2019 YZ 450FX rebuild

[MikeM1000]

What’s everyone’s opinions on rebuilding the engine. Bought bike with 100 hrs on it. Bike has 120hrs on it now. 90% is trail riding. I snowbiked it last year 6 rides I think. No issues besides clutch was cooked when I got it. Quick swap and was good all year. Has a sis engine jacket and c3 thermostat.

In my yrs of riding. Never tracked hrs. But I know my 426 had hundreds of hrs and never touched it. But people got me flinchy now.

 

[dooman92]

Mike, fwiw, I put 350 or more hrs on my 17 fx on the snow. New piston, timing chain, valve seals and valve springs at 200 hrs. None of that showed any sign of wear. The end gap of rings was just 1 thou over new stock. I sold it running strong with over 350 hrs, minimum 150hrs on that piston. Buddy had same bike, bought same time, rode together. He rode little harder. He lost crank at 100 hrs, big end heated seized from I believe lubrication failure. I was running mobil one 0-40 he was running rotela t-6 5-40. I have talked to another who lost a crank in the fx, he also was running t-6, and I believe a few others on here have had crank failures. My opinion, it is an extreme load lubrication failure and not as much related to hrs. So, if your bike never exceeded that threshold of crank load most likely good and go long time. Lugging loads big end rod at crank most.. At 200 hours my crank showed no wear. I think snow riding generally means cleaner intake air and less dirt generated wear than dirt riding so 200 hrs on dirt would most likely show more wear than my 200 hrs on snow.

 

[M5-]

I've been on YZ's for 5 or 6 years now and I rebuild my top ends every 100 hours. Timing chain, piston, rings quick hone. My 18 YZ has probably 300 hours on it now, my 11 maybe more. The 18 got a top end at 100 hours and a complete rebuild at about 250 hrs. New everything, crank, bearings, valve job, race cams, port job, stock piston etc etc. When I cracked it open, it all looked good except the gear set was screwed. I put in a low hour 2020 YZ trans in with an FX 5th gear.

At 120 hours IMO you are in no mans land if you didn't own the bike from new because you just don't know. The FX has a few different issues than the YZ. I've seen a few cracked pistons at just over 100 hours and the counter balance shaft bearings sometimes fail. Not sure why, its just what we have seen. Typically the FX is pretty bomb proof. If it were me I would rebuild it, minimum top end, timing chain, stock piston and all new bearings. I'm sure the crank is fine but Yami cranks are cheap so... and throw in a YZ 1-4 gear set, you will be way happier the FX trans sucks for a snow bike IMO.

We had a YZ go down last season in the middle of nowhere (GYTR high compression piston grenaded after about 20 hours, don't know why just bad luck) and it turned into a 5 day fiasco for everyone recovering the bike, so think about your buddies who will be helping you if it craps out somewhere. Rebuild it using Yami parts.

M5