As far as I know, they are the same as VFR400 (NC21), CBR400 (NC19 and 23), and many other small capacity Hondas of that era, and I believe all have plastic pistons.

I find it hard to believe they would swell, and more likely that, as Al says, it's muck and/or corrosion in behind the seal (in the goove).

If you have used aftermarket seals, there is a (small) possibility you have been supplied the incorrect ones, or that maybe they are not quite the same size as OEM.

As Neal says, the pistons should press in easily by hand.
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Andy.
NSR-WORLD.COM

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