The reason we use pin gauges is because the jets as marked are pretty far off. A jet marked 158 can be larger then one marked 160 for example. So say you are running a 160 jet, you read your plug/exhaust and decide to go a little leaner. So you throw in a 158 and it is actually larger. The pin gauges eliminate that. I size all my jets and pay no attention to what is marked on them.