Aluminum is NEVER lighter than plastic unless the alum part has 60-70% less area than the plastic part. As an example, a simple mini-Z MA10 motor spacers are .5 grams heavier in aluminum and they're such parts where its hard to add or remove any bulk due to the design.
Xmod knuckles are very simple, so that remains true for them. The front/rear gearbox covers/replacements are defently heavier due to the sheer bulk of them.
Bottom line is that reinforcement comes at the price of weight and money.
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