Each of the 4 pistons are cleaner along the edge on the intake side of the motor. What's up with this?

 

Mine were like that too. It's from the air/fuel mixture flowing right by it when the valves first open for the intake stroke. Air flows out of the valve and follows the contour of the combustion chamber and it sweeps down onto the piston. It's also clean around the edges of the reliefs in the pistons for the same reason.

Furthermore, you will note the clean spot goes away as you move toward the reliefs in the pistons. That is because your intake valves are extemely shrouded and not much air comes out of that area of the valve. Therefore not a lot of air sweeps down the combustion chamber in these areas and hits the pistons. As you move away from the reliefs, you see another clean spot develop. That's where the valve starts moving some air again.

 

ok. had a feeling it was from the fuel mixture flowing in there. thanks for the info. so i guess i should have my valves unshrowded for the sake of having cleaner pistons

 

And of course you'll want to add nitrous to keep the rest of the pistons clean too.

Yea I would suggest unshrouding the pockets around the valves a minimum radius of X% the diameter of the face of the valve. For the exhaust a minimum of 25%, for the intakes 30%. That's how mine are. Those numbers are ideal for a NA motor. FI the numbers are more like 30,35, or 35,40. Less compression and more flow is a win/win for FI setups. Mine will go to 30/35 next time the head is off.

 

similar:

my new block:



my old pistons looked great after steam cleaning, that's with like 10K on them:

 

Are you sure that’s not from detonation? Its hard to tell from the picture but the edge kind of looks rough.

 

Pretty sure most miata pistons look like that, and pat is on the money.

 

hey Scott, whats that thing to the left of your oil filter. I see the pressure sender but its got a huge base.

 

that's the 1.6L pressure sender.

 

my pistons looked the same way, except for gobs of gooey carbon buildup on the exhaust side. a bit rich with the SC+AFPR garbage

 

also came on the 94.

on the left is the 90-94 REAL oil pressure sender
on the right is the dummy sender that only tells you if you have some or none