So it has - well spotted that man!

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RT325 had mentioned checking into the Yamaha 490 piston as a possible donor ... back in post # 5.
He's correct, and it is doable with a little machining work.

1. The 490 piston has the same 18mm pin size and deck height, but is 10mm taller . . . So the piston skirt will need to be shortened by that amount to match the 400's height.

2. The next problem, and a bit more of a challenge to over come, is relocating the ring end locating pins, so that the ring gaps match that of the 400 piston. The 400 pins are 32mm apart, while the 490 is 42mm apart.

This must be done correctly by someone that has the equipment and experience to do it properly.
Which means, heating the piston up and then jig drilling the new pin holes (not free hand) . . . then, while still hot, pressing the new pins in with a slight interference fit.
Naturally, the new pins must be the same exact size to fit the rings, and the old pins must be either removed and their holes plugged, or "carefully" ground down to match the depth of the ring lands without damaging the ring grooves in the process.
If not done correctly, you risk the pins coming out and doing damage when the motor heats up and the piston expands.

If you can lick those two problems, then a 1st oversize (87.25) Yamaha 490 piston will work.

If you lived here is the states, then picking up a used cylinder would probably be the way to go, but availability and shipping expenses may not make sense in other parts of the world.

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