Drill Size

Boot

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I have to put a 10mm x 1.0 thread in a Brian Rupnow vertical engine head. I sent for the tap and it should arrive in a week. My problem is I don't have a 9 mm drill,which it calls out as the drill size.
I'm going to use a Letter "T" drill that I have. Is this close enough? I have every drill size known to man ,but not one Metric in my collection. Also I messed up my head by using the wrong 40° angle to cut it. I should of used the 40° from 180° not off of 90°. Stupid mistake I've made before in my 40 years of either working in a company machine shop or my own shop. I should of used a test piece instead of the real one. Some day I'll learn. Boot
 
I think either an S or a T would work. You might drill a test hole to see how the drill is going to cut then choose the closest fit.
 
A T drill will give you about 68% thread. The S drill will be almost a 100% thread. If it were my project, I might drill a few mm with the S and the remaining hole with the T to provide an easier start with maximum strength but I would expect that there wouldn't be a significant decrease going with the T drill all the way.
 
A 68% thread will give up very little strength, will be a lot easier to cut, and will be a lot less likely to break a tap. If more than a few threads are engaged, it is essentially full strength.
 
Boot, I finished building Brian’s Vertical Engine a couple months ago. I used a T drill for the 10mm X 1.0 threads.

 
I should of used the 40° from 180° not off of 90°. Stupid mistake I've made before in my 40 years of either working in a company machine shop or my own shop.

a good friend owned a sheet metal shop for many years. his employees were always making those kinds of mistakes
 
Boot, I finished building Brian’s Vertical Engine a couple months ago. I used a T drill for the 10mm X 1.0 threads.

Sleddog you did a fabulous job on building that engine. I can only hope mine comes out as good. I start making another head today.
 
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