I'm preparing to do the top end on my 99 400EX, piston, rings, needle bearing. My question.... I plan on keeping the bore stock, (unless there is cylinder scars at which time I'll go to a 416), what is the stock piston compression ratio. I keep seeing different compression ratios, 10:1, 11:1, and 12:1.

The quad is still on the standard bore. It runs great on regular gas, has an HMF exhaust with proper jetting, air box mod's. Its just that now its 10 years old, and it does burn some oil. So I want to freshen her up a bit with a new piston, rings, and needle bearings etc. What piston compression ratio should I get. I'm thinking of going with 11:1.

Thanks

 

Stock comp is 9.1:1. To get the most out of a bore do 11:1 or 12:1. 12:1 can require gas fuel or a 50/50 mix. 11:1 is safe with 93 octane.

 

personally i would go with a 10:1 compressoin piston, if you are just freshining up the engine. still run regular gas(88-89 octane) no problem, and you will have fewer problems running it on a stock cam ect.

NOTE: if it is burning a little oil you should replace the valve guide seals! they tend to leak in the 400ex's. cheap insurance, and you are already in the head

 

The more I think about it, I feel you are right, I'm going to go with a 10:1 compressoin piston. With this I don't need to worry about the octane. I was also thinking about the valve seals. What tools are needed to replace the seals. Is this something I can do in my garage with basis tools, or do I need to take it to a shop.

Thanks