I don’t have a 78t spur gear but a 72t spur gear weighs 2.7 grams.
42.1 g without a spur gear means you have too much fluid in the diff. Don’t drive the car before you have fixed this; too little fluid is not a serious problem but too much can damage the seals.
This doesn’t make sense.
If the weight is off, the amount of fluid is wrong even if the level appears fine. There might be an air bubble trapped somewhere.
You don’t necessarily have to weigh the diff to get a good build but when the amount of diff fluid is correct, the weight will match the...