[jagking]

the thread pitch is slightly larger than the old pitch if not the same.



Thats your problem slightly anything wont work it has to be exactly same !!!
M12 coarse pitch is 1.75 Fine is 1.5 or 1.25
all we really use over here now is metric and i have never seen a M13 bolt its M12 or M14
1/2 whitworth with is the old British standard had 14 threads per inch and a pitch of 1.814 invest in a pitch gauge this will help and wont cost the earth
are you sure that its not 1/2" with is 12.7000mm m12 will pick up in it and allow for the difference in pitch

[Dave5605]

Bottom line here is the bolt and the tapped hole are not the same size.
Forget the dremel, forget the drill. Forget drilling out the first couple of threads. YOU NEED THE RIGHT TAP to match the head bolt.

There are a few metric and US threads where if you try to match up mixed nuts and bolts they will SEEM to go together. However they, in fact, are not. The nut is usually a little bigger. I'm sure you can run in to the same scenario with cheap taps and dies. its also easy to thread in the wrong tap in to the existing threaded hole in certain ranges of metric/us threaded holes.

Be very careful that you don't screw up the threads in the block enough that you have to put a helicoil in.

What kind of motor are we talking about here?
Does the bolt screw in to the other holes in the block for the head? If so then its a good chance you have tapped the offending hole wrong.
Also, a real dumb question here, you aren't trying to use some galvanized bolt from the hardware store are you?

[Migman]

Get yourself a thread pitch gauge. Make sure you are using the same pitch bolt as the tapped hole. If the hole is in aluminum make sure the the threads are not galled. If they are use a threadsert or helicoil. I've never seen a clean tapped hole that the correct size bolt would not go in by hand turning only. Something is wrong somewhere. A flash light and a magnifying glass would be a good starting place.