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Austin Healey Piston Sets: Choose the Right Set | BN1-BJ8

Austin Healey piston sets for Big Healey engine rebuilding

Choose the right Austin Healey pistons before the block is machined

Austin Healey Piston Sets at a Glance

Key takeaway
Choose pistons by engine type, piston specification and measured bore size, not by the car model alone. Standard and oversize pistons may fit the same model but are not interchangeable without the corresponding cylinder dimensions.
Who this guide is for
Owners, restorers and engine builders working on four-cylinder and six-cylinder Big Healeys from Austin Healey 100 BN1-BN2 through to 3000 BJ8.
Main benefit
Identifying the correct range and size before final machining helps avoid ordering the wrong pistons or finishing the cylinder bores around an assumed size.
Important technical facts
Standard, +0.020in, +0.030in, +0.040in and +0.060in piston sets are available across the main replacement ranges. The 100M replacement range uses an approximately 8.6:1 high-compression piston specification. The 100/6 range shown here starts at engine number 40501.
The short answer
Have the engine measured first, identify the correct piston range, obtain the pistons, then establish the final cylinder dimensions using the actual pistons and the required running clearance.

The model badge only answers half the question

Choosing pistons for an Austin Healey engine sounds straightforward until the words standard, +0.020, +0.030, +0.040 and +0.060 enter the conversation. Then there is the separate high-compression piston range for 100M-type builds, an engine-number consideration on the 100/6, and forged piston choices for more specialised engines.

That means two apparently similar Healeys can need different piston sets. One BN2 engine may still be suitable for standard-size pistons, another may already have been rebored, and another may be built around the higher-compression piston specification. The badge on the bonnet is helpful, but the cylinder block gets the final say.

The important decision is therefore not simply “which pistons fit my Healey?” It is “which piston specification and size does this particular engine need?”

Start with the Austin Healey engine you are rebuilding

Use the model and engine details to choose the correct piston family first. Bore size comes next.

Austin Healey 100

BN1-BN2 standard piston range in standard and oversize options.

View 100 Pistons

Austin Healey 100/6

Six-piston sets for BN4-BN6 engines from engine number 40501 to 77766.

View 100/6 Pistons

Austin Healey 3000

Six-piston sets for BN7, BT7, BJ7 and BJ8 engines.

View 3000 Pistons

100M High Compression

Approximately 8.6:1 replacement piston specification for suitable BN1-BN2 high-compression builds.

View 100M Pistons

Forged Pistons

Separate 88mm, 84.5mm and 85mm piston routes for planned performance engine builds.

View Forged Pistons

What standard, +.020, +.030, +.040 and +.060 actually mean

The oversize figure describes the nominal increase in piston diameter compared with the standard piston size. A +0.020in piston is therefore intended for an engine being machined for that corresponding oversize. The same principle applies to +0.030in, +0.040in and +0.060in.

It does not mean that every worn engine should automatically move to the next size on the list. The cylinders need to be measured for wear, taper, condition and evidence of previous machining before the required piston size is chosen.

  • Standard size: for an engine whose cylinders remain suitable for the standard piston size.
  • Oversize: for cylinders requiring or already machined for a larger piston size.
  • Measure before ordering: an engine that has been rebuilt previously may already contain oversize pistons.
  • Use the actual replacement pistons for final machining: the finished bore and piston-to-bore clearance should be established around the pistons being installed, rather than the nominal oversize alone.

That last point is worth getting right. Removing metal from a cylinder block is rather harder to undo than changing an online basket.

Austin Healey 100 BN1 and BN2 piston with piston rings

Austin Healey 100 Pistons | BN1-BN2

The Austin Healey 100 replacement piston range covers BN1 and BN2 four-cylinder engines in standard size and four oversizes. These sets are for engines retaining the standard piston specification rather than the separate 100M-type high-compression specification.

Each set includes four pistons, piston rings, gudgeon pins and circlips. Choose the piston size only after the cylinder bores have been measured and the required machining established.

Choose Your Piston Size

Choose the standard-size set where the bores remain suitable for standard pistons. If the engine requires a rebore, select the oversize confirmed for the engine build.

Unsure which size is already fitted? Check the existing pistons and cylinder dimensions before ordering, particularly if the engine has been rebuilt previously.

High-compression piston set for Austin Healey 100 and 100M engine builds

Austin Healey 100M High-Compression Pistons

This replacement piston range is for Austin Healey 100 engines being built to the approximately 8.6:1 high-compression piston specification. Standard, +0.020in, +0.030in, +0.040in and +0.060in sizes are available.

The 8.6:1 figure describes this replacement piston range rather than the original factory 100M compression ratio. Each set includes four pistons, piston rings, gudgeon pins and circlips.

Choose Your High-Compression Piston Size

Choose the size to match the measured cylinder bores and planned machining. These are the high-compression alternative to the standard BN1-BN2 piston range.

The final installed compression ratio also depends on the complete engine build, including combustion-chamber volume, gasket thickness, deck height and previous machining.

Six-cylinder piston set for Austin Healey 100-Six BN4 and BN6

Austin Healey 100/6 Pistons | BN4-BN6

This six-cylinder piston range is for Austin Healey 100/6 BN4 and BN6 engines from engine number 40501 through to 77766. The engine number is important here, particularly on a BN4 where the model designation alone does not establish that this piston range is correct.

Standard, +0.020in, +0.030in, +0.040in and +0.060in piston sets are available. Check the existing cylinder dimensions and engine number before choosing a set.

Choose Your Piston Size

These sets cover BN4-BN6 engines from engine number 40501 to 77766. Once the engine range is confirmed, select the piston size required for the measured bores.

If the engine number falls before 40501, confirm the engine specification before ordering rather than assuming the later BN4-BN6 piston range is suitable.

Six-cylinder piston set for Austin Healey 3000 BN7 BT7 BJ7 and BJ8

Austin Healey 3000 Pistons | BN7, BT7, BJ7 & BJ8

This six-cylinder piston range covers Austin Healey 3000 BN7, BT7, BJ7 and BJ8 engines. Standard, +0.020in, +0.030in, +0.040in and +0.060in sizes are available for engines being rebuilt to the corresponding bore size.

The model identifies the piston family, but the cylinder measurements determine which size is required. An engine that has previously been rebuilt may already contain oversize pistons.

Choose Your Piston Size

Choose the standard-size set where the bores remain suitable for standard pistons, or select the oversize established from the engine measurements and planned machining.

If the engine has been rebuilt before, check the existing piston markings and bore dimensions rather than assuming it remains at standard size.

Forged piston set for Austin Healey 100, 100-Six and 3000

Forged Austin Healey Pistons for Performance Engine Builds

Forged pistons are a separate option for engines being built around a planned performance specification. Unlike the standard replacement ranges above, the choices here are defined by their intended piston diameter rather than a standard-size set followed by conventional oversizes.

Confirm the planned finished bore and complete engine specification before machining or ordering.

Austin Healey 100 | BN1-BN2

Four-cylinder forged piston set for engines being built around an 88mm bore.

88mm Forged Piston Set Austin Healey 100 BN1-BN2

Austin Healey BN4-BJ8

Six-cylinder forged piston sets are available in 84.5mm and 85mm sizes.

Forged piston diameter should form part of the planned engine specification. Confirm the required finished bore before machining the block.

Get the pistons involved before final machining

Once the correct piston family and nominal size have been identified, the engine machinist should work with the actual replacement pistons when establishing the finished cylinder dimensions and piston-to-bore clearance.

This matters because “+0.040in” identifies the nominal oversize. It is not a substitute for measuring the piston being fitted and following the appropriate clearance specification. Boring every cylinder to a number on a catalogue page and meeting the pistons afterwards is doing the job in the least useful order.

A piston ring compressor is also available for installation. It covers piston diameters from 53mm to 125mm and holds the rings within their grooves while the piston is introduced into the cylinder bore.

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