Boise Boat Works Distributor Cap & Rotor Tune-Up Kit for Mercruiser 4.3L — Thunderbolt I/IV, Brass Terminals & Gasket

A complete replacement distributor cap and rotor tune-up kit engineered to restore reliable spark for Mercruiser 4.3L V6 engines with Thunderbolt I and IV ignition systems (1985+).

This Boise Boat Works kit contains the critical components you need to refresh the ignition on GM 4.3L Mercruiser V6 engines equipped with Thunderbolt I or Thunderbolt IV systems. Designed as a direct-fit replacement for part numbers 815407Q5, 815407A2, 18-5274, and 9-29317, the kit includes a new cap with brass terminals, a rotor, a gasket, and mounting bolts. Installing these parts helps eliminate wear-related misfires, poor starting, and uneven engine performance caused by aged distributor components.

Will this kit fit my Mercruiser 4.3L V6 engine?

Yes — the kit is sized to replace factory distributor caps and rotors used on all GM 4.3L V6 Mercruiser engines that use Thunderbolt I or Thunderbolt IV ignition from 1985 onward. Confirm compatibility by matching your existing part numbers or visually comparing the cap and rotor style.

• Replaces OEM part numbers 815407Q5, 815407A2, 18-5274, 9-29317
• Fits Mercruiser V6 4.3L with Thunderbolt I and IV ignitions
• Intended for engines from 1985 and newer

How will installing this kit improve engine performance?

Replacing worn distributor parts restores accurate spark timing and consistent electrical contact, which often reduces hesitation, rough idling, and difficult starts. While not a cure-all for unrelated engine issues, a fresh cap and rotor eliminate a common source of misfires and uneven cylinder firing.

• Brass terminals provide reliable electrical conductivity
• New rotor ensures even distributor contact with plug wires
• Gasket and bolts included for a proper reseal and fit

What comes in the kit and is installation difficult?

The kit delivers everything required for a standard distributor cap and rotor service: the cap with brass contacts, rotor, a replacement gasket, and the mounting bolts. Basic hand tools and moderate mechanical experience are usually sufficient; the job is typically straightforward and can be performed in a garage or on the boat with simple instructions.

• Included: cap, rotor, gasket, mounting bolts
• Tools: screwdriver or socket set, basic ignition timing check tools if required
• Skill level: intermediate DIY or professional mechanic for best results

How can I confirm the part before ordering?

Verify by checking the part number on your current cap/rotor assembly or compare the distributor’s electrical terminal layout to the product photos and diagrams. If your existing cap lists any of the replaced part numbers or your engine uses Thunderbolt I/IV, this kit is the appropriate match.

• Compare OEM numbers printed on your old parts
• Match distributor terminal pattern visually
• Refer to your engine service manual for Thunderbolt ignition details

Who is this for: Boaters, marine mechanics, and DIY enthusiasts who maintain Mercruiser-powered vessels with the 4.3L V6 and Thunderbolt I or IV ignition systems. If your boat shows signs of ignition wear — such as stalls, rough idle, or difficult starting — or you're performing routine tune-up maintenance, this Boise Boat Works cap and rotor kit provides the direct-fit parts to restore proper distributor function.