couple thousandths maybe? I personally wouldnt bother because I like the WP seals cause they can be cleaned out and re used. the only reason i was trying the KYB seals is I was at MX-Tech and they ran out of WP seals and i was leaving the next day and wanted to take my stuff with me. another thing to consider if you were to machine the forks is the bushing, the early style were a bit different size, to tell which you have just look at the color. early ones were orange.