Cast Iron Engine Blocks: Grade Selection for Automotive and Tractor Platforms

Most cast iron engine blocks are made in grey iron, and the usual choice sits in the middle of the grade range, around EN-GJL-250 or ASTM A48 Class 35. Smaller air-cooled engines often use a lower grade. Blocks that carry heavy loads, such as tractor blocks that form part of the frame, may need a higher one.

The right grade for your block depends on its design, its loads and how it will be machined, so the grade on the designer’s drawing always comes first. This article explains how grades are named, what changes between automotive and tractor blocks, and how we confirm the grade we cast. We make air-cooled, water-cooled and combined air-water cooled blocks at our plant in Rajkot.

Why grey iron is the usual choice for a cast iron engine block

Grey iron has been the standard material for engine blocks for a long time, and for good reasons:

• It casts well. Molten grey iron flows easily into thin walls and fins and around complex cores.
• It machines well. The graphite flakes break up the chips, so bores and faces can be machined cleanly at good speeds.
• It damps vibration better than steel or ductile iron, which helps keep an engine quiet.
• It conducts heat well, carrying heat away from the bore into the fins or the coolant.
• It gives the strength most blocks need at a sensible cost.

Its main limit is low ductility. Grey iron does not stretch much before it cracks, so the design and the grade together must keep stresses within safe limits. For more background on how grey iron forms, see our article on grey iron metallurgy.

How grey iron grades are named

The same iron carries different names in different countries. In most systems, the number tells you the minimum tensile strength.

ASTM A48, used in the USA, names grades by class. The class number is the minimum tensile strength in thousands of psi, so Class 30 is about 207 MPa, Class 35 about 241 MPa and Class 40 about 276 MPa. EN 1561, the European standard, uses names such as EN-GJL-250, where 250 is the minimum tensile strength in MPa. Under the older German DIN standard, the same iron was called GG25. ISO 185 uses grade numbers from 100 to 350, and the Indian standard IS 210 uses FG grades from FG150 to FG400.

Here is how the grades most often used for blocks line up:

ISO 185	EN 1561	Older DIN name	Close ASTM A48 equivalent
200	EN-GJL-200	GG20	Class 30
250	EN-GJL-250	GG25	Class 35
300	EN-GJL-300	GG30	Class 40 to 45

Two cautions. These equivalents are close, not exact, because each standard has its own test rules. And the strength figure usually comes from a separately cast test bar, not from the block itself. A thick section of a block cools more slowly and can be noticeably weaker than the test bar, while a thin section can be stronger.

We cast grey iron from ISO 185 grade 100 to 350 and cross-reference every grade across 15 national standards, including ISO, ASTM, DIN, EN, JIS, BS and IS. The full table is on our iron casting page.

Automotive and tractor engine blocks: what changes

The aim is not to pick the highest grade you can find. A higher grade is stronger, but it is also harder to machine, more prone to shrinkage in heavy sections and less forgiving in thin ones. The right grade is the one that suits the block’s job.

An automotive engine block has to be compact, light and quiet, and it is usually made in large numbers. Walls are kept thin to save weight, so the iron must fill thin sections reliably and machine cleanly at volume. Medium grades such as EN-GJL-250 are a common choice.

A tractor engine block often has a second job. On many tractors the engine is bolted between the front axle support and the gearbox, so the block forms part of the tractor’s frame and carries the weight and forces of the machine. Tractor blocks therefore tend to have heavier sections and may call for a higher grade, such as EN-GJL-300. Tractor engines also work at high load for long hours, often in dust and heat.

Air-cooled blocks rely on their fins for cooling. The iron must flow into thin fins without misruns and carry heat away from the bore. Lower to medium grades often suit this well, because they cast more easily and conduct heat better. We covered the other checks for these blocks in our article on what to verify before you commit to tooling.

Water-cooled blocks have coolant passages formed by sand cores. The walls between the bore and the coolant must be sound, so the grade has to give enough strength while still casting cleanly around the cores.

As a general guide:

Block type	Typical grade range	Main reason
Small air-cooled or agricultural engine	Lower to medium, for example EN-GJL-200	Fills thin fins and conducts heat well
Water-cooled automotive block	Medium, for example EN-GJL-250	Balance of strength, castability and machining
Tractor block that forms part of the frame	Medium to higher, for example EN-GJL-300	Carries frame loads as well as engine loads

Please treat this table as a starting point for discussion, not a rule. The engine designer’s drawing and test requirements decide the grade.

Where ductile iron fits in

Ductile iron, also called SG iron, has round graphite nodules instead of flakes. This makes it much stronger and tougher than grey iron, and it bends before it breaks. On the other hand, it is harder to cast in thin, complex sections, it damps vibration less and it conducts heat less well. For these reasons it is used less often for complete blocks and more often for parts that carry high loads, such as bearing caps and brackets.

If your design calls for it, we cast ductile iron to ISO 1083 grades from 400-15 up to 900-2, and their equivalents in other standards.

What a grade on the drawing does not tell you

A grade name on its own leaves gaps. When a drawing reaches us, we confirm the grade, the process and the feasibility, and these are three details worth adding if they are missing:

• Where the strength matters. A test bar result and the strength in a thick section of the block are not the same thing. If one area of the block carries a high load, the drawing should say so.
• Microstructure limits. For bores and running surfaces, graphite type and flake size matter as much as tensile strength. Our microscope checks graphite against these limits: types A and B with flake size 4 to 8, types D and E up to 20%, and free ferrite no more than 5%. Our article on graphite morphology explains what these types mean.
• A hardness range at a named point. A hardness range checked at a stated location is easy to verify on every lot, and it tells you a good deal about machining and wear.

How we confirm the grade

Naming a grade is half the job. The other half is proving it. At our plant:

• The chemistry of every heat is checked on a SPECTROMAXX spectrometer before pouring.
• Graphite structure and ferrite are checked under a metallurgical microscope.
• Tensile strength is tested on computerised universal testing machines rated 400 kN and 1,000 kN.
• Hardness is checked on our hardness testers.
• Every lot ships with a material certificate and an inspection report, and castings can be traced back to the heat they were poured from.

You can see these instruments on our quality page.

Frequently asked questions

What does ASTM A48 Class 30 mean?

It is a grey iron with a minimum tensile strength of 30,000 psi, about 207 MPa, measured on a test bar.

Is GG25 the same as EN-GJL-250?

Yes. GG25 is the older German DIN name for the grade that the European standard EN 1561 now calls EN-GJL-250.

Is a higher grade always better for an engine block?

No. A higher grade is stronger, but it is harder to machine and harder to cast in thin sections. The best grade is the one that suits the block’s loads, wall sections and machining.

Can you cast to my country’s standard?

Yes. We cast to the standard named on your drawing, whether ISO, ASTM, DIN, EN, JIS, BS, IS or another national standard. Our grades are cross-referenced across 15 standards.

Do you send a material certificate?

Yes. A material certificate and an inspection report go out with every lot.

Which engine blocks do you make?

We make air-cooled, water-cooled and combined air-water cooled blocks, along with more than 350 models of cylinder liners and sleeves.

Not sure which grade your block needs?

If you are planning a block and the grade is still open, we are happy to help. Please send your drawing or sample, the engine type and your expected yearly quantity through our enquiry page,  Buyers outside India can write to exports@esteemauto.com. If your drawing names a grade, we cast to it. If it does not, we will tell you which grade the application calls for.

About the Author

This article was written by the Esteem Auto team in Rajkot, Gujarat. We have run our own foundry since 1990 and cast grey iron from ISO 185 grade 100 to 350, with every grade cross-referenced across 15 national standards. The chemistry of every heat is checked on a spectrometer before pouring, and each lot ships with a material certificate. We make air-cooled, water-cooled and combined air-water cooled blocks, and we export to more than 50 countries.