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Regency Generators Knowledge Base

Diesel generator SAE size? What is a flywheel SAE size and why is it important?

What is an SAE Flywheel Size?

SAE stands for "Society of Automotive Engineers", this is an American organisation that sets standards for engineering professionals. Just one of the standards it sets are those of flywheel housings and flywheels on engines. Engine flywheel housings in the generator industry are usually directly coupled to single bearing alternators, which therefore must have the same sizes as the engine.

The flywheel housing is static and sits around the outside of the flywheel. The flywheel is the part of the engine that spins and transmits the power to the alternator.

Rather than everybody having different flywheel sizes, this standard breaks down the sizes into numerical values and therefore the choices into far more limited numbers.

These are expressed as two numbers, for example SAE 00 / 18, where the first number is the flywheel housing and the second is the flywheel.

Here you can see a image of the engine flywheel housing and flywheel. It is from a Perkins 4016-61TRG3 engine and has a SAE 00 / 18 coupling arrangement.

here you can see an alternator coupling on a Stamford HCI544D. This is a SAE 0.5 / 14 arrangement.

This diagram shows you the measurement and how the holes are arranged.

What are the SAE flywheel housing sizes?

The SAE flywheel housing sizes are 00, 0, 0.5, 1, 2, 3, 4, 5 and 6. In the Perkins engine range for example, the smallest engine we supply is a 9kVA. This engine uses the SAE 5 flywheel housing.

Here is a table of the housings and the respective sizes, in mm and the tapped hole sizes in inches (a metric bolt won't fit in the holes!)

SAE Number XD XB Number of Tapped Holes Tapped Hole Size
00 787mm 851mm 16 1/2"
0 678mm 679mm 16 1/2"
0.5 584mm 619mm 12 1/2"
1 511mm 530mm 12 7/16"
2 448mm 467mm 12 3/8"
3 410mm 429mm 12 3/8"
4 362mm 381mm 12 3/8"
5 314mm 333mm 8 3/8"
6 267mm 283mm 8 3/8"


What are the flywheel sizes?

The largest flywheel size is SAE 21 and the smallest 6 1/2. All dimensions in mm except hole size is in inches, as these are bolt sizes (again, a metric bolt won't fit in the holes!)

SAE Number XE XF XG Tapped Hole Quantity Hole Size
21 673mm 641mm 0mm 12 5/8"
18 572mm 543mm 16mm 6 5/8"
14 467mm 438mm 25mm 8 1/2"
11 1/2 352mm 333mm 40mm 8 3/8"
10 314mm 295mm 54mm 8 3/8"
8 264mm 244mm 62mm 6 3/8"
7 1/2 241mm 222mm 30mm 8 5/16"
6 1/2 210mm 200mm 30mm 6 5/16"