Philips electric shaver heads should be replaced every 2-18 months depending on use and hair thickness. When replacing the shaver head be sure to use the same shaver head that was on the shaver previously. You can find this out by looking at the "HQ" number of each model. If you have an HQ2 be sure to replace with the HQ2. Although, the heads look similar they may not fit correctly and you may damage the heads and/or the shaver.
Philips and Norelco replacement heads are interchangeable, just be sure to select the correct replacement head!
 PHILIPS NORELCO HQ55 SHAVING HEADS NEW  US $22.99
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 Norelco HQ9 Philips ELECTRIC SHAVER HEADS hq 9 New US $18.50
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 Philips Norelco RQ10 shave heads New US $25.49
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