yz450f top end maintenance, when to rebuild ?

i have a well used yz450f,i'm wanting to freshen it up over the winter,i have a couple of question first. whats the best way to tell when the timing chain needs replaced and second, what is the specs. on piston and ring wear? the third question is does anyone ever install the weisco 13.5 to 1 piston in an totally stock yz450f. any help on these three questions would be greatly welcomed. thanks

If you don't find any kinking or binding links, the degree to which you can bend the chain sideways is an indication of the wear at the pins and bores. However, let's be realistic; the chain is a $15 part. Replace it.

It sounds like you don't have a manual to look your specs up in. You can download one here:

http://www.yamaha-motor-europe.com/community/service/manuals.jsp

The cylinders are aluminum plated with Nikasil. They should be replaced, or bored and replated, if there is any measurable wear, or any score marks that won't come out in 4 quick passes with a ball hone.

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