used parts

What's acceptable will depend on the particular component and what you want it to do in service. What is acceptable for a low power N/A build is maybe not ideal for a maximum effort turbo drag engine. It's also a little harder to mark or damage a conrod than it is a piston skirt so you need to consider the material the parts are made from. I'm looking for any sign that the components have been dropped or had other parts knock them. Essentially any signs of this would be a problem. This is normally a problem when I'm delivered an engine and engine parts from a customer rather than an engine I've personally stripped.

On a crankshaft journal you're looking for any scratches that you can feel with your fingernail. If you can feel a scratch with your fingernail then the journal will need polishing and quite possibly grinding. With used pistons we're looking for scuffing on the skirts where they have been damaged by debris or potentially a partial seizure. You'll also want to confirm the skirt diameter is still within spec and make sure the rings are free in the ring lands. An inspection of the crown for damage from detonation is also important.

What kind of damage are you likely to see from land damage? piston skirts again the finger nail rule apply? Light marks are normal lines vertical i mean light marks signs thanks andre appreciate your feed back

If the piston has been undergoing detonation it's common for the ring lands to show a sandblasted appearance. In worse cases there may be clearer signs of melting. This also tends to nip up the ring grooves making the rings tight or hard to move freely on the pistons. As for the skirts, some minor scratching isn't uncommon. If there's any serious scratching or material transfer obvious then this could indicate the part should be replaced.