You have three measurements to consider - the pitch of the chain, its gauge and the number of links.
The pitch is the distance between any three rivets divided by two - could be in inches, 1/4" , 3/8" or decimal .325", .404".
The gauge is the width of the drive link - the bit that runs in the slot in the bar.
The bar should be stamped with this information, and on most chains there's also a reference stamped on the chain on each link or on the tie straps.
The number of links depends on the length of the bar and the type of machine - the distance between the start of the bar and the drive sprocket is important too.
Each manufacturer has a different sizing system for their chains - for example the Husqvarna H42 is the same as the Oregon 73LP - 3/8" pitch, .058" / 1.5mm gauge.
It's important to get the right chain for your machine and the bar - you may also consider whether you want a chisel or semi-chisel cutter profile.
You should be able to find all the information for your machine on-line on the manufacturer's website - they all have manuals on-line.
If you are really stuck after this another alternative is to take it to a good service centre. They'll be able to do all the measurements and even make up a chain of the right length if need be.
Wes, March 2009
