Bored And Stroked

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Revision as of 18:24, 9 January 2012

Boring and stroking an engine is the way to increase the size of the engine. Boring means to bore the cylinders larger and fit oversized pistons, while stroking means modifying the crankshaft for a longer throw. For example, Nissan created the A15 by stroking the A14 engine -- the cylinder bore is the same, but the crankshaft throws are bigger.

Bored

When the cylinder bores are damaged, the block is normally bored the next size up overbore (73.5 for an A12) and oversize pistons fitted. This is a repair process that uses commonly available service pistons. It adds very little capacity to the engine, and is not a performance option. For example:

A12 +0.5 overbore: adds 17cc

When building a racing or performance engine, the bore may be taken out several millimeters. This increases displacment by a large amount, and may require custom pistons. For example A12 with stock 73mm bore (1171cc) becomes 1372cc with 79mm pistons.

A13 Bored A12 Block

The early "A13" works race engines, as well as the 1978-up A13 engine used a standard A12 crankshaft, but a slightly larger bore. All A12, A12A and A13 engines (except 1974 A13) used an interchangeable crankshaft.

Factory A13(1980): 
Works A13 TS-A cup racing: 

A15 Overbore

1607.8 cc is commonly gotten from A15 blocks by a combination of 79mm bore and std 82 mm stroke

B120dat was working on this. See A14 engine2

Discussion: Bored Out an A15?

Overbore

Stock bore size
73mm: A10/A12/A13(1974)
75mm: A13(1980)
76mm: A14/A15

Pistons are available in 1/2 mm increments:

+0.5 overbore
+1.0 overbore
+1.5 overbore

Maximum Overbore

A12 blocks can usually be bored out to 79mm. 80mm may be possible but the particular block should be sonic-tested to verify.

79mm pistons
A12 stroke (70mm) with 79mm pistons: 1372.5 cc
A14 stroke (77mm) with 79mm pistons: 1509.7 cc
A15 stroke (82mm) with 79mm pistons: 1607.8 cc

80mm pistons
A12 stroke with 80mm pistons: 1509.7 cc
A14 stroke with 80mm pistons: 1548.2 cc
A15 stroke with 80mm pistons: 1648.7 cc

80mm overbore A14

With a cylinder liner, it *may be* possible to go up to 81mm sleeved before combustion chambers are off the mark from too much offset.

Stroked

A14 Crankshaft into A12 Block

Discussion: A14 crank into a12

You can not swap the A14 reciprocating parts (pistons, etc) into an A12 block, but you can use the A14 crankshaft as part of an overall larger capacity rebuild.

To do this you need:

• A12 block
• A14 crank
• A15 pistons
• Optionally, A12A rods, or new A13 (short stroke version) rods

You can use the A12 rods by boring the small end out to 19mm to take the larger wrist pins of the new pistons. Dodgeman chose to get the right rods from the US, as he didn't want to risk anything, but he bored out the 1200 rods in my 1200GX when he used the A14 pistons with the stock crank & they have been in service for about 300,000k without trouble.

The crank needs to be reduced in overall diameter in a lathe by a small amount to prevent the counterweights from striking the pistons at BDC.

Discussion

Stroked A15 Crankshaft

Offset-ground Crankshaft Journal

Discussion