If you rebuilt the engine yourself you might just be out of luck. I've built a few motors and it's a meticulous process even if you've done it many times before. Sometimes things like crank and rod bearings are out of spec, and any number of small things can go wrong. But ... if someone ELSE rebuilt it ... Give em a swift kick in the bum and make em do it again!! lol
If you're rebuilding it yourself, and you have to again. Get some "plastiguage" and test fit all your main bearings and rod bearings. This saved us easily $2,000 on our Formula Ford when we got a set of bearings that were out of spec. Would have easily chewed up a VERY expensive crankshaft. You put the gauge in the bearings and tighten them to spec, then remove it and measure the thickness. It will correspond to a specific oil clearance that should be listed in a good repair manual. This is one of those crucial steps that 90% of the professional shops out there neglect to do. They can afford the 10% out of spec rate by just rebuilding 10% of their motors twice, then 10% of those three times etc. It may only cost $2 per engine or so to do the plastiguage, but it's labor and time intesive so it's then not cost effective. If anyone rebuilds their own motor, don't risk it. Just buy the stuff! Anyway ... sorry about the bad luck dude. That sucks!!
