What Does My Pump Motor’s Frame (FR) Rating Mean?

Your pump motor’s frame (FR) rating notes three key parts of your motor’s design which will help you match your current motor to a replacement. The three key parts are:

  • Barrel Size
  • Flange Shape
  • Shaft-End Type

As we know from previous blogs and how-to guides, the quickest and easiest way to find your pump’s motor replacement is to input the motor’s , (CAT) or number into Inyo’s search feature. However, over time pump motor labels fade, tear or smudge making the part or catalog number unreadable. Knowing your motor’s frame rating, in conjunction with its horsepower, service factor and amps, will help you find a match.

Without further ado, let’s get started.

Barrel Size

A pool pump motor casing (also known as the motor barrel) is made in two sizes. This is noted on the label as:

  • 48 = 5-7/8” Barrel Diameter (approx.)Pool Pump Motor Frame Types
  • 56 = 6-3/4” Barrel Diameter (approx.)

The 48 and 56 barrel sizes are interchangeable as long as the frame shape matches (see below). To switch from 48 to 56 or vice versa requires a simple adjustment to the motor stand. Depending on your pump model, this may be as simple as adding or subtracting an extension piece to support the motor, or you may have to modify the foot stand manually. “Manually modify” is fancy talk for taking a hacksaw to remove a piece or adding a rubber gap filler. Manufacturers have slowly but surely started to include the rubber gap filler in 48-frame motors to prevent the need for pool owners to rig a solution.

Frame Shape & Shaft-End Type

The number after the 48 or 56 on your frame rating denotes the shape of your flange size and also the type of shaft end. The five most common pool pump motor frame shapes are:

Century B2853-5Century UST1152-5b122 motor56z motorCentury BN24V1-5


  • Y (Square with Threaded Shaft) – Y-frame motors feature a square metal flange for bolting to the pump housing.
  • J (Round Flange with Threaded Shaft) – This is the standard for round flange motors for modern pumps. The J-frame features a threaded shaft for impeller installation.
  • C (Round Flange with Keyed Shaft) – These motors have a notch that runs the length of the exposed motor shaft that is called a key. The keyway allows for the brass key extension to be slid over the motor shaft. It is secured in place by two or three screws. It is mostly used on cast iron and bronze pumps. The keyed shaft motor is being phased out in favor of the standard threaded shaft.
  • Z (Round Flange with Flat Shaft) – The Z-frame uses a flatted shaft end on which the impeller is to be secured. Z-frame motors are usually made in batches to be fitted to a pump manufacturer’s specialty pump. The tricky thing about the flatted shaft is that the width of the flat side varies. So, not all flat shaft Z-frame motors are interchangeable.
  • Flex (Thru Bolt Design with Threaded Shaft) – An offshoot of the Y-frame motors, the Flex frame is used on above ground pool and spa pumps. The Flex-frame motors are usually labeled as 48Y, but will have the distinct pedestal at its base. The Flex is unique because it uses four thin partially threaded screws (known as thru bolts) that are slid the length of the motor through the back end and screwed into the seal plate.

Prefix Letter

I stated in this article’s introduction that there are three key points noted on the motor label, but on some labels there is a mysterious prefix. The letter is usually a Y or a P, for example:

  • Y48Y
  • P56J

I stated in this article’s introduction that there are three key points noted on the motor label, but on some labels there is a mysterious prefix. The letter is usually a Y or a P, for example:

Ignore the prefix letters, which indicate overall motor length, as there are no industry standards.  Check the length of the suggested replacement if a longer motor would be a problem in the installation

I receive questions about this letter often, but all I know is that it does not serve any true purpose for matching a motor. But since I care about all of you, I asked the manufacturers and this was their response. In summary, don’t worry about it.

If you have any questions on pool motor frames then we would be happy to help, feel free to give us a call at 1-877-372-6038 or email us at upload@inyopools.com If you liked this article then make sure to sign up for our Blog and get our Free 128 Page Pool Care Guide.


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10 thoughts on “What Does My Pump Motor’s Frame (FR) Rating Mean?

  1. Matthew,
    I have a motor that went out and It was a 3/4 hp Centurion by Century. I ordered a replacement which did not identify the frame. The dead motor was a 56Y and the company shipped me a 56J. Are these incompatible or will the new 56J work?

  2. Thank you young man, without dimensions I could not tell if the bolt patterns and face to shaft end are the same with J and Y frames.
    As charity, I’m replacing a 1hp Pentair motor and seals but prefer to come out of my pocket as light as possibly and help them with a more energy efficient motor, maybe 3/4hp.
    What say you?
    Thanks, JS

  3. Hello!! I’m in need of a HyFlo pump: .75CD
    The SN: F48M93A15
    AMB: 40 C

    V: 115
    A: 5.6
    HZ: 60
    FR: 48

    CODE: C
    INS: B
    PH: 1


    Hopefully you can help me out.

    Thank you,

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