Core Shift is when the structure inside the engine core, made of sand and (in the reverse image of the engine – like a negative) when it is cast, a core component actually shifts between the time it is made and the time iron is pored in it. When the camshaft hole isn’t centered in the rear of the block it has actually shifted and the cam bore holes, from front to the back of the engine are at an angle to the centerline of the engine. Not in line as they should be.

This causes problems because the camshaft needs more force to turn, it isn’t centered so the valve timing will be off and it will never really produce maximum horsepower and torque because of the parasitic drag of a core shift block.

I chose this used 2-bolt instead of another 4-bolt block. The 4-bolt had a lot of core shift while the one I bought has none. Once it was machined the engine went together easy and delivered the power level I expected. To gain more strength for the main caps, I added ARP studs and followed their strict tightening protocol. It was assembled and running in three days.

Since I have experience running up to 450 HP through a 2-bolt block; I’m not worried.  Neither should you if you have to choose between a close to zero core shift 2-bolt block over one with visible core shift.

The engine is now in my 1985 El Camino in the spot the stock 305ci used to reside. This engine is delivering a lot of performance with just 2-bolt mains and it is running better as I get more miles on the clock. I have right at 375 RWP; that works out to +400 at the flywheel. This is plenty of power to drive on the street and run autocross events.

Check this out when you are looking to buy any used engine regardless of the brand: GM, Mopar, Ford, or any other brand you run into. With the new casting technology this is not a problem anymore, but check it out away.