New LS1 Owners - Newbie Tech - 402 vs 408

Im planning a new 6.0 iron block build but i'm on a budget. Are there any big differences in the 2 besides the 408 is bored .030? So my question is wouldnt it be cheaper to just get the 402 rotating assembly instead of the 408 to save money on the extra machining?

If the block doesn't need to be bored I would go with the 402ci. This would also allow you to bore it .030 later in life if you ever need to.

There is virtually no power difference between the two. Like Conan said going 4.005 now gives you the chance to go farther later if you need to.

do what they said i wouldve if my block permitted but chose 408 bcuz needed to be bored .30 over to clean it up but i recommend a wiseco/eagle for motor/n20 motor as i bought a stroker from tsp :-) but for some reason any type of failure in a 402 allows u to go up to 408 no problem

There's pro's and con's to both. With the 402 you have futher options for re-freshening the motor. With the 408, the increase in bore size can increase air flow by decreasing valve shrouding. There are better options for cyl heads with a 4.030" bore. Personally, I would choose the 408 as you have the ability to create a more efficient combo.

Do the 402, as others have said you can bore it out to 408 later if need be for refreshening.

so no machining work needed for a 402........

Your still going to need Machining, your just not going to have it bored out.

It will get a minute amount of material honed from the cylinder bore, but that's it.

Like Conan said going 4.005 now gives you the chance to go farther later if you need to.


But iron blocks can go out further than 4.030 bore size right? So even if he runs into a problem with a 408, he can still bore it out further if need be. Might be less options for pistons, but it's not like that's it for the block.

Al block sure, once you get past 4.010 you're kinda done, but not so with iron blocks.

It all depends on what heads you go with and the size of the valves. If you're running stock sized 2.00 valves then no need to go bigger than say 4.010.

But if you're going with some LS3 heads and their 2.165 valves, then it might be better to bore it out to a 408 for added clearance to the valves.

Once you go larger than 4.030" piston choice becomes minimal until you hit the LS3 sizes 4.065"<. I am not saying they are not out there but just that you will have a more difficult time finding them. If you do not have a requirement for the 4.030" bore save the cyl wall and material for strength and go 4.005" 402ci. It is always nice to be able to say oops with a piston vs a piston and block combo. Neither size should effect price, It certainly will not with us at Thompson Motorsports.

thanks for all the info guys. just trying to save money here. What if I went with forged 364 internals instead of 408/402? Whats the power potential with a 364 vs 408? My goal is at least 500 RWHP and like i said, just want a nasty street car, not an all out drag car. I just bough the block today from TSP, next up is which assembly i want!

390 ftw!