The timing-chain cover off a 500 WILL physically fit a 126 engine (and vise versa) BUT the oil pumps are different in depth. Therefore, if you fit a 500 timing-chain cover off one of your 500 engines onto your 126 engine, you MUST use the 500's oil-pump, and again, if you fit a 126 timing -chain cover ontoa 500 engine, you must use a 126 oil-pump---basically, never 'mix and match'