Is this scratch on the block surface tolerable?
 

I really hope this doesn't require taking the block to machinist...

http://i202.photobucket.com/albums/a...o/P1000776.jpg
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http://i202.photobucket.com/albums/a...o/P1000774.jpg
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I can feel the scratches with my fingernails when i slide across them.

I'm going to say YES, looks insignificant to me BUT I am absolutely just guessing, I do not know. :D

Finish cleaning the block and then look for the scratch.

I would take a block sander with some 320 wet dry sandpaper lubricated with some atf or wd40 and sand it till the raised areas are knocked down, then not worry anymore.

Tom W

OOO, never use sand paper wet or oiled on an assembled block.

I see rust on the piston and a black mark on the cyl wall (corrosion?)

I'd be more concerned with that.

Pistons are aluminum so no rust. Let the owner make the calls here.

Looks like rust to me. The piston is aluminum but it must have come from somewhere.

Maybe that's why the head is off.

How do you know it's rust?

Thank you all for the replies.

The cylinder wall is not rusted. it's just dirty from oil/brake cleaner mixture.
I'm just going to clean the block surface more using brake cleaner and solvents.
I don't want to use sand paper cause that might leave more scratches..

Next time I do any block surface cleaning, i'll just be more careful. I didn't think the razor blade would scratch through the steel block.

You're getting anal here. Use 180 grit sandpaper, dry. It will load up like crazy, which is what you want as the swarf will be trapped in the paper. Follow the curve of the cylinder when you clean so no streaks run on a radius. After the block is cleaned of old gasket and corrosion go over it with a rag and solvent and blow out the cylinders with compressed air and clean out the bolt holes thoroughly.

I would do it manually and use some Scotch Brite sponge for removing all stuff until block surface is clean and as said by Chas-H following the curve of the cylinder and all the other holes (3 fluids are there: combustion; coolant and oil as far as I know).
Good luck