Mercruiser Thunderbolt Ignition HEI V8 Tune Up Kit | Sierra 18-5273 Tune-up kit includes cap and rotor fits Mercruiser V8 marine engines with Thunderbolt distributors. For Mercruiser V8 HEI Thunderbolt ignitions Includes 18-5395 distributor cap and 18-5431 rotor Replaces Mercury 805759T3, 805759Q3 Replaces Mallory 9-29350 Replaces Sierra 18-5249