im building a 4.310 bore 4 inch stroke 6.385 rod blower motor b&m 420 mega blower with chiller my ? is will the 2 bolt block handle the abuse we are gonna use ARP studs where not gonna make that much power are we or should we spend the money and splay the caps
   PLEASE HELP NEVER PLAYED WITH BLOWERS

In my personal opinion the two bolt (if it is a good one) will survive if used in a sane manner, however, if she is full tilt and gets a good gulp of air (cavitates the pump) and comes in hard, deflection can and will occur that "could" cause a failure..

I am of the opinion that a quality two bolt block is almost as strong as a four bolt because there is more meat in the block webs (not drilled for a bolt....) the biggest issue is the mating surfaces of the caps and the condition of the main bores (round is key)....

It has always been my gut feeling after seeing conversions to splayed  billet caps and the machine work involved that it leaves the block in somewhat of a weakened state and really requires a main stud girdle that ties the caps to the pan rail to prevent the walk. 

Unless you're making over 1,000 HP at a stupid RPM, I believe you will be fine with just quality studs and a very precision align hone and cap mating surface...

I am sure others will disagree, but I have never seen actual cap walk or distortion on some pretty aggressive builds on the two bolt platform unless there was an issue with the original assembly. 

Gnats-ass is just close enough to get you into the ballpark....

Another thing to keep in mind, there really should be little to no thrust issues if installed in a pump boat properly...The crankshaft and rods is where my funds would go...

GT

Boy are you in for a LOT of education. Granted, it's a tiny blower, and little to me is my 6-71! But still pretty damn efficient at what it will do.

At the RPM these boats work at, do yourself a favour and go 4 bolt, Minimum, Forged crank at minimum, and some damn good rods. You not only have to deal with the increased cylinder pressures on the compression side, but the significantly increased violence after the ignition side within the cylinder. Then theres the (relatively) minor issue of increased strain on the crank to drive the blower at speed...........Cap walk will break cranks!

It's really basic physics how it all works, but just simply looking at any V-8 engine should tell you that since all the pressure, violence and action is from the top side of one, the bottom better be strong enough to handle it. 

FWIW, splayed caps are the greatest thing I've ever seen! They originate from the very rare 340 T/A Mopar blocks that were derived from top fuel Hemi's. If ever there was a factory block asking to be blown, it was the 70-71 340 T/A block! 

Ray

Told you....

I would by a World block before spending the dough on a conversion...Just my .02c

You may be money ahead depending on what your machinist is going to want for the labor...I do know the parts are far from reasonable from a cost perspective...Call Dave at A and D before you make a final decision.



GT

Before your project gets expensive, with all that fancy machine work.  I have a dart 4.500, eagle 4.500, eagle 6.535, JE -10cc short I would let go for $6k the crank is cut for the extra key 1/4 @ 180. It is not assembled and of course everything is brand new. I built it as a spare, but I haven't had any problems with my current setup. When I had the machine work done I wasn't sure if I was going back to a blower so I had it cut for the keyway. No O-rings in the block.

And there is no block fill in it.

2bolt cap BBChevys with ARP studs, and a PROPERLY line honed block, will live a long and useful life at 600-650 HP assuming NO detonation. Above that HP level or with detonation the caps will "walk" side to side and spall metal back and forth to the block and the cap. When this side to side "walk" occurs, the cap becomes pinched in at the parting line, and that will close up the bearing clearance across it. When this closing up occurs for long enough, the edge of the bearing will scrape the oil from the crank, and this will cause the bearing to weld itself to the crank, and the result is a spun bearing. Modern bearings, especially CL-77s are made with a large, shallow chamfer for the last 1/4" or so to allow that to happen a small amount before destruction. This is called a "Delta wall" bearing. The more HP a bearing is designed for, the more Delta wall is included. We use CL-77 "P" (for up to 500-550 HP in a big block) or "H" (for over that HP level) in every engine we build. The "H" bearing has more "Delta" and more crush to retain the bearing and promote heat transfer. Remember- if the line hone is big by .001" the crush is reduced by .00314" and disaster will occur at a lower HP level. TIMINATOR