Tapping a Hole - Numpty Question

[virden]

I am intending to fit a breather elbow into the side of my X/F cast alloy rocker box.   The brass elbow thread is 3/8"  and the tap thread is the same width.    Question: what size of hole should I drill in the casting?  I assume given the thinness of casting wall, 5mm, I should adopt aircraft practice and start with a small hole, increasing size in stages?

Thanks

[SM25T]

Example .... M6 x 1mm pitch thread ... you drill a 5.0mm dia hole. So you need to know the pitch of your 3/8" thread. You are intending to tap the casting, rather than drill a clearance hole?

[virden]

Its a BSPT thread, hope that answer the question.  I intend tapping the casting because of the proximity of the rockers and push rods.  Also I am concerned that there is a danger of any nut in there vibrating loose

[ECR]

Google: tapping drill sizes for bspt threads

[SM25T]

So your elbow has a 3/8" male taper thread. Assume you are going to drill and tap a parallel thread. Spot of thread sealant or loctite.

 

[SM25T]

Table shows drill sizes. 

[SM25T]

Resolution not great ... here is a link. Scroll down a bit for the table. You are going to tap a parallel thread ... so you want the black column.

See https://www.newmantools.com/tapdrill.htm

[elie boone]

If you drill in the middle of the valve cover make sure it's dead center between the valves as there isn't much clearance between valve cover and rockers.

[virden]

 Many thanks all.

Elie  I am proposing to drill at the front end of the cover, directly opposite the 2nd rocker bearing block, this seems to offer most room  and easiest routing for the breather  under the heater pipe