I removed the head and cylinder to measure the piston clearance because there was a lot of top end noise. Things were worse than I expected, both the piston and cylinder were scored. The bike was probably run without oil at some point. Possibly, the injector tank ran empty or the injector hose was left off the carburetor.

Fortunately, 0.5mm oversize piston cleared up the cylinder. TY175 uses the same piston as a 1974-76 DT175.  Pistons are no longer available from Yamaha, but they are available from several piston manufacturers.  I used a piston from Wossner. Wiseco also still makes pistons that fit.

While the cylinder was being rebored, I decided to take the bike and engine the rest of the way apart and do the restoration I’d been planning for the winter.