Engine rebuild

Hi, Im looking at my pistons and after cleaning them up i see they the crown is not very smooth, is there value in polishing the pistons to be very very smooth?..i.e glass smooth? especially what looks like the cast point in the centre...

 

Just clean the carbon off.

 

^^^Ditto here. Don't do anything else but clean off the carbon.

 

any particular reason?

 

No idea, myself, so I Googled this.

Quickly found one thread where it was claimed that aircraft engine manufacturers found a higher failure (burn through) rate in polished pistons than in unpolished ones. The theory there was that polished pistons absort heat more readily than rough ones.

No idea if this is true, but it may be, as I can't remember seeing even high--end aftermarket pistons with polished crowns.

 

When something is polished, you loose surface area and heat exchange takes place through the surface. Additionally, rough surfaces can trap gasses allowing for them to in effect "insulate" the metal leading to less exchange.

I don't know if this works on a piston, but it sure worked on the skin of a SR-71 blackbird. From afar this plane looks smooth and sleek, but get close up and you can see intentional ripples to take the effects into account.

 

These are all valid reasons to decarbonize only. Another reason is that care must be taken around the ring groove area. Too much "cleaning" in here and the rings no longer seal properly or if material is removed from the piston crown, foreign material has more opportunity to collect between the piston and cylinder wall and potentially cause damage

 

Thanks Guys...all makes sense.