I'm making a replacement battery box and wondering how big the drain holes are - I know some sort of rubber plug goes in there but I can't remember what size (and I don't have a car to measure). Can anyone help me?

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Just a suggestion,

If your making your own, rather then make it to the standard size, which may be hard to find new plugs for? Why not find some readily available plugs and make the holes to suit?

The one thing I don't like about the replacement boxes currently on offer is that they have three drain holes, rather than the original two.
If you could knock up exact copies, there could be a market for them!

My replacement battery box has no drain holes.
Probably should drill one in eh?

Especially, if your boot leaks!

Why don't you drill a 1/4"hole in the 4 corners of the bottom of the box section?

Only need one drain hole if its in a corner, it'll drain when the cars parked on the right angle.

Ah, too smart by half! Will have to look into this, I would like to keep it all simple and easy to do / replace. Will drop by Bunnings at some point.

What size are plugs in the firewall and front floorpan? 1 inch?

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Floorpan ones in my car are only half inch I think, definitely not 1 inch.
Perhaps a good plug to use would be the ones that cover the grease fitting on the rear trailing arm, thats about the right size. Put some newies on the covers where you see them and use the old ones on the battery box.

Another way of looking at the problem is.... what size drill bits do you own?

I would make them half inch because that is the biggest drill bit I own.

Well, will a 1/4" hole drain any better than a 1/2" hole? How much water do you have in your boot!?