Modern Vespa

Swapped out my VMC SuperG for a BGM 177. Installation went smoothly; other than the squish being a bit on the loose side although I think that is related to my case. Thinnest base gasket got me to 1.15mm; spec is 0.8-1.2mm.

However I neglected to check the ring gap, as I have only messed with cheaper cast iron big bores on all the scooters I've messed with so far and never checked on any of those either. This was the first alu+nikasil bore I have used. My previous post was asking about a strange new tapping/plinking noise and out of curiosity I pulled the head again. Lots of scrapes from the rings overexpanding!!

Asked a friend and he asked if I gapped the rings, which I replied no with my tail tucked between my legs

I used a bar hone oiled with 10w30 automotive oil to scuff the bore up with just a couple of swipes as per his recommendation. I then centered the rings with my old piston and measured them with a feeler gauge. They were between 0.23mm and 0.245mm. Since I could not find a spec in the included manual, I gapped them to 0.31mm per a formula I found online based on bore size, although I took one a bit too far to 0.325mm. The wider one went on the top of the piston as it will experience more heat.

Nothing else I can do but put it all back together and hope for the best, right?

Owning your screw up, digging in and doing your best to fix the problem, and crossing your fingers hoping it works knowing you'll be shelling out more coin if it doesn't. Kudos, I am rooting for your smart fix!!! And lesson earned & learned.

The ring gap was fine. Those vertical marks in the plating is quite common for nikasil cylinders and is just visual. I wouldn't have honed the cylinder. I'd replace those rings you gapped too much and replace them with a new set that's closer to a .23mm. If you're hearing plinking noises, double check the squish band (as in use the correct method to check squish), double check the timing, then check for jetting.

.22-.25 for the end gap. Factory had it right.

Good save.

The ones above the exhaust port? I can feel them with my fingernail; they aren't just visual. I sent these pics to a dirtbike mechanic friend of mine and he said its telltale sign the ring gap was too tight, the rings got hot, expanded and scratched into the bore. He told me 0.31mm minimum for a 63mm bore; and I found a few different formulas online that all agreed around the 0.29mm-0.32mm range for a safe minimum, with a larger gap for race bikes.

What does the piston itself look like?

Engine is back together and i only got pics of the crown and top side. The small marks near the center are from the TDC tool I had in the sparkplug hole. I checked around the entire piston and saw no other marks or signs of wear.

I'm not reading all that, it looks like it passed the finger nail test, no harm no foul.
Hopefully cheers!

Recently I pulled a standard 200 cylinder with a GS piston which has rings with some kind of chrome coating. The wall had identical vertical streaks. Ring gap is fine but ring plating has melted and created wide streaks on the wall. I don't know what your rings are coated with but my guess is that the material cooked and started to come off. From the pictures I think the BGM piston has a coating as well.

I don't know what you ended up doing with your engine, but I would not re-use the old rings as the coating is cooked and I would strongly recommend honing the cylinder to remove the smeared material. Since the cylinder is either nicasil or ceramic, you can't remove any of the cylinder coating with any hand operated honing stones/brushes. Most of that smearing will clean off.