I turn one plug by hand and then turn back socket wrench but forget if i did 1/2 or 3/4 ( i wanted to do 3/4)... so I unscrewed it and tried again. But I think now the washer is flatten so when I screw by hand, it's tight already?

 

No just tighten it. The washer will flatten, but not enough for you to not reuse it. Just do it hand tight then 1/4 turn with the socket.

 

With washer instead of conical type sparkplugs, its better to use a torque wrench if you can, but go on the low side of spec. Aluminum heads are not forgiving to ham-fisted mechanics.

 

Also, it would be a good idea to use a little anti-seize lubricant on the threads so that it will make it easier to get them out later on.

 

Amen on this, if you leave plugs in there for a long time with no anti-seize on there the threads of the plugs (steel) can gall against the head (aluminum) and since steel is stronger than aluminum guess what gets ripped out? Yeah heads are expensive to replace. A little prevention goes a long way.

 

Anti-seize and torque to 13ft/lb.