Moderators: Random Orbital Bob, nev, CHJ, Noel, Charley

By J-G
novocaine wrote:... that centres it in your 3.75mm hole. ...

Most of what Novocaine says is good advice but I do hope that Annette did not drill a 3.75mm Ø hole for the M4 fine thread!

Even for the sloppiest 7H fit that is 3% too large. The correct core dia. is 3.46 - 3.57mm (5H fit) and that difference will be significant in this application where the specific issue is one of being gas tight.
By novocaine
my bad, I'm claiming tiredness. :) my head jumped to a 3.5mm hole, then I checked it on a table on the phone and didn't read the right value. should have got the tables out properly. :)

J-G you are 100% correct on that one.
By Gerry
If I was making a venturi out of delrin or even soft Aluminium I would make a jig to hold it out of a piece of MDF/Ply then just use a router and a trimming bit to flatten it off around the hole. It's a tiny area of material that you need to remove to seat the needle valve and that doesn't provide an airtight seal you will need something on there to stop a vacuum leak.
I would imagine at this point your venturi is worth more than the Magnum GP :twisted:

By J-G
novocaine wrote:J-G you are 100% correct on that one.

:) That's a first :)
I normally make a silly mistake when I pontificate! I did check this a number of times since my first reaction was based upon M4 Course rather than Fine :(

For about 8 years, when I worked for Herbert Small Tools, my life was 'Threads' - specifically, Coventry, Namco, Fette etc. Dies and Thread Rolls - so whenever I see a mention of threads, errors show up like a red mist :oops: