Q: Help! I need more bobbins for my sewing machine, but they all look alike in the store and online. Does it matter what size I get, and if so, how do I know which ones to buy?

—Harriet Brown, Greenwich, Connecticut

A: Carol J. Fresia, Threads’ senior technical editor, has gone round and round on this issue herself: It definitely does matter what size you use. The wrong size bobbin can cause jamming, poor stitch quality, or damage to your machine. Start by checking your machine’s manual. The bobbin size and type should be listed there. If it isn’t, but you have one of the bobbins that originally came with the machine, take it to a local machine dealer (preferably one who sells your machine brand) and ask for the same type.

I realize this may be easier said than done, since many of us don’t live near a sewing machine dealership and are obliged to buy accessories online. If you find yourself in this position, do some research. SewingPartsOnline.com carries a remarkable range of bobbins, and you can search by machine brand and model to find the type that fits your machine.

If your machine is a newer model intended for home use (not an industrial machine or long-arm quilting machine), there’s a good chance that it takes one of three commonly available styles. Class 15 (also called A style or SA156) is 13/16 inch (about 20 mm) in diameter (approximately the size of a nickel) and 7/16 inch (about 12 mm) high. Its top and bottom, called the flanges, are flat. The L style (sometimes referred to as SA155) has the same diameter and flat flanges but is only 5/16 inch (about 9 mm) high.
Other bobbins you’ll find in many chain fabric stores are the Class 15J and Class 66. These are close in diameter to the Class 15 and L-style bobbins but have slightly convex flanges. Though these four types don’t look substantially different, they are not interchangeable.

An online search for bobbins may turn up the temptingly named “universal bobbin.” Avoid these, unless the description clarifies the bobbin style (usually a Class 15, but not always). You’ll also find a vast selection of non-original manufacturer bobbins. These are not necessarily dangerous to your machine or poorly made. If you have doubts, order from a reputable company with a good return policy.

Many bobbin styles are available in metal or plastic. Choose the same material as the machine’s original bobbins. The bobbin thread tension may be sensitive to the bobbin weight, and you’ll get more satisfactory results with the version intended for the machine.

Some European sewing machine brands, such as Bernina and Viking, use brand-specific bobbins. Don’t substitute other sizes. If your machine is vintage, seek advice from an online community that focuses on your machine type. Should you buy or inherit an older machine, it may come with a mysterious assortment of bobbins that may or may not fit. Before installing one, exercise due diligence so you don’t damage your heirloom machine.