Place cucumbers and onions in a large Dutch oven; sprinkle evenly with salt, and add water to cover. Cover and let stand 3 hours. Drain well.
Combine vinegar, sugar, mustard seeds, turmeric, and cloves in a large container; simmer 5 minutes. Add cucumbers and onions; bring to a boil. Remove from heat.
Pack cucumber mixture into hot sterilized jars, leaving 1/4- inch head space. Cover at once with metal lids, and screw bands tight. Process in a boiling- water bath for 10 minutes. Chill pickles before serving.