By Volgen Power Team | Last Updated: May 22, 2026 | Reading Time: 15 min
Volgen Power has supplied Cummins 6CT cylinder heads and parts to fleet operators and diesel shops across North America since 2010. The information below reflects OEM specifications verified against Cummins QuickServe data (parts.cummins.com).
Quick Answer: Cummins 6CT 8.3L Cylinder Head
8.3L / 505 cu in
114 x 135 mm
3973493 / 3973494
28 x M11 TTY
Key Takeaway: The Cummins 6CT (8.3L) uses a different cylinder head design than the 6BT, with larger displacement and higher power output. Common failures include injector sleeve corrosion and head gasket blowthrough. Always use new TTY head bolts when reinstalling.
Table of Contents
Is Your 6CT Cylinder Head Failing? Check These Symptoms
- White exhaust smoke (coolant entering combustion chamber)
- Loss of coolant with no visible external leak
- Engine overheating or running hot repeatedly
- Low coolant warning light after short drives
- Oil turning milky or frothy (coolant contaminating the oil gallery)
- Misfire on one or more cylinders
- Reduced engine power or poor acceleration
- Oil pressure dropping under load
6CT vs 6BT: Key Differences
| Specification | 6CT (8.3L) | 6BT (5.9L) |
|---|---|---|
| Displacement | 8.3L (505 cu in) | 5.9L (359 cu in) |
| Bore x Stroke | 114 x 135 mm | 102 x 120 mm |
| Horsepower | 250-330 HP | 160-250 HP |
| Head Bolts | 28 x M11 TTY | 22 x M11 TTY |
| OEM Head Part # | 3973493 / 3973494 | 3966454 / 3966455 |
| Injector Sleeves | Copper (3934158) | Pressed steel |
Note: The 6CT uses different injector sleeves (coppervs. steel) that are prone to corrosion if coolant is not maintained. See our 6BT Cylinder Head Guide for more details.
The Cummins 6CT 8.3L is the bigger displacement sibling of the legendary 6BT 5.9L. Introduced to meet mid-range power requirements for commercial vehicles, the 6CT shares the 114mm bore with the 6BT but uses a longer 135mm stroke to displace 8.3 liters. Its 2-valve-per-cylinder overhead cam design made it a workhorse in school buses (Thomas Built, Blue Bird), medium-duty trucks (International, Ford), and industrial equipment.
| Specification | Value |
|---|---|
| Engine Series | Cummins 6CT (B Series) |
| Displacement | 8.3 Liters / 505 cu in |
| Configuration | Inline-6, 2-valve per cylinder |
| Bore x Stroke | 114 x 135 mm |
| Power Output | 210–300 hp |
| Peak Torque | 600–800 lb-ft |
| Fuel System | PT (Pressure-Time) pump, mechanically governed |
| Typical Applications | School bus (Thomas Built, Blue Bird), medium truck, industrial generator, construction equipment |
| Cylinder Head Part Number (assy) | 3973493 |
| Cylinder Head Part Number (bare) | 3973494 |
OEM Part Numbers and Cross-Reference
Cummins 6CT Cylinder Head — Complete Parts List
| Part Description | OEM Part Number | Quantity per Engine |
|---|---|---|
| Cylinder Head Assembly (complete, with valves & springs) | 3973493 | 1 |
| Cylinder Head (bare casting, no valve train) | 3973494 | 1 |
| Head Gasket Set | 3802420 / 3802421 | 1 |
| Valve Cover Gasket | 3802423 | 1 |
| Head Bolt Set (28 bolts, torque-to-yield) | 3934510 | 1 |
| Intake Valve | 3802920 | 6 |
| Exhaust Valve | 3802921 | 6 |
| Valve Seat (intake) | 3802915 | 6 |
| Valve Seat (exhaust) | 3802916 | 6 |
| Valve Guide | 3802917 | 12 |
| Injector Sleeve (copper, interference fit) | 3934158 | 6 |
| EGR Valve Gasket | 4936086 | 1 |
Cross-Reference Table
| Volgen Power Part Code | Equivalent OEM Part Numbers | Notes |
|---|---|---|
| VPC-3973493 | 3973493 / 3973494 | Full assembly vs. bare casting — confirm which you need |
| VPC-3934158 | 3934158 | Copper injector sleeve, sold individually or as set of 6 |
| VPC-3802420 | 3802420 / 3802421 | Head gasket set — verify thickness for your application |
Common Failure Modes and Diagnosis
1. Injector Sleeve Corrosion
Injector sleeve failure is unique to the 6CT design. For a detailed guide to diagnosing and fixing diesel cylinder head problems, read our Diesel Cylinder Head Problems: 7 Common Issues & OEM Solutions.
The 6CT’s copper injector sleeves (part 3934158) sit in a water jacket and are subject to galvanic corrosion over time. This is one of the most common reasons for coolant contamination in 6CT engines. The sleeve’s interference fit can loosen from repeated thermal cycling, allowing coolant to seep past the seal and enter the combustion chamber or oil passages.
Signs of injector sleeve failure:
- Coolant loss with no external leak — suspect injector sleeves first on a 6CT
- White smoke from one specific cylinder
- Oil turning brown/milky (coolant entering oil gallery around injector bore)
- Injector cup showing a rust-colored deposit ring around the base
Repair requires pressing out the old sleeves, cleaning the bore, and pressing in new sleeves with the correct interference fit. Do not reuse old sleeves. On the 6CT, replacing all six sleeves is standard practice even if only one has failed — the others are likely close behind.
2. Head Gasket Blowthrough
The head gasket (3802420/3802421) seals the combustion chamber, coolant passages, and oil galleries between the cylinder head and engine block. On the 6CT, blowthrough failures typically occur between the combustion chamber and coolant passage — causing coolant to enter the cylinder or exhaust.
Signs of head gasket failure:
- White sweet-smelling exhaust smoke (coolant in combustion)
- Coolant level dropping and/or external coolant leak at the head gasket area
- Oil contaminated with coolant (check dipstick — mayonnaise-like emulsion)
- Engine overheating
- Loss of engine power
On the 6CT, head gasket failure is often triggered by overheating from a failing water pump, thermostat, or coolant leak elsewhere in the system. Always address the root cause before installing a new head gasket. Never reuse torque-to-yield head bolts — they stretch and lose clamping force on reuse.
3. Valve Seat and Valve Guide Wear
The 6CT’s exhaust valve seats (3802916) and valve guides (3802917) wear over time, particularly in engines that have seen extended service or infrequent use. Valve seat recession reduces compression and causes exhaust smoke. Valve guide wear allows oil to be drawn into the combustion chamber, causing blue/white smoke at idle that clears at higher RPMs.
Signs include compression loss, oil consumption between oil changes, and exhaust smoke. Repair requires machining the cylinder head — valve seat grinding and guide replacement — and should be done by a qualified diesel machine shop.
4. Surface Warpage from Overheating
The 6CT cylinder head is an iron casting, which provides durability but can warp under severe or repeated overheating. Warpage is most common between cylinders and around the injector bores. Even minor warpage (0.002″ or 0.05mm) can cause head gasket leaks and injector sleeve sealing failures.
How to check for warpage:
- Remove the cylinder head from the engine
- Clean all old gasket material from the deck surface
- Place a precision straight edge across the deck surface in multiple directions
- Check gaps with a feeler gauge — 0.003″ (0.076mm) or greater indicates the head needs milling
- Also, check the injector sleeve bosses and valve seat contact surfaces
If warpage exceeds 0.005″ (0.127mm), the head should be sent to a machine shop for milling. Most 6CT heads can be machined once or twice before reaching the service limit — your machine shop can verify.
Torque Specifications
For comparison with other Cummins engines, see our Cummins 6BT Cylinder Head Torque Specs and NT855/KTA19 Torque Reference. Torque Specifications
Proper torque sequence is critical on the 6CT. The cylinder head uses 28 torque-to-yield head bolts (part 3934510) that stretch during tightening and must be replaced after each removal. Follow the three-step sequence below exactly — deviating from factory specs is a common cause of repeat head gasket failures.
Cummins 6CT Cylinder Head Torque Sequence
| Step | Torque Value | Newton-Meters | Notes |
|---|---|---|---|
| Step 1 | 60 ft-lbs | 81 Nm | All 28 bolts, in sequence (see diagram below) |
| Step 2 | 80 ft-lbs | 108 Nm | All 28 bolts, in sequence |
| Step 3 | 90 ft-lbs + 90° turn | 122 Nm + 90° | Angle torque — use a torque angle gauge. Always use NEW bolts. |
⚠️ Critical: The 6CT head bolts are torque-to-yield (TTY) design. They MUST be replaced every time the head is removed. Reusing stretched bolts can cause head gasket failure within the first few thousand miles. Never use old head bolts on a 6CT.
Additional 6CT Torque Specifications
| Fastener | Torque Value | Newton-Meters |
|---|---|---|
| Rocker Arm Shaft Bolts | 24 ft-lbs | 32 Nm |
| Injector Hold-Down Bolt | 15 ft-lbs | 20 Nm |
| Valve Cover Bolts | 10 ft-lbs | 14 Nm |
| Intake Manifold Bolts | 35 ft-lbs | 47 Nm |
| Exhaust Manifold Bolts | 35 ft-lbs | 47 Nm |
| Cross-over Tube Bolts | 24 ft-lbs | 32 Nm |
Cylinder Head Repair vs. Replacement: Decision Guide
Not sure whether to repair or replace? Our Cylinder Head Repair vs. Replacement Guide covers when to machine and when to buy new. Cylinder Head Repair vs. Replacement: Decision Guide
Not every failed 6CT cylinder head needs full replacement. Use this guide to decide which path makes sense for your engine and budget.
| Situation | Recommended Action | Estimated Cost Range |
|---|---|---|
| Cracked cylinder head casting | Replace with OEM assembly (3973493) | $1,800–$3,500 parts + labor |
| Injector sleeve failure (all sleeves) | Rebuild head — new sleeves + valve job | $800–$1,500 machine shop + head gasket kit |
| Head gasket failure only (no other damage) | Replace head gasket (3802420/3802421) + head bolts (3934510) | $300–$600 parts + labor |
| Surface warpage > 0.005″ | Machining (milling) — usually viable if within service limits | $200–$500 machine shop |
| Valve seat/guide wear | Valve job at machine shop (no head replacement needed) | $300–$700 machine shop |
| Bare head cracked + worn beyond machine limits | Replace with complete assembly (3973493) | $1,800–$3,500 parts + labor |
Rule of thumb: If the cylinder head casting has a visible crack between valve seats, through a coolant passage, or around an injector bore, replacement is required — do not attempt to weld iron 6CT heads. If the damage is limited to consumable components (gaskets, sleeves, valves, guides), a rebuild is usually the more cost-effective choice.
For help identifying your specific failure, consult a qualified diesel technician or Cummins QuickServe Online for service bulletin CS-1089 (6CT cylinder head inspection and repair guidelines).
Compare with other Cummins cylinder heads: 6BT 5.9L | ISF 2.8/3.8 | NT855/KTA19
Cummins 6CT Engine Spare Parts | Related & Featured Product
OEM: 3973493
OEM: 3973494
OEM: 3802420
OEM: 3934158
OEM: 3934510
OEM: 3802423
