[07DX]

The Turbotec 26T pulley arrived in the mail yesterday. It came with a tap for the trans flange to enlarge the holes to accept the new larger hardware that came with the kit.

Ive never tapped anything in my life. Do I drill out the holes first? Or just go to town with the tap and hope for the best?

Any help is appreciated.

Rodric

 

[myvrodrocks]

I am sure that you have to drill first. If not, the tap might grab the wrong groove and nopt pull straight. But it depends on the size of the hole...there is a special size bit for pre-drilling for a tap. Nowtsure about SAE but metric is like .7 So for a M8 thread, you drill with a 6.7 bit, for a M6 you drill with a 4.7.

 

[rc4man]

Drilling an existing hole, the bit will tend to self center and follow the existing hole.
This is not to say that you can't get it through crooked but generally the bit will go true enough to do the job.
Of course you can always run the tap in crooked as well which is more likely for most people.
Also because you will be drilling with only the outer most lip of the drill flutes there will a marked tendancy for the drill to "grab" and pull into the work piece with possible damage to either the work or the drill bit.
A bit modification technique often used for drilling cast iron, brass and other materials that have a tendancy for grabbing the bit is pictured below. It is basically flattening or reducing the rake angle of the bit.
Use car to ensure that you do not make the edge any more than flat. Genrally just reducing the angle by half is sufficient.
Be careful.