The area where the tip protrudes must be flat. If it looks dished out around the tip than you would waste your time cleaning them. Cost to rebuild varies from $35-$45 ea. They replace the nozzle and tip and reshim the injector so that it pops off at the right PSI. And make sure you use a good additive because now days the low sulfur diesel has zero lubricity in it killing our fuel systems fast. For what they charge to clean and put new tips and all it isn't worth messing with yourself. Any diesel shop that works on pumps injectors can rebid them and get the parts. I HIGHLY RECOMMEND asking for ZEXEL tips as the DENSO tips are now china and they suck!