What pistons are in my 390

[74Torino]

I’m replacing a 390 short block and I’m not familiar with these pistons I found in my replacement engine. They look to me like lower compression pistons. The engine came from a ‘70 mercury monterey. according to the original specs, a 390 from one did have a regular 9.5 compression ratio, and that was with regular c8ae-h heads. I attached photos here, the closer up one is of the pistons I’m curious about, the other photo shows the pistons I’m used to seeing in 9.5 ratio 390s. Do you guys think these will have the same ratio?

[6TNCRZY]

That looks like 9.5 pistons, as I remember from 40 years ago the 10.5 pistons were flat top with 4 valve reliefs.

[eschaider]

Those are cast pistons if that is important to you.


Ed

[olddog]

Top picture pistons look to be significantly deeper dished, compared to the bottom picture.

The diameter of the dish in the top picture looks to be a larger diameter dish, as well.

If my observations from the pictures is correct, the top piston is much lower compression. Maybe as low as 8.5:1, which the factory likely used in the late 70's.

If you have both pistons available, fill them with children's putty, and compare to prove it. If you could set them on a level surface the correct way is to fill them with water and then suck the water into syringe to measure the CC.

[74Torino]

Ok, glad to hear it. Thanks for the help

[74Torino]

Originally Posted by olddog

Top picture pistons look to be significantly deeper dished, compared to the bottom picture.

The diameter of the dish in the top picture looks to be a larger diameter dish, as well.

If my observations from the pictures is correct, the top piston is much lower compression. Maybe as low as 8.5:1, which the factory likely used in the late 70's.

If you have both pistons available, fill them with children's putty, and compare to prove it. If you could set them on a level surface the correct way is to fill them with water and then suck the water into syringe to measure the CC.

I would think they’d be low compression pistons if the engine was later 70s, but it’s a 1970. The other picture is a 1971 ltd 390

[74Torino]

I dont know what I’m talking about here, but could the piston be a bit taller to make up for a bigger dish with these?

[74Torino]

Another source says that all 1970 2v 390s are 9.5 ratios, and all 1971 2v 390s are 8.6 ratios. By that info, the first picture is a 9.5 ratio and the second is actually and 8.6 ratio. The source was erareplicas.com. Does that seem right? By how these look it seems the opposite

[olddog]

Unless you are certain that the engine was never touched and totally factory, looking up what the engine came out of is useless. I can put anything I want into any engine, so you never know what has been done to the engine.

[74Torino]

Originally Posted by olddog

Unless you are certain that the engine was never touched and totally factory, looking up what the engine came out of is useless. I can put anything I want into any engine, so you never know what has been done to the engine.

You’re right, but the engine definitely doesn’t look touched. Everything inside and the dirt buildup on the outside looks factory

[74Torino]

I’m looking at the original head gasket here, it’s a thin steel gasket that’s .02 thick. Obviously with a used 50 year old gasket the thickness isn’t accurate, but compared to my .05 thick fel-pro gasket, it still seems like it was much thinner originally. I think I’ll look for head gaskets that are thinner and closer match this original shim steel one

[olddog]

Originally Posted by 74Torino

You’re right, but the engine definitely doesn’t look touched. Everything inside and the dirt buildup on the outside looks factory

50 years is a long time. If it was rebuilt in 1980, there have been 40 years of grime accumulated. Are you really sure you can look at an engine and say it was untouched? You have the pistons right in front of you, and you are doubting what you are seeing, because you are reading things that don't even matter, if you don't know the engines history.

No idea what your intent is, but if all you want is an engine that runs and you don't want to pull it apart, put it together and run it. Who cares what the compression is? It will run.

If you care about horse power, then keep the block and pitch most everything else. Now you are talking serious money, when you go down this road. You will be rewarded with serious power at the end. Your choice.

[olddog]

Originally Posted by 74Torino

The engine came from a ‘70 mercury monterey.

What makes you believe that engine was the original engine that came in that car? It was very common to swap engines out back in those days.

While you are at it, measure the bore and stroke. This engine may be bored over, or it may not even be a 390.