Fitted bolts are interference fit bolts. Normal bolts when tightened are liable to reduction in their cross sectional area. But this reduction in diameter may shear off the bolt where large torque is transmitted. In a fitted bolt, the holes are initially drilled to undersize and are reamed in the assembling place before the bolts are tightened in place. Accuracy is required in the machining. These bolts could be of two types one having a slight taper of about 1:100 on diameter and the other having a large taper of about 1:15 on diameter. However, the holes in either case are reamed in final place.
Sometimes an oversized bolt is stretched hydraulically reducing the bolt diameter. When tightened down and the hydraulic pressure is released the final exact fit in the bolt hole is achieved, like a pilgrim nut on coupling shafts.
Fitted bolts use onboard ship :
These bolts find place in engine and boiler mountings, coupling shafts etc. and form a rigid fixture. On main engine mounting these are situated aft of the engine in the way of thrust block. This system of foundation bolts offers the rigid seating with provision for thermal expansion towards fore end.