DACCA, EAST PAKISTAN. - An impending crisis in grain distribution is expected to grip East Pakistan soon, further sapping the health and resistance of much of the war-torn nations population.

Experts on food aid in Dacca and visiting teams of specialists say the shortages, expected to begin appearing in the next three months, will claim most of their victims from among poor rural people.

The shortages will affect even those parts of the country normally considered food-surplus areas, the experts agree.

The huge problem facing the government and the foreign relief aid teams is partly one of shipping in enough grain. The east's two ports, Chittagong and Khulna, have a limited capacity. In normal times their best processing rate has been 170,000 tons of aid food grains a month. Some experts here believe that even if the ports, now hit by labor shortages and storage problems, can reach this top capacity, it may not be enough.

But the crux of the problem is the distribution of the grain. Officials here are less worried about physical damage to the communications network or shortages of ships, boats and road vehicles than they are about the capacity of the administrative machinery.

One aid official commented that the major problem is always organization and management, and that the provincial government of East Pakistan is one of the weakest in the world at the best of times.

September is the month in which the crisis may begin. In September the rice crop now being planted will be harvested. This crop normally accounts for about 75 per cent of the east's annual production of about 11.5 million tons of food grains.

Even the government estimates that this harvest will be 10 per cent below normal because of haphazard planting and the flight to India of peasants and of financiers, who give necessary credit to peasant families at planting time.

Some aid officials believe that the shortfall could be significantly higher and that it will not be offset either by the unusually good winter crop just being harvested or by the absence of several millions of the population, who have become refugees in India.