namasgt

I took the pistons out from the short block Im using to replace the broken engine for my 88 S4. a few of the pistons have the coating gone. this one was the worst one (pics are from both sides):

namasgt

the engine should have around 130 to 160 ,000 miles, the rod bearings looked good but Im changing them any way.

Rob Edwards

FWIW, here are a set of '91 GT pistons that I pulled and Chemdipped them. After these pics I had Greg ultrasonically clean them to get the rest of the carbon off them. Though they look a little scary to my untrained eye, he said the coating on these skirts is just fine:

I am utterly unqualified to comment on this, other than what I've been told.

namasgt

Thanks for the pictures Rob, they do look like mine. I was also told to use chem tool (berry man). I also did read on an old post that someone sujested paint tinner.

I do have the old engine with the cracked cylinder wall on #8, if these look bad after cleaning I will take the other engine apart.

Thanks

James Bailey

All used 928 pistons look much like that. The tin/iron coating is said to be a break in sacrificial coating only and is designed to wear in quickly.

namasgt

Thanks for the info

I just finished cleaning the pistons with berry man and have noticed some discoloration spots on the piston tops like bluish purple 2 or 3 of them have this and for one of them the piston pin has blue spots on one of the faces (I did not seperate the rods from the pistons).

namasgt

2 of the pistons have small dimples on the top any idea if this is bad.

namasgt

on pic #1 the pin has turned blue

#2 & 3 are the same piston , the surface has dimples (there is another piston in the same state but with less damage)

#4 blue spot on the corner (is this normal or did the chamber got too hot using wrong spark plugs)

does any one know if any of these are bad.

Thanks

GregBBRD

The sides are fine. Those pits, on the top, are from the engine sitting with moisture getting into the engine (exhaust valves open). The moisture turns the carbon acidic and attacks the surface. I'd guess that pin got that way when it was ground originally.

Clean, assemble.

namasgt

Thanks alot Greg.

namasgt

I made a gauge on the side of the rod, will it break if I use it /
I do have another engine sitting .

blown 87

I am going to defer to Greg Brown on that, but from what I see (you need a Macro lens) that is nothing a sheet of glass and a very little 2000 sandpaper will not fix.

namasgt

Hi Greg
I can actually feel it with my nail.
it happened by the drill hitting it while I was trying to remove a rust spot on the rod using a nylon brush, but then the drill slipt from my hand and the chuck hit it.

docmirror

I'll give you a rod if you're concerned. I don't think that damage would propagate a crack, but stranger things have happened. PM me if you need it.

blown 87

Smooth it out, it will be fine, or at least I would not worry to much about it.