(Decking) Whats the limit

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mopar4x4stroker

mopar4x4stroker

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I'm building a 408 stroker using 68cc RHS heads and .030 KB/Icon pistons. I need to keep it pump crap happy(91). Looking at the 27cc IC983's (9.1:1), there designed for the mag blocks with a 9.585 deck height vs the 9.599 LA. Which means I'd have to shave .030 to achieve zero deck for a desired quench of .039. Is this too much? How will this effect intake and gasket alignment? Should it be done to prevent distortion, two .015 or three .010 passes ?

I could go with IC745's (+20.5cc) and shave .010, but 9.6:1 will be high. This will be for a street driven 4x4 5,700lbs with o/d, 4.10 gears and 33" tires and 2200-2400 stall. I live in ND @ 900ft, even though its flat, it can often get hot and windy. I dont tow anything big 2500-4000lbs, but I still dont want to gamble since its my first motor.

Any thoughts or opinions would be great :)
 
The cam choice is most important for you really. If it was me, I'd go with the IC745 dished because it's a truck, and you'll want a cam that makes high cylinder pressure. Using those, zero decking them, and running the .039 gasket gives just shy of 10:1, and you'll want to run a cam larger than 245° @ .050 range to keep the dynamic ratio around 8.6:1. That should mean pump 90 or better will be fine. Tight quench and a big enough cam are the most important factors to me.
Now, if you have and want to keep power brakes, you may want to either run a vacuum can (Some RC had this anyway) or enlarge the chambers of the RHS heads slightly by hand which is very easy. The slightly larger chambers will allow for a smaller cam, that will run the brakes better.
As far as decking, .030 is nothing. Most normal block are at least .020 "tall" anyway because the factory cut them "flat and close"... Not to a specific blueprint deck height. The 9.59 is a blueprint spec. A CNC type mill can take it in a few passes. A horizontal miller that uses a table, fixture, and rotating multi-bit cutting wheel will need to make more passes and IMO no shop should be using any more. As far as intake fitting. I think you will end up trimming. How far, I'd say wait and see how it fits with the gaskets you want to use. Then have it cut to fit.
 
Wouldn't the 745's put me at 9.6:1? The rhs heads are 67-68cc. Either way, I've decided to go with hypers. After talking with KB this morning, they turned me on to the kb416's that are a 25cc dish that should put me at 9.3:1(perfect) and require less milling. He said being that I live in ND and plan to drive it in the winter, that forges requiring twice the cylinder wall clearance do to expansion and can actually shrink in the cold resulting in rocking and hitting the head with tight quench. He said hes even seen forges in really extreme cold conditions shrink so much they gall the rist pin. He said hes seen hypers live up to 600hp on a healthy tune and good tolerances. Not to mention there $100 cheaper and use 5/64th's rings. Seems like a good piston for a 475-500hp street motor that will never see laughing gas or boost or 6500 rpm. You just need to fallow directions with ring gap. since the sylicon transfers the heat to the rings more than a std cast or forge piston. This is getting off topic though, I just wanted to know how far a guy can mill the heads or block b4 you have to stat shaving the intake. Idk if there a general guide line.

Thanks moper
 
I used the KB 356 hypers, milled my j heads .040 to 60cc no problem.

10.1 comp

to directly answer your question, .030 is not even close to being too much, go for it.
 
I just decked my stock bore stock stroke 360 .065, shouldn't have to do anything to the intake.

Edit:
I think mine was a little on the tall side, it is a '71 360 and by using 65cc J heads I brought the comp to 9.4:1
 
Awesome, thanks guys. Wasnt sure of the limit b4 you have to touch the intake. I know sb's are notoriously inconsistent with factory finished deck hights. Can someone tell me if they were more on the tall or short side.

Thanks
 
Tall..........

Not sure why either, it feels like a 20HP difference in the ones that are .010 down the hole compared to the ones that are .080..... MPG too.

Guess it's just saving moolah on machining at the factory.
 
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