A cracked cylinder head is either an aluminum casting that needs TIG welding with ER4043 filler and 300F preheat, or a cast iron casting that needs nickel stick rod (ENi-CI) with 400-500F preheat and slow cooling. Both repairs follow the same basic sequence: identify the crack, prep it completely, preheat the entire head, weld with the correct filler, slow-cool, and then have the head resurfaced at a machine shop. The difference between a successful repair and a head that cracks again within 1,000 miles is almost always in the preheat and cool-down procedures.
Before you commit to the repair, compare the cost of welding plus machine work against the cost of a replacement head. On common engines, replacement is often cheaper. On rare, discontinued, or expensive heads, repair is the only option.
Identifying the Crack
Common Crack Locations
| Location | Cause | Repairability |
|---|---|---|
| Between cylinders (deck surface) | Thermal cycling, overheating | Good (most common repair) |
| Valve seat to water jacket | Overheating, thermal shock | Moderate (seat may need replacing after weld) |
| Exhaust port to water jacket | Thermal cycling at the hottest point | Good |
| Spark plug hole to water jacket | Over-torquing, corrosion | Good (may need thread insert after) |
| Cam tower or bearing surface | Oil starvation, overheating | Poor (precision surface damaged) |
| Multiple cracks in thin sections | Severe overheating event | Poor (head likely weakened throughout) |
Crack Detection
Visual inspection catches obvious cracks, but many cylinder head cracks are hairline and invisible to the naked eye, especially when the head is dirty.
Pressure testing (wet method): Seal all coolant passages, pressurize the water jacket to 30-40 psi, and submerge the head in hot water (180F). Bubbles emerging from a crack indicate its location. This method also finds cracks that are only open when the head is at operating temperature.
Dye penetrant testing (DPI): Apply penetrant, allow dwell time, clean the surface, and apply developer. Cracks show as red lines (with red dye penetrant) against the white developer background. Clean the surface thoroughly before DPI. Oil and carbon deposits hide cracks.
Magnetic particle inspection (MPI): Works only on cast iron heads (ferromagnetic). A magnetic field applied to the head causes iron particles to cluster at cracks. Very sensitive to surface and near-surface cracks.
Aluminum Cylinder Head Repair
Most modern automotive cylinder heads are cast aluminum alloy, typically A356 (Al-7Si-0.3Mg) or 319 (Al-6Si-3.5Cu). Both are weldable with the correct procedure.
Preparation
- Strip the head completely. Remove valves, springs, retainers, seals, and all hardware. The head should be bare casting.
- Clean the head. Hot-tank or chemically clean to remove oil, carbon, and coolant deposits. The repair area must be absolutely clean. Any contamination causes porosity.
- Locate the full crack. Use DPI after cleaning. Cracks in aluminum castings often extend further than they appear.
- Grind out the crack. Use a carbide burr to create a V-groove along the full crack. On deck surface cracks, groove to about 2/3 of the deck thickness. Don’t grind through into the water jacket if the crack hasn’t already penetrated. If it has, plan to weld from both sides.
- Verify crack removal. DPI again. If dye bleeds from the groove, keep grinding.
Preheat
Temperature: 300F (149C). Use an oven for uniform heating. If using a torch, heat the entire head evenly. Do not exceed 400F (204C) on aluminum, or the casting begins to soften and warp.
Verification: Use a temperature crayon (300F Tempilstik) on several locations across the head. The entire casting should be at temperature, not just the repair area.
Soak time: Hold at temperature for 30-60 minutes before welding. This ensures the heat has penetrated through thick sections.
Welding
Process: TIG (GTAW), AC polarity.
Equipment setup:
- Tungsten: 3/32-inch 2% lanthanated, balled tip (AC forms the ball naturally)
- Cup: #8 or #10 alumina, or gas lens with #12 cup
- Gas: 100% argon, 20-25 CFH
- Filler: ER4043, 3/32 or 1/8-inch diameter (ER4043 matches the silicon content of most head alloys)
Settings:
| Section Thickness | Amperage | Filler Size | Technique |
|---|---|---|---|
| 3/16" (thin deck bridge) | 100-130A | 3/32" | Fast travel, minimal heat input |
| 1/4-3/8" (typical deck) | 130-170A | 3/32" or 1/8" | Moderate travel, fill groove in 2-3 passes |
| 1/2"+ (thick sections) | 160-200A | 1/8" | Multiple passes, allow cooling between |
Technique:
- Start the arc on the groove sidewall, not the root
- Build the puddle on the sidewall and flow it across the groove
- Add filler rod liberally. Cast aluminum is porous and absorbs filler. Don’t try to run autogenous.
- Work at a steady pace. Too slow and you overheat. Too fast and you get poor fusion.
- Fill the groove slightly proud of the surface (0.020-0.030 inch above deck). This material gets machined off during resurfacing.
Slow Cooling
After welding, return the head to the oven at 300F and let it cool slowly by reducing temperature gradually over 2-4 hours. If no oven is available, wrap the head in a welding blanket and let it cool undisturbed for 4-6 hours minimum.
Do not quench. Do not blow compressed air on the weld. Rapid cooling causes shrinkage cracks in the weld and HAZ.
Cast Iron Cylinder Head Repair
Cast iron heads are less common on modern engines but still prevalent on older vehicles, industrial engines, and diesel applications. The repair procedure follows standard cast iron welding practice with preheat and nickel rod.
Preparation
Same as aluminum: strip, clean, groove, DPI verify.
Additional step for cast iron: Bake the head at 500F (260C) for 1-2 hours before welding. Cast iron absorbs oil and coolant into its porous structure over years of service. This contamination vaporizes during welding and causes porosity. Baking drives it out.
Preheat
Temperature: 400-500F (204-260C). Uniform heat across the entire head.
Cast iron is far more crack-sensitive than aluminum. Uneven heating creates thermal gradients that crack the casting before you even start welding.
Welding
Process: Stick (SMAW) with ENi-CI or ENiFe-CI rod.
Settings: ENi-CI 3/32-inch at 60-80A, DCEP preferred. AC acceptable if DCEP isn’t available.
Technique:
- 1-inch maximum bead length per pass
- Peen each bead immediately while still red/orange with a ball-peen hammer
- Let the bead cool to the preheat temperature (not below) before the next pass
- Fill the groove in multiple short passes, peening each one
- Don’t weave. Stringer beads only. Weaving overheats the HAZ.
Slow Cooling
This is the critical step for cast iron. Slow cooling prevents the martensite and iron carbide formation that makes the HAZ brittle.
- After the final weld, maintain the casting at preheat temperature for 30 minutes
- Reduce temperature by 50F per hour if using an oven
- If wrapping in a blanket or burying in sand, allow 8-12 hours to reach ambient temperature
- Do not remove the blanket to “check on it.” Every time you unwrap, you introduce a thermal shock.
Post-Weld Machine Work
Every cylinder head repair requires machine work after welding.
Deck Resurfacing
The weld repair and the heating/cooling cycle warp the deck surface. A Blanchard grinder or surface grinder machines the deck flat to the original specification (typically within 0.002-0.003 inch flatness over the full deck length).
Cost: $75-150 per deck surface at most machine shops.
Material removal: The resurfacing cut removes the weld crown and brings the deck to a flat, true surface. If the weld is recessed below the deck (didn’t fill enough), you’ll need additional welding before resurfacing.
Valve Seat Recutting
If the crack extended near a valve seat, or if the welding heat distorted the seat area, the valve seats need to be recut. A three-angle valve job (typically 30-45-60 degrees) restores the sealing surface.
Cost: $50-150 per head depending on the number of valves.
Pressure Testing (Verification)
After all machine work is complete, pressure test the head again:
- Block all coolant passages except the test port
- Pressurize to 30-40 psi with the head submerged in hot water (180F)
- Watch for bubbles for at least 5 minutes
- Any leakage means the repair failed and needs to be redone
Cost Analysis: Repair vs. Replacement
| Head Type | Repair Cost (weld + machine) | Remanufactured Replacement | New OEM | Recommendation |
|---|---|---|---|---|
| Common 4-cyl cast iron | $250-500 | $150-350 | $400-800 | Replace (reman) |
| Common V8 cast iron | $300-600 | $200-500/ea | $500-1,200/ea | Replace if available |
| Common aluminum 4-cyl | $300-600 | $250-600 | $600-1,500 | Compare on price |
| Performance aluminum (LS, Hemi, etc) | $400-800 | $500-1,200 | $1,000-3,000 | Repair if single crack |
| Diesel head (medium/heavy duty) | $500-1,200 | $1,000-3,000 | $2,000-6,000 | Repair (usually worth it) |
| Rare/vintage head | $400-1,000 | Often unavailable | Unavailable | Repair (only option) |
Additional costs to consider: Gasket set ($50-200), head bolts if TTY type ($30-100), coolant and fluids, and labor for removal and reinstallation if you’re paying a shop.
When a Head Isn’t Repairable
Walk away from the repair when:
- Multiple cracks across the casting. A head with cracks in 3-4 locations has been severely overheated. The entire casting is compromised, and repairing one crack just means another one opens.
- Crack through a cam bearing surface. Welding in a cam journal area warps the precision bore. The bearing surface can be re-bored, but the combined cost of welding and re-boring usually exceeds replacement.
- Extensive deck erosion from coolant leak. If coolant has been leaking through a crack for a long time, the deck surface may be pitted and eroded beyond what a single resurfacing pass can fix.
- Head has been previously welded and cracked again. A second crack at or near a previous repair indicates the casting can’t handle the thermal stress at that location. Further welding attempts have diminishing returns.
- Crack into the oil galley or cam tower. Oil-side cracks are difficult to clean and contaminate easily. The tight spaces make welding access poor.
Doing It Yourself vs. Machine Shop
DIY welding + machine shop finishing is the typical approach for skilled hobbyist welders. You handle the crack prep, preheat, and welding. The machine shop handles resurfacing, valve work, and pressure testing.
Full machine shop repair (some shops do the welding too) is the turnkey approach. More expensive in labor, but the shop has ovens for uniform preheat and slow cool, experience with hundreds of head repairs, and the equipment for post-weld machining all under one roof.
If you’ve never welded a casting before, a cracked cylinder head is not the place to learn. Practice on scrap castings first. The cost of ruining a head by improper welding exceeds the cost of having a shop do it right.
Common Mistakes
Welding without full preheat. Spot-heating just the crack area creates a thermal gradient that cracks the rest of the head. The entire casting needs to be at uniform temperature.
Not removing the entire crack. Welding over a crack that extends deeper than the groove. DPI after grinding is the only way to confirm the crack is completely removed.
Overheating aluminum. Running too hot or too slow on an aluminum head melts the casting beyond the groove boundaries. Keep travel speed up and amperage at the low end of the working range.
Skipping the slow cool. Pulling the welding blanket off after an hour because you’re impatient. The cooling rate in the first few hours determines whether the weld and HAZ are ductile or brittle.
Not planning for machine work. Welding the head and bolting it back on without resurfacing. The warped deck doesn’t seal against the gasket, and the head gasket blows again within weeks.
Cylinder head crack repair is a specialized subset of casting repair that requires the right filler metal, controlled preheat, and follow-up machine work. Done correctly, a repaired head performs identically to a new one. Done incorrectly, it cracks again faster than the original.
For more repair topics, see the repair & maintenance overview and our guide to broken casting repair.