Putting together a 410 and have the 381NP pistons. Heads are stock C7. I assumed that these pistons would give my the stock compression ratio of around 10.00-10.5:1. They look and measure (except for bore) the same as the 428 pistons I just removed from a 66 428, same dish with valve reliefs. I noticed today on e-bay they have a set and advertise the CP as 8.2:1, that's a long way from 10.5:1, anyone know what the CP would be with this piston set? Thanks

 

We could give you a accurate compression ratio with the following information...

Piston pin height

Deck Height

Piston volume cc

Cylinder head CC

Bore size

Rod length

Head gasket bore size

Head gasket compressed thickness

all must be accurate measurements...then we can calculate the Compression ratio ..............

 

The piston pin height will determine if those pistons give you the 10 to 1 or the 8 to 1. Measure the pin height on the 428 pistons and then measure the pin height on you 410 pistons. If they are the same then 10 to 1. If the pin height is shorter then less than 10 to 1

 

Thanks, checked them today, looks like they are both in the neighborhood of 1.67-1.69". I assume pin height is the measurement from pin center to top of piston also referred as compression height?

 

1.78 should get you at Zero deck Without decking the block a large amount

Most 390 engines have pistons .050-.100 in the hole = No compression

 

In the 60's 390 pistons had the 1.78 or so pin height and in the 70's it went down to the 1.68 height if I'm not mistaken. This reduced the comp from 9.5 or 10:1 to 8:1. And it was accomplished by using old 410 pistons then recast as 390 truck pistons. If you try to order pistons they almost always will try to sell you the low comp version. Cant really remember what pistons got the 410 up to quench, the brain is failing at 4:30am. I'm sure a search of the forum will get you all the info you need as this topic has been covered numerous times.

 

Since the pin height measures the same as the 428 pistons from the 66 block is it safe to assume then that the CP would be 10.5:1? Book I have shows the compression height's of the 66-67 410 and the 428 to be the same, 1.687, also advertises the CP to be 10.5:1.

 

The book may show that those were the original specs, but the piston manufacturers have rolled over to poor gas and the EPA and changed the specs on the pistons they now produce, even though they still claim them as "direct replacements". Also compression is also effected by combustion chamber size and head gasket thickness.