If you can weld, weld a nut to the broken stud/bolt, don't worry about welding the nut or stud to the block......you can't.....the block is aluminium, you will be welding a steel nut to a steel bolt, the heat will help to free off the seized bolt and the nut will give you something to hold, the trouble with drilling is that the broken bolt is harder than the aluminium block and its easy to go 'off centre', as a by line most of the threads in an O.M.C. are U.N.C.

There are plenty of UNF thread available,any good engineering company will be able help you.
I would use 8.8 high tensile bolts. Your biggest problem is going to be extracting the broken stud without damaging the threads in the hole.If there is an engineering works near hand I would use them as they would have a lathe.Or if the threads do get damaged,drill the hole out and use a helicoil insert.
http://sizes.com/tools/thread_american.htm Might pass a few minutes for you.
https://www.spaldingfasteners.co.uk/cat ... -unf-bolts

I agree with all that's been said before but would add that, as se7en mentioned, heat will help. I my opinion repeated heat/cool cycles along with penetrating fluid (a 50/50 mix of transmission fluid & acetone is reckoned to be the best) is the best way to remove seized/snapped off studs or bolts. The aluminium will expand more than the steel stud/bolt making it easier to remove. If you are using a propane torch you will be fine but be careful if you use oxy/acetylene as it is hot enough to melt the ali.

It also helps to give the snapped of bolt/stud a good clout a few times (don't break the casing though) along with all the above.

Good luck & be PATIENT!

Cheers, I'll have another go at removing them then instead of drilling, my welding isn't up to much and closely resembles pigeon s@#* so I probably won't attempt that
I've got some screw extractors so perhaps a small hole in the bolt and one of them would work best? Plus some of them are probably long enough to get some mole grips on so I'll try that too.
Thanks for the link haventaclue that will prove handy if I don't have any luck removing them!
In the mean time out with the blow torch tomorrow!

Worthwhile investment for the workshop http://www.ebay.co.uk/itm/Zeus-Precisio ... 48389edb50 all you need to know about screw threads plus other handy info, decimal equivalents, how to solve triangles, bend allowances etc etc

Christ I haven't seen one of them for a while! That takes me back to college days you're right though that's probably a worthwhile investment. Whilst on the subject of measuring what's the best way/tool for measuring the bore both for size and ovality?

I have a copy in the toolbox and one in my work desk and find it quicker than tinternet.

For bore measuring it depends on your budget but for ovality and bore an internal micrometer wil work well http://www.ebay.co.uk/itm/Mitutoyo-50-2 ... 27d470743a

A venier calliper will give you a reading but not far into the bore. You could also use internal callipers or a telescopic Gauge ( http://www.ebay.co.uk/itm/MITUTOYO-TELE ... 1e7ca070b9) measured using a micrometer or vernier calliper but these require more "feel"

Alacrity wrote:I my opinion repeated heat/cool cycles along with penetrating fluid (a 50/50 mix of transmission fluid & acetone is reckoned to be the best) is the best way to remove seized/snapped off studs or bolts.

Any particular method recommended for this Geoff? It's just a transmission fluid/acetone mix combined with a naked flame sounds a bit scary
Do I apply that after heating?

Cheers

A quick update on progress ... I managed to remove 3 broken bolts with blowtorch-ing/oiling these were the ones with a bit of thread still protruding so I managed to get some mole grips on them. The others however I couldn't remove with screw extractors so today I drilled and helicoiled them. I used the exhaust plate as a guide for the drill so they didn't wander off course, the drill size for the helicoil was the same size as the clearance hole on the exhaust plate so it worked out pretty well
End result is all thread issues are now sorted ... Thank god!