My late father was a machinist and maintenance supervisor for a company that used quite few Russian made machines such as bulldozers, trenchers, backhoes etc. Problem was that, due to typical communist mismanagement, there were no spare part to buy. So all parts were made in the shop my father managed. Since many tractors were copies of machines made in the USA only nuts and bolts were metric and the rest of dimensions we in "imperial" units. It was impossible to use standard sizes in example for reamers or drill bits. They had to be custom made. In other words the machines were maintenance nightmare.