89-98 5.9L 12 Valve Cummins HD Manton Conical Valve Spring Kit

The Manton D-105-Kit-12v Heavy Duty Conical Valve Spring Kit is a complete conical spring, billet retainer, and valve lock upgrade for the 1989-1998 5.9L 12V Cummins. The kit includes 12 conical valve springs, 12 billet steel retainers, and 24 valve locks - everything needed for the core valve spring conversion. Designed for engines running over 40 PSI and 3,700 RPM, these springs protect valves from being pushed open under high boost while remaining safe for daily driving. Stronger pushrods are recommended when running this kit. Heat-treated shims for adjusting seat pressure above the base spec are available separately.

What This Kit Is and What It Solves

Under high boost on the 12V Cummins, cylinder pressure can overcome factory valve spring seat pressure and force intake or exhaust valves partially open - a condition that robs efficiency and causes instability at the top of the rev range. The Manton conical spring design addresses this directly with 170 lbs seat pressure and 375 lbs open pressure, engineered to hold the valve firmly closed against high boost cylinder pressure up to 6,500 RPM.

The conical shape is the key difference over a standard straight valve spring. A conical spring has a progressively wound coil that resists coil clash and can accommodate higher lift without binding at the same installed height. The 7-degree billet steel retainers in this kit are matched to the conical spring shape - they are not interchangeable with standard straight-spring retainers. The included 7-degree valve locks are designed specifically for these retainers. Everything in the kit is matched to work together. For builds running over 40 PSI and 3,700 RPM on a 12V, this is the correct valvetrain upgrade path - confirm your pushrod upgrade plan, then proceed with confidence.

What Comes in the Kit

The D-105-Kit-12v includes the three core components for a complete valve spring conversion on the 12V Cummins.

• (12) Manton Conical Valve Springs - 170 lbs seat / 375 lbs open, .650" max lift, 6,500 RPM rated
• (12) Billet Steel Retainers - 7-degree, matched to conical spring shape, stronger than OEM retainers
• (24) Valve Locks - 7-degree, designed for use with these specific retainers

The core valve spring conversion - springs, retainers, and locks - requires no additional sourcing. Pushrods and shims are separate decisions based on your build.

D-105-Kit-12v Specs

Verify the spring specs and kit contents below match your cam lift and build goals before ordering.

If your engine, cam lift, and boost level fall within the specs above and your pushrod upgrade is planned, this is the correct kit to order for your 12V Cummins.

Complete the 12V Valvetrain Upgrade

This kit handles the spring, retainer, and lock conversion. These companion Manton components complete the full 12V valvetrain upgrade.

• Manton Series 3 Pushrods - 89-98 5.9L 12V Cummins - the recommended stronger pushrod upgrade to pair with this kit
• Manton Series 5 Pushrods - 89-98 5.9L 12V Cummins - maximum-strength pushrod option for competition builds running this kit

Avoid These Common Ordering Mistakes

• Ordering for a 24V Cummins (1998.5+) - this kit fits the 12V (1989-1998) only
• Assuming pushrods are included - stronger pushrods are recommended with this kit and must be ordered separately
• Assuming shims are included - heat-treated seat pressure shims are available separately; contact Pure Diesel Power if your boost level requires seat pressure adjustment beyond the base spring spec
• Running a cam with lift exceeding .650" without verifying clearance - max valve lift accommodation is .650" at 1.940" installed height
• Using standard straight-spring retainers with these conical springs - the included 7-degree billet retainers are matched to the conical spring shape and are required for correct function
• Placing an order with a California or Colorado shipping address - eligible out-of-state address required

Ready to Order - Confirm These First

• Engine is 1989-1998 5.9L 12V Cummins - not the 24V
• Cam lift is at or under .650" max valve lift accommodation
• Stronger pushrods are planned - sold separately, required for this build level
• Shim need assessed - contact Pure Diesel Power if your boost requires seat pressure adjustment
• Eligible (non-California, non-Colorado) shipping address confirmed

For a 1989-1998 5.9L 12V Cummins running over 40 PSI and 3,700 RPM - whether a stock rebuild or a race-capable build - the Manton D-105-Kit-12v is the correct conical spring conversion kit to order. Confirm your pushrod plan and shim needs, then add to cart.

Frequently Asked Questions