Tjhopper24

Will 243 heads bolt directly to my 5.3 that had the crappy .706 castech heads on it?

redtan

iTrader: (5)

Yes 100%. Later 5.3s came with 243s (or 799s) stock from the factory.

Tjhopper24

I've read a few different answers some said yes others said they had to be milled to get compression back

MonmouthCtyLS7

iTrader: (6)

Yes they will.....

MuhThugga

The 243s will physically bolt up, but you will want to mill/go with a thinner head gasket to keep compression up.

bobfig

Yes they will bolt on. Just did that to my truck that had a cracked 706 head. They need to be milled some as the chambers are 64-65 cc vs the 706 heads with 61cc. Had mine milled 0.030" to get the chambers to 59-60cc and used the stock ls1 mls head gasket that compresses to 0.051 to give me about 9.9:1 compression.

Also when doing this you need to check your pushrod leangth as you are changing the valvetrain geometry around.