what would be some of the pros/cons of 18:1 pistons oversized .030?

If im not wrong and i might be i think the blocks in the 6.5 have some pretty thin cylinder walls and webbing. Someone else will come and give more imply then me.

The big advantage would be the ability to run higher boost from lower compression. If you are going all out go for it. However if the stock pistons are ok I would keep them and get HD thicker head gaskets to lower the compression.

Typically the point of boring a block for oversized pistons is to fix a problem in the bore ie... scored bore, out of round, too worn etc.

I am pretty happy with 12 psi turbomaster and GLE chip. JMHO.

Good Luck

well i want to custom build a 6.5 for hauling and drop it in a 73-87 chevy crew cab dually, and i want 18.1 compression pistons for more power but i was thinking that the ones that are .030 oversized would allow for slightly more intake into the combustion chambers.

A drop in compression wont make more power, it will make less if anything. You will need more boost to compensate for the lower compression. If its worth it to spend the money to have 18:1 comp just so you can see the needle move higher then go ahead but i'd personally leave it alone. Plus the lower comp will effect how well the 6.5 starts in the cold weather.

you measure the bore, and then overbore to minimum of what it needs. boring out won't make a heck of a difference to how much more you can get in the intake. going to 18:1 will allow you to crank the turbo up and have less stress on your heads and gaskets. trying to start in the cold, becomes another issue though

I let my machine shop make the call on overboring my 929 block during my recent rebuild. For whatever reason they bored it out .040 oversize. I didn't realize it until I got it back. Nothing I could do about it except put it back together. Now have 2000 miles on it but not enough time or heavy use on it to tell if it will hold up or crack through or whatever.

The Army tech manual says .030 is max.

I just had my 141 block bored .030 over to fix some pitting issues, the machine shop said definitely not more than that. In fact, there still is a slight pit, but he said he really didn't want to go more... so I still have a tiny tiny tiny pit in cylinder 7. We'll see how she holds up, I'm still in the process of reinstalling.

Oh, and I hope he didn't install the 18:1 pistons...he said he installed regular Mahle pistons... so???