how often do i need to replace the timming chain?

i have an 01 wr426f and i bought a new timming chain for it but havent installed it yet. im planning on going riddind this weekend and im scared i could possibly blow up my engine if i dont replace it. i bought an 02 yz426f in the summer and the timming chain skipped and it broke a valve and ruined the head.:bonk: i heard every 40 or 50 hrs oh ride time. the 02 yz426f i had was bought new in 05 so it was only ridden for 4 years. how often should i replace it? how long does it take to replace?

you can probably go 100 hours safely, I do mine every time I do rings or piston/rings which is around 80 hours. 40 hours is relatively low IMO.

yamaha says on a 09 yz450 20 hours

klx250/300 manual gives a 20 link standard length of 127.0-127.4 mm and a replacement spec of 128.9 mm.

i measure ten links on top since that can be done without removing anything but the cover.

the chain "looks" the same on my wr but i haven't checked yet or found a spec.

i have read else where that high revs wear the timing chain faster so i'm not sure mileage or hours are a good measure to go by.

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