Piston Ring Measurement

[dduclaux]

I have a 3.0L SC previously rebuilt by another owner. I'm in the process of a rebuild on the same engine and I'm trying to figure out whether my pistons are 78-79 vintage or 80-83 vintage for the purpose of purchasing new rings. The Pelican Parts site states that 78-79 rings are 1.5x1.75x4.0mm, whereas the 80-83 rings are 1.5x1.75x3.5mm.

What do those specs represent, particularly the 4.0mm or 3.5mm measurement? Is this a measurement I can do to the old rings or the piston itself to determine the vintage?

Possibly, there is a way I can cross check the part number on the piston to determine its vintage?

Thanks in advance for any insights,
Duane
'79SC Targa

[1980SC]

I'm going through the same thing right now. I have a previously rebuilt '79 which had it's pistons replaced with used ones. I don't know what year they are?

The 3.5 vs 4.0 is the width of the bottom oil control ring. Supposedly the 4.0mm was used on '78- july'80, and then the 3.5mm after july'80.

I measured my oil control rings, and they are 4mm, so you can do that to figure out which replacements to order.

I'm trying to figure out if the earlier pistons used the "++" and "--" weight markings, or if that was just '80-'89 ?

-Rob
1980sc

[Wayne 962]

These are the thickness of the three rings.

As for the "++" and "--" - what is the context for trying to figure this out? These markings are only really used for replacement of a single piston when the engine is being half-torn apart...

-Wayne

[1980SC]

My engine has 5 pistons marked "++" and one piston marked "+".

Since they have the 4mm oil control ring, then I figured they are probably '78-'79 pistons. Your rebuild book has the weights listed for the '80+, but not the '78-'79.

I'd just like to know if all SC pistons use these "++" style weight marks, or does it mean my pistons are '80-'83.

I'd really like to know if they are 8.5:1 or 9.3:1.

-Rob
1980sc

[dduclaux]

Gentlemen, thank you for your insight.

One clarification please:
References to "width" or "thickness" would require a measurement: (a) from the inside edge of the ring to the outside edge of the ring; or (b) the outside edge of the ring from the top edge to the bottom edge?

-- Duane

[1980SC]

I measured mine outside edge to outside edge.

I'm interested in what is marked on your pistons.

My pistons say either :

C021 93L7 ++

or

C011 93L7 +

-Rob
1980sc