“Sorry — it’s cracked. You’ll need a new head.”
If you’ve heard those words from a machine shop, you’re not alone. Most shops pressure-test a cylinder head, find a crack, and hand it straight back. And for them, that’s the honest answer — because they don’t have the equipment or the skills to repair it.
We do. CRS has been repairing cracked castings in Durban since 1977 — it’s literally the business we’re named for. So here’s the straight story on cracked cylinder heads: why they crack, which ones can be saved, how the repair actually works, and when replacement really is the better call.
Why cylinder heads crack in the first place
Almost every cracked head we see comes down to heat:
- Overheating — a failed water pump, stuck thermostat, radiator leak or low coolant. The head gets hotter than the casting was designed for.
- Thermal cycling — years of heating up and cooling down slowly fatigue the metal, especially on hard-worked diesels.
- A blown head gasket left too long — combustion gas cuts into the casting and cooking continues until something gives.
- Rapid cooling — topping up a boiling engine with cold water is a classic way to crack a head on the spot.
Cracks almost always appear where the casting is thinnest and hottest: between the valve seats, in the combustion chamber, or around injector and spark plug bores.
A head can be cracked with no visible sign at all. If your engine overheated — even once, even briefly — have the head crack tested and pressure tested before it goes back on. Fitting a new gasket to a cracked head wastes the gasket, the labour, and usually the second gasket too.
How we find cracks (and why testing comes first)
Before anyone talks repair or replacement, the head needs proper testing. At CRS every suspect head is:
- Stripped and chemically cleaned — you can’t test what you can’t see. Carbon and scale hide cracks.
- Crack tested — to reveal surface cracks invisible to the eye.
- Pressure tested — the water jacket is sealed and pressurised to find internal leaks no surface test can see.
- Checked for warp — overheating that cracks a head usually warps it too; a straight edge and feeler gauge tell us if skimming is needed.
Only after testing can anyone honestly tell you what the head needs. Be wary of a verdict — either “it’s fine” or “it’s scrap” — given without it.
The repair most shops can’t offer: metal stitching
For cast-iron heads — most diesels, trucks, plant, marine and older engines — the gold-standard repair is metal stitching (also called metal locking).
Instead of welding, specially designed locks and studs are mechanically inserted across and along the crack, sealing it and knitting the casting back together — completely cold. That “cold” part is the whole point:
- No heat means no distortion. Welding cast iron puts enormous heat into a casting that hates it — it warps machined faces and often cracks again beside the weld as it cools. Stitching does neither.
- It’s permanent. Metal stitching has been used for a century on engine blocks, pump housings, press frames and ship castings. Done properly, the repair outlives the engine.
- It’s proven before it leaves. Every stitched head is pressure tested again after repair. It goes back to you sealed — demonstrated, not promised.
For aluminium heads — most modern petrol engines and bakkies — cracks are repaired by specialist argon-arc welding, then the head is re-machined: seats recut, face skimmed, and pressure tested the same way.
WhatsApp us photos of the crack and the engine make — we’ll tell you honestly if it can be saved, usually the same day.
Repair vs replace: how the maths works
| Situation | Usually the right call |
|---|---|
| Common passenger-car engine, replacement heads cheap and on the shelf | Replace — sometimes a new head genuinely costs less than a full repair. We’ll tell you when that’s the case. |
| Diesel, truck, plant or marine engine — replacement head expensive or weeks away | Repair — stitching or welding is typically far cheaper and much faster than importing a head while your machine stands. |
| Older, imported or discontinued engine — no replacement exists | Repair is the only option. This is exactly what casting repair was invented for, and it’s our home ground. |
| Crack runs through a critical, highly stressed area beyond economical repair | Replace — an honest shop knows when to say no. We test, we tell you straight, and we don’t repair what shouldn’t be repaired. |
For fleet operators the deciding factor is rarely the part price — it’s downtime. A truck or excavator standing for three weeks waiting on an imported head costs far more than the repair that would have had it running by Friday.
While the head is off — the checklist
A cracked head rarely travels alone. Whatever caused the overheating has usually left fingerprints elsewhere, so while the head is with us it pays to:
- Skim the face — overheating warps heads; a fresh, flat face is cheap insurance for the new gasket. Part of our standard cylinder head reconditioning.
- Do the valve work — seats, guides and valves suffer in an overheat. Doing them now costs a fraction of doing them after reassembly.
- Check the block deck — if the head warped, the block face may have too. And yes — cracked blocks can be stitched as well.
- Fix the cause — thermostat, water pump, radiator, fans. A perfect repair won’t survive the same overheat twice.
Frequently asked questions
Can a cracked cylinder head really be repaired permanently?
What is metal stitching?
Is it cheaper to repair or replace?
Why did my head crack?
WhatsApp us photos and the engine make — we’ll tell you honestly whether it can be stitched, welded or skimmed, and quote it fast.