How to measure piston?

[sdblaster]

How do you measure what size piston you need,I can not read the old one it has melted alum on top.Can it be done from home?

 

[Wildcard2]

You will need a caliper or micrometer. Measure it accross the diameter of the bore. If you melted a piston there is a good possibility that your bore has been damaged, I'd recommend you have a machine shop look it over and tell you if it is good or needs bored and what size piston to get.