An upper frame assembly is mounted to a lower frame assembly to define a main frame assembly for a work machine, such as a skid steer loader, which has distributed loading throughout the mounting therebetween. The distribution of loading is achieved by mounting the upper frame assembly along a pair of side members which extend along its length to a respective pair of side rails of the lower frame assembly which extend along its length. The side members and respective side rails of the upper and lower frame assemblies are connected by exclusive horizontal bolting therebetween.