These are handy, too:
https://smile.amazon.com/IRWIN-HANSON-BOLT-GRIP-Extractor-394001/dp/B0000CCXVZ/
About 10 years ago, I ended up needing a set to get the drain plug off my Monte Carlo ('04, with the piston slap-happy 3.4L V6) after the previous owner had almost completely rounded off the bolt head. I didn't have access to a welder, so this was after trying Quick Steel putty on a socket, which I was able to hammer into place, but it broke as soon as I started to turn it.
With the extractor sockets, the fucker came right out on the first attempt.
You should really have been saturating the starter end of the bolts with penetrant too, since that's where the threads are.
There won't be any threads in the transmission side of the connection because if there were, you'd never be able to tighten the starter to the transmission. That said, it'll be a major challenge to dremel those bolt heads off with their location. Its doubtful, but what happens if you manage to get the heads off and its the smooth parts of the bolts that are corroded to the transmission housing?
I seriously hope you started with a 6 point socket. There's pretty much zero reason for a mechanic's toolbox to contain a 12 point socket. Even a 12 point combination wrench is something to be avoided if possible.
I'd suggest another rounded-nut remover like https://smile.amazon.com/IRWIN-HANSON-BOLT-GRIP-Extractor-394001/dp/B0000CCXVZ , but there's not much room to swing a hammer to properly start it.
Been there done that. Those installers suck! I'd try an extractor like this.
If you do end up having to remove the balancer, I'd recommend finding some all thread, a thick washer (3/8" steel plate), and a nut. Then when you're done installing you can just use vicegrips to pull the all thread out. Or weld a nut to the end of it if you use it often enough.
https://www.amazon.com/IRWIN-HANSON-BOLT-GRIP-Extractor-394001/dp/B0000CCXVZ
The tool of last resort. They served me well as a mechanic for swollen wheel nuts on cars and rounded off u-joint bolts on trucks. Seat the socket with a hammer so the teeth bite, spin it off with an impact. Heat makes a difference.
Wait. You’re just trying to get the oil filter cap off? If you are just use these: nut extractor I’ve used them on turbos before and they work. Just hammer it on and twist it off
I've heard really good things about these, but never used them myself.... https://www.amazon.com/IRWIN-HANSON-BOLT-GRIP-Extractor-394001/dp/B0000CCXVZ/ref=sr_1_1?ie=UTF8&qid=1500352874&sr=8-1&keywords=irwin+nut+remover
I'd still go with the nut splitter...assuming you really need to pull the ball joint at all.
These are awesome. Get those shitty bolts out and replace them with something better. Also, I'd use a socket wrench.
IRWIN HANSON BOLT-GRIP Bolt Extractor Base Set, 5 Piece, 394001 https://www.amazon.com/dp/B0000CCXVZ/ref=cm_sw_r_cp_apa_ftr3ybCRYHJCW
well, the good news is that there's still a little bit left to grab on there. the bad news is that it's still gonna be a cork soaker to get out.
if you're lucky and have the room to get a "bolt-out" on there that would be my first choice. chances are there's not the room to get pliers on it, and really all vice grips are gonna do is maul it.
the way i see it, is using a bolt out first. and, failing that weld a nut to the stub and extract it that way. obviously, the bolt out is easier and DIY. but if you don't have the room then the motor has to come out.
Bolt extractor if you have rounded it. Amazon link
Edit to say if you are talking about getting the belt off, you put a square drive from a 1/2" ratchet in that square hole near the pulley.
I have a set of special socket heads with the hex inners in a kind or spiral - they're designed specifically for undoing rounded nuts and bolts and they work.
Something like this:-
http://www.amazon.co.uk/Irwin-Bolt-Grip-Remover-394001/dp/B0000CCXVZ
Irwin Industrial Tools 394001 Bolt-Grip Bolt Extractor Base Set, 5-Piece https://www.amazon.com/dp/B0000CCXVZ/ref=cm_sw_r_awd_1J-ivb0SYM6TM
This is what you need. Make sure to buy the right size. Hit the extractor on with a hammer. It doesn't sound like you have a lot of tools. So you'll also need to buy a racheting wrench, take care to buy the appropriate size as well.
Honestly, I'd ask a handier friend (that already owns the tools) to remove it for you so you don't spend $40-50 in tools to remove it.
I'd use a saws all to just take that problem end off then use a hammer to tap it out.
If it's threaded in, get a set of these (EVERYONE should have these, seriously life saving) and they'll let you loosen it out no problem. http://www.amazon.com/Irwin-Industrial-Tools-394001-Bolt-Grip/dp/B0000CCXVZ
This is a very easy one. There are sockets by the company Irwin which makes removals for these. Check it out: https://www.amazon.com/IRWIN-HANSON-BOLT-GRIP-Extractor-394001/dp/B0000CCXVZ
Or dremmel the head to fit a flat head bit on a impact .
Instead of ruining an expensive e-torx socket you're unlikely to have around anyway, use a socket actually designed for this eg. https://www.amazon.com/IRWIN-HANSON-BOLT-GRIP-Extractor-394001/dp/B0000CCXVZ/
These have been recommended many times here. I haven't used them myself.
Forgot to mention perhaps the best mix of easiest and most likely to work....
https://www.amazon.com/IRWIN-HANSON-BOLT-GRIP-Extractor-394001/dp/B0000CCXVZ/ref=sr_1_5
https://www.amazon.com/IRWIN-HANSON-BOLT-GRIP-Extractor-394001/dp/B0000CCXVZ You had to use something like this to get that bolt out correct?
Get bolt extractors
you could try one of those stripped bolt removers
http://www.amazon.com/Irwin-Industrial-Tools-394001-Bolt-Grip/dp/B0000CCXVZ
i have no idea if that would work or even be possible but it does exist...
Otherwise, grab yer drill.
Something like these will probably work. If not, you may need to weld a nut to it and turn it with a wrench.