The Accordion Nominal v. Actual Sizes
Ever grab a 2×4 lumber stud and wonder why it feels suspiciously undersized? Or install piping labeled “½-inch” only to find leaks at every joint? You’ve collided with the *nominal vs. actual* divide—a manufacturing sleight-of-hand older than the Industrial Revolution.
This gap isn’t malice—it’s legacy. Sawmills historically sold rough-cut “2x4s” (nominal), planing them smooth to 1.5″x3.5″ (actual) post-purchase. Piping IDs shrank as walls thickened for pressure ratings.
Most every manufacturer uses nominal sizes. Nominal sizes are marketing poetry, typically a whole number rounded up from the “actual” size.  The takeaway, check spec sheets ruthlessly—especially when it comes to filters. Â
The Accordion by AFX Manufacturing uses nominal sizes for just 5 parts. All other parts are actual sizes +/- .125″ variance.
| Â NOMINALÂ | Â ACTUALÂ |
| 1m | 39.25″ |
| 36″ | 35.25″ |
| 20″ | 19.625″ |
| 18″ | 17.625″ |
| 29″ | 28.75″ |
Due to historical numbering, the sizes in the table are “nominal” by sku (1MS, 36S, 20S, 18S, 2972S). All other custom size skus are actual (+/- 0.125″ variance). If you require an “actual” size, send the inside dimensions of your frame. AFX will provide the appropriate size/part number. Â
Do you need a custom size?Â
Measure the inside of your exhaust frame for length and width (or height). Send us your actual inside dimensions and we will provide the manufacturer part number!