Purpose of hole is Piston

[Salmonseeker]

I was checking my spark plugs on a 2004 Johnson 50HP 2 stroke and decided to look into the cylinder through the plug hole. The top of the piston was nice and black and I noticed what looked like a little bump. I used a wooden skewer used for cooking to poke at the bump and found that it was actually a small hole.

I checked the other piston and it had the same thing. The hole is about 1/8" in diameter and about 1/8" deep. I was a bit concerned and did a search on replacement piston and noticed some come like this. "http://www.masonmarine.net/product_info.php?cPath=24_39_48_117&products_id=1404"

Is there a reason for this? Is it just to help in the machining process of the piston, no center to remove on a lathe?
Also what is the coating on the top of the piston in the picture? Mine is black but I think it is just carbon.

Thanks

 

[F_R]

Re: Purpose of hole is Piston

The hole is a lathe center hole. I'm not familiar with the pistons in the picture, but I would guess some sort of protective coating against preignition damage (??)

 

[Salmonseeker]

Re: Purpose of hole is Piston

Makes sense, Thanks