Hey I want to gear down my 04 400ex and I was looking at buying a 39 tooth rear sprocket instead of the stock 38. I was wondering if I would need to get a new chain for this mod. Thanks

 

go smaller in front... its alot easyer

 

No it is not necessary to change out the Chain
I have two 400ex Hondas
On both I have regeared Going from the origional 15/38
Went to the 14 on the front
Mine to pull the dunes easier with 22" Paddles
Wifes bike make easier for her to get over rocks trail riding
That worked so good I tried the 39 tooth on the rear
Even better
I have even gon as far as to get a 13 tooth for the front of my 400
Havent tried as yet at the dunes but will find out how it works in about 3 weeks
Still running origional length chain
Ray

 

So I should go with a 14 tooth front instead of 39 rear?

 

You won't really notice a difference just going up one tooth on rear. You'll only go from 2.53 to 2.6, but if you drop to a 14T on the engine you'll be at 2.78 ratio. Here's a handy gear calculator you can just enter your #s in and it will calculate automatically.
GearCalculator

 

Your biggest change will be the front sprocket
The rear sprocket is small change
The only problem with the rear sprocket is clearance
Not only ground but chain gaurd
Calculating the ratio is simple and a great tool
It would also be great if there were the same with tire diameter calculated in there

I know there is with Cars and gear ratio to tire size