Pitted piston top - what causes this? Detonation?

[rbastedo]

I've been soaking the pistons in gas for a few weeks and got around to cleaning them up this morning.

#1 - #3 look very good, like new!

 

#4 looks really bad, it was also the most crusted with carbon.

 

Now it's decision time, buy a set of new pistons or just the one to replace this one?

 

Also what do the numbers mean? Some have a "2" like this and the others have a "3".

This was a spare A15 motor that came with a type 60 5 speed I bought from a guy, I'm rebuilding this to replace the motor in my green Midget.

 

[datsunrides]

That is foreign object damage.

[rbastedo]

Have to confess I didn't even think of that!

Wonder what got in there, wasn't anything in there when I pulled it apart.

[datzenmike]

That is foreign object damage.

 

FOD for sure.

 

It passed out the exhaust valve long ago. No need to replace the piston unless the bore is being over sized and new over size pistons are in the future. Look inside the air filter housing. There are two (at least) temp sensors for controlling the warmed exhaust air and the over heat air bleed to control an over rich idle condition. Maybe a screw fell out?

[DanielC]

There are very slight tolerance differences in bore diameters, even in bores of the "same" diameter  The factory measures the bores, and assigns a number, based on the slight differences. 

When the block gets pistons for the first time, they choose a piston, from several different slightly different sizes to fit that bore.

The "2" pistons are probably slightly smaller than the "3" pistons, and I am talking a few ten thousands of an inch.

 

The block may have a "2" by the bore of the pistons, and a "3" by the bore of the "3" pistons.
 

[rbastedo]

Thanks Mike and Daniel,

I ordered a set of pistons already.

 

I have a digital caliper and my measurement of the cylinders shows very slight differences.

Interestingly there are numbers stamped in the face of the block.

The more you learn. B)