This spring drops in on the older steel guides and fits well-(set). It also fits newer guns synthetic guide but not as tight because the steel guide is 15 thou bigger. However it fits both guides well for use by the average consumer wanting a drop in spring.
avg from 36-37 coils
.550-.555 ID
Wire varies .125-.127"
Works in all Marksman 60-61 too
Can be spaced up for power- .400" +