Crankshaft scratches
 

After taking my engine apart recently and i noticed scratches on my crankshaft, i know these are from dirt getting into the engine but i dont understand how it is getting in. I replace by air filters after use and even wrap them in vetwrap after every race, clean the airfilter housing with soap etc. I check the inner filter regularly and no dirt makes it passed the outer layer at all. The scratches are limited to the crankshaft and not my piston and sleeve from what i can see and the bearings are smooth. Could it be my fuel tank letting it in? Its a d817 clunk tank without an external filter.

Ive already cleaned the bearing, it wasnt gritty before but i went over everything anyway. I cant take a pic right now but when i had it was like a darker discoloring of the barrel under where the carb is, inline with the intake port on the crankshaft. I couldnt feel them under my finger nail but it looked like tiny scratches for sure

The bearings werent dirty i just cleaned my whole engine including bearings, sorry if i confused you. My airfilters are always maintained and zip tied as tight as i can. I used to use RB filter oil (extremely thick) now to motul which is also very thick and tacky

Yea it had been sitting for a while and has 15 litres through it, just thought it would be a good idea to check it before i start running it again

There is just 0.05mm up to 0.1mm space (depending brand) between the crankshaft and crank case. Probably you will not imagine it but the crankshaft does flex. With an unbalance by default with a 1 cilinder engine and maybe from the clutch the crankshaft can band that much to touch the crank case. But just keep in mind with foam filters it is possible that dirt will come through.
I know in the large scale with K&N air filters they have a dust sack to put over the filter, maybe something to try.

Nothing significant can get through a properly done foam filter.
OP, perhaps just run the engine and enjoy it. It's just a toy, nothing to stress over. If the engine is running fine, then have at it.

99% of off-road engines you strip down will have some light scuffing or at least dullness directly under the carb. Unless your seeing some deeper scratching it's nothing to really worry about.
The ingestion of microscopic dust particles is very hard to complely eliminate.
You also have some introduction of dust through the front bearing which gets transported through the vaccum oil return gallery (behind the front bearing) and exits directly into the area beneath the carb.
Even relatively new front bearing will start passing dust after a gallon or so. You can see the result of this as a black oily film behind the bearing when it's pulled. Some of this black film will find its way through the oil return gallery and onto the crank.
Running something like the Buku front bearing cap will stop a lot of this happening.
You can also block off the oil return gallery. All this will result in is a little pooling of oil behind the front bearing which may weep a little after use.
Some believe in doing this - others don't agree.
Something to think about if your worried about the crank getting scuffed.

Yes ive had a rip in my carb boots before, those soft losi ones and it was from using scissors to remove the zip tie. I noticed it when i did it at the time though and i got a new one which has no holes. I prepare my airfilters as best as i can so im thinking something got in side when i removed it for cleaning or something. Engine still runs so it could be worse

You still might want to post a picture.
If you think about it, if dirt was getting in during air filter maintenance, there might be a scratch here or there. If you have the typical cloudy, smeared fingerprint appearance on your crank, thats not from the occasional speck of dirt from swapping air filters. It's from a long continuous supply of super fine particles getting through the filter over time.
I still say nothing of significance gets through a properly done foam filter, those tiny particles are likely harmless, and you should just run the engine.