This could be...

[lonerider41life]

Hello.
I just want to do a maintenance to my bike, because I was experienced in leaking oil and coolant at the engine cylinder head cover, camshaft oil seal, coolant pump, cylinder head conected metal hose and thermostat housing. So I bought some like this: https://www.ebay.com/itm/155221997714?_ ... BM3PGgpu1k, took the engine out of the frame and started working. I was very surprised, when I removed the left cylinder head (to replace the gasket, the valve seals, the camshaft seal), to see a spot of corrosion in cylinder no. 4. Where can it come from? I had no problems with oil consumption or low or unbalanced compression compared to the other cylinders. I'm actually surprised by what I found and I can't find an explanation. There are no visible signs that someone has ever dismantled the engine... I haven't disassembled the right side yet. I hope the surprises don't continue... Any ideas...

[Rambozo]

More than likely this is from the bike sitting for an extended period. Condensation in the cylinder collects at the lowest point.
Have you tried to clean it up with solvent? It doesn't look like rust in the pictures, more like dried oil film or other crud.

[lonerider41life]

It is rust/corosion. It is deeper than cylinder mirror/surface. I will ask someone for a micrometer and try to measure the diference. How long time needed to happen that? Could be one winter time or longer (stored bike outside)?

[Tparr]

My .02 cents worth. Do your best to sand/hone/wire brush the offending rust and move on. Ride and enjoy. These engines are robust and even with this small flaw this bike will outlast you and your sons.

[Rambozo]

I agree, if it is not affecting compression or cylinder leakage, clean it up as best you can and ride on. I would expect it would be years of storage, but it really depends on the storage conditions. Temperature swings and humidity as well as pressure changes control how much moisture can migrate. Another candidate is to check the air filter housing for possible mouse damage. There is a whole air filter hall of shame page that has tons of examples of mice nesting in air filters. Mouse urine is super corrosive, and I could see that draining down into a cylinder with an open valve and causing something like that.