348 pistons

[Gregg St martin]

I have been told a truck 348 has lower compression than a car motor. Can I just change the pistons to increase the compression?

[Bungy]

Sure, That's the main difference between the truck and passenger engines. Just swap in some higher comp pistons and you're good to go. :)

[CDNpontiac409guy]

Greg... GM built the truck 348 two ways. The "good" one uses the same block as a car, just the pistons are different. With this one, absolutely... just change to proper 10-10.5:1 car pistons, and you're 90% there !

On the other hand, if you are the unfortunate soul to be in possession of a genuine "truck block", you are in trouble. These can be easily identified by the two MASSIVE machined cutouts at the tops of the cylinder ( combustion chamber ). If you install the 12:1 piston that would be required in order to get 10:1, you end up with a homely little pocket on the side of the dome of the piston, which collects oil and carbon deposits, ultimately creating a hot spot which can cause detonation against the side of the piston.
Sorry, if it's real truck block.... take it ALL apart. Pull the main caps off and keep them for future needs. Then paint the block, and use it for an ornament :p

[Tic's60]

Is there a pic of the different pistons somewhere? I have a set of 348 ones that are domed with a slight bevel on each side. The block is cut on the bottom of the cylinder rather than the top.

Here's mine

http://www.tictoc.us/piston001.bmp

http://www.tictoc.us/piston002.bmp

http://www.tictoc.us/piston003.bmp

thanks

[CDNpontiac409guy]

Tic, those are the regular 250/280 HP cast 9.5:1 car pistons. They were also used in the truck 348 block that has the MASSIVE cutouts that I was referring to.

[Mr Goodwrench]

dont forget about the .060 deep combustion chamber in the 333 truck heads guys, dont know how much that lowers compression but it has to be some.

[348NUT]

Here's the low compression 348 truck piston. I also have a 348 truck block with the two large cut outs. The car blocks have one smaller one. Easy to tell the difference. NUT

[Gregg St martin]

If I am only concerned with driving the car and not racing it there should not be a problem with the lower compression

[threeimpalas]

The "true" truck block with its lower compression for a given piston would be ideal for a supercharged or turbocharged application.

[348NUT]

My 348 has aprox. 120# compression. It runs good, has plenty of power for everyday driving, and gets 18-20mpg, so it's not the end of the world to have lower compression, but on the other hand, if I was putting the money into a rebuild, I'd sure try to boost the compression at least to 9 to 1, preferably to 10 to 1. If I was going to use a true truck block because that's what I had to work with, I'd use the stock 9.5 to 1 pistons and enjoy driving it! NUT