A borescope that will fit through the spark plug holes costs about 30 bucks on ebay. I use mine all the time for inspecting engines, and it even fits down barrels as small as .22 to inspect the condition of the rifles I buy. Mine has a mirror attachment that lets me put it through a hole and look sideways.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!