If an air cooled block manufacturer gets the fin geometry wrong, you risk an engine that runs hot, a bore that distorts, fins that crack, and tooling money that is already spent. An air-cooled engine has no water jacket, so most of the cooling depends on the fins. That is why the checks before tooling matter so much.
We make air-cooled, water-cooled and combined air-water cooled blocks at our plant in Rajkot, and our IATF 16949:2016 certification covers air-cooled blocks along with liners and sleeves. This article sets out what we would check if we were in your place.
Why the fins matter so much
An air-cooled block sheds its heat through the fins on its outside, straight into the air moving past. For that to work, the fins need the right thickness, height and spacing, and they need to be the same all the way round the cylinder. Small errors add up quickly:
- Fins that are too thick add weight and leave less space for air to pass between them.
- Fins that are too thin may not fill properly in the mould, and they can crack in service.
- Uneven fins make one side of the cylinder run hotter than the other, and the bore can lose its roundness.
- Short or broken fins reduce the cooling area, often where it is needed most.
The point where each fin meets the cylinder wall matters too. A sharp corner there concentrates stress and is a common place for cracks and shrinkage, so a sensible root radius on the drawing helps both the casting and the engine.
A block with these faults can look perfectly good on the shelf and still run hot in the field. Often nobody finds out until engines come back. For a simple comparison of the two cooling methods, see our article on air-cooled and water-cooled engines.
What usually goes wrong at the foundry
Most fin problems start in the mould or during pouring. These are the common ones:
- Misruns and cold laps happen because thin fins cool fast. If the iron is too cold or flows too slowly, it freezes before the fin is full, leaving a short fin or a weak seam.
- Fins can snap when the casting is knocked out of the mould or cleaned.
- A core can shift during pouring, leaving the wall around the bore thicker on one side.
- Rough or sandy surfaces between fins trap dirt, are hard to clean and can restrict airflow in tight gaps.
- Shrinkage can form at the fin root, where a thin fin meets a thick wall and the metal shrinks as it cools.
How we make air-cooled blocks
We cast our air and water cooled blocks in shell moulds. A heated metal pattern is covered with resin-coated sand, and the heat sets the resin into a thin, firm shell. Shell moulds hold closer dimensions and give a smoother surface than ordinary sand moulds, which helps with thin fins and the narrow gaps between them. Our cores are made on a core shooter machine, so core shapes stay consistent from one block to the next. You can read more about our foundry on our infrastructure page.
The iron is melted in our induction furnaces. We run four, from 300 kg up to a 900 kg dual-track unit, and the chemistry of every heat is checked on the spectrometer before it is poured.
Machining is done in the same plant. Our machine shop runs CNC turning centres, special purpose boring machines and 4-axis and 5-axis machining centres. Machined dimensions are checked on our CMM against your drawing, and the rest of our inspection equipment is shown on our quality page. With the foundry and the machine shop under one roof, the part does not pass between separate companies on its way to you.
Choosing a grade that suits thin fins
Thin fins need iron that flows well into narrow sections and carries heat away from the bore. Lower to medium grey iron grades usually do both better than high-strength grades, which are harder to run into thin sections without misruns or hard spots. As with any cast iron engine block, the grade still has to suit the loads, so it is a balance that the drawing should settle. If your drawing names a grade, we cast to it. If it does not, we will tell you which grade the application calls for.
What to verify with an air cooled block manufacturer before you commit to tooling
Tooling is the part of a project that is hardest to undo. Before you approve it, we suggest checking these eight points with any supplier, including us.
- Experience with this kind of part. Ask whether they already make air-cooled blocks, and for which engines. If they mention a certificate, check that its scope actually includes blocks.
- The moulding process, and the reason for it. Ask which process will be used for the fins and why. For thin fins and close spacing, shell moulding is usually the safer choice.
- A feasibility review before the quote. A careful supplier studies your drawing first and raises concerns about thin sections, sharp corners or tight gaps. When you send us a drawing or a sample, we confirm the grade, the process and the feasibility before we quote.
- A named grade. “Cast iron” is not a specification. The drawing or the quote should name a grade, for example ASTM A48 Class 30, and the supplier should be able to give you the equivalent in your own national standard. We cross-reference our grades across 15 national standards, as shown on our iron casting page.
- Tooling cost shown separately. You should know what you are paying for patterns and core boxes, apart from the piece price. Our quotations show tooling separately.
- A first article with a dimensional report. Before series supply, ask for sample blocks and a full dimensional report. Check fin thickness, fin height, fin spacing and the wall around the bore, not only the machined sizes.
- Inspection on every lot. Ask what is checked on each lot and what documents come with it. Our lots ship with a material certificate and an inspection report.
- Casting and machining in one place. When the foundry and the machine shop are separate companies, problems can fall between them. Ask who is responsible for the finished part.
Risks and checks at a glance
| Risk | What it can do in service | What to check before tooling |
| Thin or misrun fins | Less cooling area, cracked fins | Moulding process, feasibility review, fin sizes on the first article |
| Uneven fin spacing | Hot spots and a distorted bore | Pattern design, fin measurements on the first article |
| Core shift | Uneven wall around the bore | Core making method, wall thickness on the first article |
| Unnamed or wrong grade | Wear, cracking, poor machining | Named grade on the drawing, material certificate with every lot |
| Tooling cost hidden in the piece price | You cannot see what you paid for tooling | Tooling quoted as a separate line |
What to send with your enquiry
A complete enquiry lets us review the part properly the first time. Please include:
- Your drawing, or a sample if no drawing exists
- The engine make and model the block is for
- The material grade, if it is known
- Your expected yearly quantity
- Whether you need the block as-cast or machined
- Any inspection, testing or certification requirements
Frequently asked questions
What is an air-cooled block?
An air-cooled engine block cools through fins on its outside instead of a water jacket. It needs no radiator, coolant or water pump, which is why it is common in smaller, portable and agricultural engines.
Why do you use shell moulding for air-cooled blocks?
Shell moulds hold closer dimensions and give a smoother surface than ordinary sand moulds. That helps with thin fins, narrow gaps between fins and the accuracy of the surfaces that carry heat away.
Can you develop a block from a sample?
Yes. Where there is no drawing, we take dimensions from the sample on our CMM and profile projector and identify the material on the spectrometer. Patterns and tooling are then made in-house.
Do you also make water-cooled blocks?
Yes. We make air-cooled, water-cooled and combined air-water cooled blocks, and we will cover water-cooled blocks in a separate article.
Planning an air-cooled block?
If you are planning an air-cooled block, please send us the details listed above. You can use our enquiry page or email sales@esteemauto.com, and buyers outside India are welcome to write to exports@esteemauto.com. We will go through the fins, the grade and the process with you before any tooling is made. Our full range is on our cylinder liners, sleeves and cooled blocks page.
About the Author
This article was written by the Esteem Auto team in Rajkot, Gujarat. We started in 1980 as a machine shop and have run our own foundry since 1990. Today we make air-cooled, water-cooled and combined air-water cooled blocks, along with more than 350 models of cylinder liners and sleeves. Our blocks are shell moulded and machined in our own plant, and our IATF 16949:2016 certification covers air-cooled blocks, liners and sleeves. We export to more than 50 countries.