Most workshops treat a cracked or leaking exhaust manifold as a nuisance job. It ticks, it blows, it fails a noise check — get to it when there’s time.
We see the other end of that decision. Cylinder heads arrive at our workshop damaged, and the damage is not evenly distributed. On aluminium heads the warp is consistently worse on the exhaust manifold side. On cast iron heads, it shows up as cracking instead.
That is not a coincidence.
What is actually happening
When an exhaust manifold cracks, or the flange stops sealing against the head, exhaust gas escapes where it should be leaving through the system. Gases are not expelled properly, and heat concentrates around the exhaust ports instead of moving away from them.
Cylinder heads are designed to run hot, but evenly. Introduce a hot spot on one side and you introduce uneven expansion. Run that through enough heat cycles and the head distorts — pulling more on the exhaust side than the inlet side, because that is where the heat is sitting.
By the time the head comes off, the manifold is no longer the problem. It has already caused one.
What we see on the bench
We measure every cylinder head that comes through our workshop before we touch it. Across the heads that arrive damaged — including many sent to us by other engineering shops that could not resolve them — the pattern is consistent, and it splits by material.
On roughly nine out of ten warped aluminium heads we see, the exhaust manifold side is the worse side. Aluminium moves readily with heat, and a hot spot on one side of the head produces uneven distortion across it.
Cast iron resists warping far better, so instead of moving, it fails. A faulty exhaust manifold leaves the head running considerably hotter than it should, and then fuelling introduces rapid flash cooling in the cylinder area. That cycle of high heat followed by sudden cooling is what cracks cast iron heads, and a leaking manifold makes it happen far sooner than it otherwise would.
Same root cause, two different outcomes. Either way the manifold gets there first.
We should be clear about where our knowledge ends. We are machinists, not mechanics — we do not diagnose engines on the vehicle, and we are not going to tell you what to listen for on a cold start. What we can tell you is what these heads look like once they reach our bench, and how often the exhaust side is the side that has given up.
If you are running a workshop, that is the useful part: when a manifold is cracked or not sealing, the head is already being cooked, whether or not the vehicle is telling you about it yet.
A head that came in last week
A customer who sends us his truck heads brought in his own vehicle’s head — a five-cylinder 2.5. The job he asked for was simple: set the tappet clearances. The head had already been overhauled by another machine shop.
The head was clean, so we ran the pre-checks at no charge. What we found:
- The head had been skimmed while it was still bent
- It had been skimmed 1mm below standard specification
- The valves had not been refaced true to the valve stems, so they were not seating
- The valve seats had never been cut — work that should have formed part of an overhaul
The customer sent his mechanic to look, and asked us for a report. Gordon’s answer was that we would document what we measured, but we would not get involved in a dispute with another machine shop. That is not what we are here for.
What the head actually needed: straightening, valves refaced correctly to the stems, seats cut, the head re-skimmed true — and then the tappets set.
We recommended a shim plate to compensate for the 1mm that had already been removed. Taking that much off a head changes compression ratio, valve timing and piston-to-valve clearance. The mechanic declined, on the basis that he had run plenty of these engines with 1mm skimmed off. That is his call, and it is on record.
The point is not that another shop got it wrong. The point is that the customer paid for an overhaul, believed he had one, and only found out because someone measured the head before touching it.
The mistake that turns a repair into a replacement
Here is where a manageable problem becomes an expensive one.
A warped head cannot simply be skimmed flat and returned to service. On an overhead cam engine, skimming a head that is bent does not correct the bend — it removes material from a distorted casting. The result is a cam tunnel that no longer runs true. The camshaft binds or wears, and the head that could have been saved is now scrap.
We regularly receive heads that have already been skimmed by another engineering shop or a backyard operator, and are still bent. The skim was carried out exactly as the customer asked. Nobody checked whether it should have been done at all.
That is not a machining failure. It is a process failure — the checks were never done.
Why we check before we cut
Every cylinder head that comes into Casting Repair Services gets a pre-check report before any machining is quoted or carried out. That covers crack detection, measuring the head for distortion, checking valve seating, and establishing whether the head is straight, whether it can be straightened, and whether skimming is the correct repair at all.
There is a charge for it as a standalone service. On a full cylinder head overhaul it is not an extra at all — every one of those checks has to be carried out as part of the overhaul anyway, so it is already in the price. The separate charge only applies where someone wants the head assessed and then takes it away.
That is worth being clear about, because the pre-check has a reputation among some mechanics as an added-on cost that inflates a quote to the vehicle owner. On the jobs mechanics actually send us, it usually isn’t one.
Set against the full cost of a cylinder head job or an engine overhaul — parts, labour, gaskets, the vehicle standing — the pre-check is a small fraction of the total. What it removes is the possibility of the entire job being wasted. Machining a head that was never going to be serviceable costs the customer everything spent on it, and then costs them a replacement head on top.
There is also a question of who carries that risk. When a rebuilt engine develops a problem six months later, the vehicle owner does not phone the machine shop. They phone their mechanic. A pre-check report is documented evidence of the condition of the head before any work was done and what was corrected — which is worth having when a comeback lands on your bench.
Other industries have already settled this. Take a television in for repair and you are told upfront that anything under a set amount is simply carried out and charged. The diagnosis is not optional and nobody argues about it, because everyone understands you cannot quote a repair you have not looked at. A cylinder head is a considerably more expensive component than a television.
Put simply: we would rather spend the time measuring than hand back a head that fails.
Repairing the manifold itself
The manifold pictured here came to us in two pieces — a complete break through the casting. A replacement means downtime and, on older or imported machinery, a wait for a part that may no longer be made.
We welded it back together and machined the mounting face flat so it seals correctly against the head. It went back into service with years of life left in it. That is the repair that should happen before the head is the one on the bench.
What to do about it
- Do not defer a leaking or cracked exhaust manifold. It is a cheap repair now and an expensive one later. Manifolds can usually be welded and the mounting face machined flat, rather than replaced.
- If a head is coming off, have it measured. Not skimmed on request — measured first, then a decision.
- If you are quoted a skim with no check, ask what checks are included. A price that low usually means none.
- Ask what has already been taken off. A head that has been skimmed before has less material left, and every skim changes compression ratio, valve timing and piston-to-valve clearance. If it has gone below standard specification, a shim plate should be on the table.
- Ask for the numbers. You should be told what the distortion measured and whether the head is within limits for the make.
Send us a photo on WhatsApp. We will tell you honestly whether it can be saved, what it needs, and what it will cost.
Manifold and cylinder head work at CRS
We repair exhaust manifolds by welding and machining the mounting face flat, and we carry out full cylinder head pre-check reports, straightening, skimming, valve and seat work, crack repair and reconditioning. Durban-based since 1977, with work coming in from all over South Africa.
Bring the part in or send us a photo, and we will tell you straight what it needs.