I've read all of the existing lightsaber build discussions I can find on here, and I'm confused.

Say you have a Basic Lightsaber hilt, which has 5 HP. Then you put an Ilum crystal in it, which uses 2 HP.

Don't you only have 3 HP left for modifications then? The builds I'm seeing suggest that you still have the full 5 HP, which seems to contradict the statement that a crystal uses 2 of the weapon's hard points.

What am I missing?

As of Keeping the Peace there is a Cyclic Crystal Array mod that allows 2 (and with a further mod, 3) lightsaber crystals in one saber. It only costs 2 HP

So basically, the HP taken up by a crystal doesn't count?

The core rulebook says that the crystal takes up 2 of the hilt's hard points, so if you have a lightsaber with a crystal it only has 3 hard points. If you build your own and it doesn't have a crystal, then it has a default of 5 (of course, if you're using the lightsaber crafting rules, it may have less or more).

There are things that can increase the Hard Points for a lightsaber. If you roll a Triumph on your roll to construct the hilt, you can add 1 HP per Triumph.

If the character is a Sentinel, they may have Intuitive Improvements, allowing them to add up to 2 additional HP, and would also likely be able to add 1 more with Tinkerer.

All told, a basic Lightsaber hilt could potentially have 10 or more Hard Points if the owner is a Sentinel. But installing any sort of crystal will always take up at least 2 of your hilt's HP.

So it's possible that the builds you were looking at were wrong, or that they assumed they'd have access to one or more of the tricks I mentioned above.

OK, maybe they were assuming those tricks. Interesting, thanks!