambers make a world of difference, even TZ350 ones will bolt on but they are bit noisy