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You don't have to drill under the bracket.
Here is the best spot. It's at the lowest point in the diff and yet you still miss the pinion race boss and bracket by a bunch.
Nice thing about a drain, you can flush to clean the inside a bit.
I think you've got the winning location. I don't recall ever seeing a case failure that began in that area.
I also like your bolt for a drain plug. Pipe plugs are tapered and act like a wedge in the hole. Seems to me that a straight thread would put less stress on the case and you could ensure a good seal with one of those copper brake line washers.
Are you planning to offer the drill, tap and bolt with washer, along with a pre-drilled cardboard template for $7.70? If you don't, I bet a kit from China will be available trough Corvette Freight in a couple of months.
Looks like you drill and tap a hole in the bottom of the diff with a paper template to mark where to drill. What I can't figure out is how you drill all the way into the diff without getting shavings inside the diff.
That's why you don't drain the oil. When you break thru the oil flushes out any chips, also the spiral flutes of the drill pull out 99% and the tap usually hold the small chips.
The hole saw is for making a through hole in the strut bracket. The hole in the bracket needs to be slightly larger than the pipe plug in the diff housing. The holes are spotted a bit off center so that you miss the ring gear.
Sucky-sucky machine does not work that well for this.
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My sucky sucky works very well. I just used the pump that comes on the fuel line for outboard motors. Attach some vacuum hose on each end and it pumps just about anything from anywhere. In saying that I have dumped diff oil plenty of times and never found anything but clean oil. No need to drill a drain plug.