bodies don't last in acidic manx soil - most poor people were merely buried in a linen shroud or sheet - wood was expensive and you often see in 18th wills where the dales (ie planks) are to come from to make a coffin. However I suspect even a wooden coffin lasted no more than 40 years at most

Then there is the minor matter of just what grave you exhume - I ssume you would be interested in old Ballaugh - most stones are now stumps - graves also were extensively re-used.