Replacement brush head compatibility: the dimensions, the test, and how to write the claim

Replacement heads are the most profitable line in the category and the one most often handled carelessly. This note covers the interface families that matter, the dimensions that decide a fit, why a compatibility claim is a legal question as much as a technical one, and how to test a head before you commit to a mould.

The interface families#

• Shaft-driven sonic. The head couples onto the handle's output shaft, either by pushing onto a splined shaft or by a screw-on collar. Retention is by friction or thread, and the head must be radially aligned or the sweep becomes asymmetric.
• Round push-fit. A circular head seats on a drive shaft and is held by a push-fit coupling. The critical dimensions are shaft diameter, coupling flat and seating depth.
• Proprietary snap-on. Brand-specific clips, magnets or bayonet fittings. Cheap to inject, expensive to reverse-engineer, and the most exposed legally.

The dimensions that decide whether it fits#

These are injection-moulded parts, and tolerances accumulate across the mould, the material shrink rate and the machine — so the same drawing produced in two tool shops can fit differently. That is why a first-article inspection and a fit test are not optional even when the mould is a copy of an existing one.

Writing a compatibility claim without creating a legal problem#

Naming another company's product on your packaging is a trademark question, not a technical one. Three rules keep most sellers out of trouble.

1. Use “fits”, not “for”, and never the other brand's logo. “Fits Sonicare handles” describes a fact. A pack carrying the Sonicare wordmark reads as an endorsement — or as counterfeit, depending on the market.
2. State the model list explicitly and date it. “Fits models X, Y, Z — confirmed [month, year]” is checkable. “Universal” is not, and it is the first thing a marketplace complaint will quote.
3. Keep the disclaimer short and visible. “Not manufactured by or endorsed by the brand owner; brand names are used for identification only.” Put it on the retail pack, not only in the manual.

This is a commercial-risk summary, not legal clearance. Have your own counsel review packaging that names a third-party trademark before you print — the cost of reprinting a container of packaging exceeds the cost of that review by an order of magnitude.

How to test a head before you commit#

• Fit test in quantity. Not one handle — at least ten, from different production batches, because handle-side tolerances move too.
• Retention cycles. Fit and remove the head a few hundred times on a rig and record pull-off force at the start, the middle and the end. A head that fits new and releases at cycle 80 will produce returns and one-star reviews.
• Tip wobble. Measure lateral displacement of the bristle tip under the brush's own drive. Anything beyond the handle's own baseline shows up as noise and uneven wear.
• Tuft retention. Pull-test tufts after the head has been through the hot-water or sterilisation exposure your channel will use.
• Review mining. Before tooling, read the one- and two-star reviews of the three best-selling compatible heads for that interface. It is a free failure-mode list.

What we can quote#

We tool and mould replacement heads for both shaft-driven and round push-fit interfaces, in filament grades from soft to medium-hard, with custom retail packaging. Head tooling is quoted separately from the per-unit price so that ownership is unambiguous. If you are evaluating a head for an interface we have not run before, send us the handle and we will return a fit report with the measurements above before you commit to a mould.