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Cylinder Block

5 min read

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Engine Construction
Building an engine
Current
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Cylinder Block
Casting and boring

The cylinder block is the single casting that holds the cylinders, the cooling passages and the crankshaft bearing supports, and it is the part of an engine a settlement is least able to make from scratch. Casting it sound needs cores for the water jacket and even walls, and boring the cylinders round and straight needs a rigid machine; for most settlements the practical route is to salvage a block and check its bores for wear, since a 1 mm overbore of an 86 mm cylinder raises displacement by 2.3%.

Part of Internal Combustion

What the block does, how it is cast and bored, how to measure a worn bore, and the failure modes of cracking, warping and wear.

One Casting Does Five Jobs At Once

The block carries the cylinder bores, the passages for coolant or the fins for air (see Air Cooling and Water Cooling), the bearing supports for the crankshaft, the mounting for the head, and the oil passages. A flaw in any of these shows as a leak, a seizure or a crack.

Cast iron is the traditional block material because it casts well, machines to a good wearing surface and tolerates heat. The corpus covers the casting process in Sand Casting and Casting: green sand around a reusable pattern, metal poured through a sprue, and a riser to avoid trapped air. A block needs more than that: cores that form the hollow water jacket have to be supported in the mould and must not shift, or the wall thickness varies and one side becomes thin. Aluminium blocks are lighter and cool well, and the corpus alloy article, Aluminum Alloys, warns that remelted mixed scrap rarely keeps the strength of a proper alloy, so an aluminium block cast from scrap is a risk.

Boring Is Where The Engine Gets Its Seal

After casting, each cylinder is bored round, straight and to size, then finished smooth. The corpus describes the same operation for steam cylinders in Cylinder Boring and the finishing of bores in Boring and Finishing: a rigid bar, light finishing cuts, and checking against the shaft or gauge. An engine’s rings must seal against that wall, so out-of-round or tapered bores leak gas past the piston.

The corpus tolerances for a steam cylinder are not tolerances for an engine, which runs hotter and sealed with rings fitted to a maker’s figures. This page gives no bore tolerance. A rebuilt engine takes its fits from the maker’s data or from a measured good engine of the same type.

What You Need

Method: Measure A Salvaged Bore

Step 1: Clean the bore and check for ridge, scoring and rust. A hard ridge at the top of the piston-ring travel shows the bore has worn below it.

Step 2: Measure the diameter at the top, middle and bottom, and at two directions at right angles, in each place.

Step 3: Read the differences. The difference between top and bottom is taper; the difference between the two directions is out-of-round. Compare the largest reading with the maker’s bore size if you have it, or with a fresh cylinder of the same engine.

Step 4: Decide. A bore with only light wear can be re-ringed. A bore with deep scoring, large taper or out-of-round needs boring oversize with a matching oversize piston, or a sleeve. When it is wrong, the engine uses oil, blows smoke and loses compression.

Step 5: Water-test the jacket. Block all openings, fill with water, and look for weeping. Cracks from freezing show here.

Worked Example: What An Overbore Costs In Displacement

Take the corpus 2-litre four-cylinder with 500 cc per cylinder and a bore of 86 mm from Engine Modifications.

  • Bore area: π ÷ 4 × 86² = 5,809 mm².
  • Stroke implied by 500 cc: 500,000 ÷ 5,809 = 86.1 mm, a nearly square engine.
  • Bore 87 mm after a 1 mm overbore: area scales as (87 ÷ 86)² = 1.0234, so displacement rises 2.3% to 500 × 1.0234 = 511.7 cc per cylinder.
  • If the combustion chamber volume were unchanged at the corpus 71.4 cc, the compression ratio would rise from 8.0 to (511.7 + 71.4) ÷ 71.4 = 8.17.

So a small overbore slightly raises both power and compression, and the oversize piston must be paired with it. The chamber volume is the corpus model, not a measured head.

Common Mistakes

  • Boring one cylinder and not the others. The pistons then differ; the engine runs rough.
  • Not checking the deck for warp. A head gasket seals only against flat surfaces.
  • Ignoring a frozen jacket. Water expands when it freezes and cracks a block; drain before frost.
  • Using unknown scrap aluminium. The casting can be weaker than it looks.

Safety

Foundry work, heavy castings and the test fluid all hurt

Molten metal poured into damp moulds can throw metal; keep moulds dry and wear protection, as the corpus casting articles advise. A bare block weighs enough to crush a foot, so lift with help and a stand. Boring and honing throw metal and abrasive dust; wear eye and breathing protection. A water test of the jacket is done with water, not air.

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