If your LT engine came with a factory engine oil cooler and you're deleting it, you will have a water jacket exposed. This plug threads into there using a large 17mm allen.
SPECS: Thread- 27mm X 1.0mm, Zinc plated steel, 17mm Allen
PART NUMBER: 11611351
NOTE: Fits some LS's but most LS's we've seen have a brass plug and 28mm threads.
If yours has 28mm threads, use part number 12561663
Key words: LT1 LT4 LT5 LSA