How hard to install new top end?

I was wondering how difficult it is to install a new piston on a 2008 YZ450?

I am a fairly competent mechanic. I had a WR450, 06 YZ450 now an 08, I have adjusted the valves on all these bikes, pulled the carb a million times, replaced the timing chain ect.....

Is there a good step by step link somewhere? YES! I have the manual.....

Any advice? In the past I have paid a local guy to do it for a couple hundred bucks.

Interested to hear any replies as well. I have an '09 450 myself I'm getting ready to do the top end on as well as cam chain. I'm new to 4 strokes, but I had a yz250 that I put 14 pistons in over 12 years. Curious as to how different they may be. I've heard the most common mistake is to get the cams out of time.

Interested to hear any replies as well. I have an '09 450 myself I'm getting ready to do the top end on as well as cam chain. I'm new to 4 strokes, but I had a yz250 that I put 14 pistons in over 12 years. Curious as to how different they may be. I've heard the most common mistake is to get the cams out of time.

+1 Biggest mistake = cam timing.

Not hard to do & like any other engine if the above is remembered.

There not hard just double check your work as you go a long don't install the spark plug and turn the motor by hand with a socket and ratchet on the crank bolt over slowly to make sure nothing binds up and no valves hit etc. before you try to kick it over with the weight of your body.

I also with the plug still out kick the motor over until I get oil though one of the banjo fitting just to make sure it getting oil to the top end before I would start the bike.

Make sure you timing marks line up and count the pins on the timing chain between the cams write it down before you tear it apart.

If you done a 2 stoke it about the same but you don't have to worry about having the rings clocked for the anti rotation pins in the ring land of the piston, But make sure you stager the ring gaps on your oil rings if you use a 2 ring compression ring piston stager the ring gap on them also. (Staggering the ring gap is the Clocking of the gaps put on gap at 12 O Clock put the other at 3 o Clock and so on).

Make sure you Step torque the bolts use the torque pattern by the book.

Last thing I can tell you Clean, Clean, Clean, is what you want any time you have a motor apart and putting it back together.

I use assembly lube my self on the piston and cylinder and cams when I put things back together new oil filter and oil run 1 tank f gas through the new motor then pull the filter check the filter for any abnormal shavings put new filter and oil.

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