your mech dosn't know what he's doing.    it sounds like he tried to put it in with a hammer and broke it so he made that shaZam! up.

i will agree with the T.B.
shoud be a NEG 0.004 fit if not more(less)

My Clymer manual is on loan to a friend but He looked in it & said that it said something about the hole having to be bored out 12mm.  Can anyone confirm this? Thanks

Yes, the Clymer manual mentions reaming the guide hole with a 12.3 mm reamer. This is technically different than boring but that is probably what your mech meant. The new guide is a specific size and the hole must be at a particular diameter in order to achieve a fit that doesn't allow the guide to loosen or distort. Love the little GS, Acura (Integras are cool), and this forum.

C........

Replacement guides are oversized, so the hole does need to be reamed out.
The stock guides are cast iron and have a groove for a clip. The clip bottoms out in the head when the guide is inserted so the depth is correct.
This is a weak spot on the guide, so it can break when it's hammered in.
If the guide could be pressed in it would be less likely to break. The angle makes this tricky though.
The valve seat will have to be re cut after the guide is installed.  The valve will be off a hair, so you need to do a valve job to get the valve to seat properly again.