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Clearance Holes

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vcutajar:
Hi guys

I have another silly question which might be obvious to everybody except myself.  Bear in mind that when I started this hobby I did not have an engineering or machining background.

Today I was drilling the clearance holes in the rear cylinder cover of the Corliss and was thinking of what type of fastening I should use for this model.  Studs or bolts.  I am sort of leaning on using studs but the last time I tried using homemade studs (on the Kiwi) the experience was not positive.  I could not line up the two halves of the Kiwi crankcase with my studs.  When I used threaded rod instead I had no problem at all to line them up.

Which brings me to my question which portrays my level of ignorance.  What should be the clearance hole for a 3mm stud??

I have always assumed it to be 3mm but now I am having doubts about it.

Regards

Vince

peatoluser:
I generally use a drill 0.2mm larger than the nominal thread diameter for small sizes, but there's no hard and fast rules about clearance sizes. A cylinder cover with 10 studs might need larger clearance holes just to be able to fit it over them without it binding, whereas a cover with just 4 studs, 0.2mm might be just fine.
interestingly, commercial counter bores , for socket head screws often have a guide dia. 0.5mm bigger than the nominal thread dia. , so clearance hole has to be at least 5.5, 6.5 etc. 

if your studs are binding in the holes and commercial rod isn't, it sounds like the threads are not square with the stud dia.

yours

peter

Jasonb:
Or more likely the commercial stud is undersize as it often is.

J

Ramon Wilson:
Yes, agreed, (edit - with Peter) and you'll probably find Vince that your studding is slightly under 3mm dia too which might be another reason that your studs were binding on the Kiwi.

As Peter says the actual clearance isn't that important unless its a 'fitted' bolt. Anywhere between 0.2 - 0.4 should suffice on most things.

Just in case - If you don't have an accurate means of dividing both parts separately then drill the cover with the clearance hole first and use as a drill jig to spot the holes to a depth such that the lip of the drill takes out approximately one thread - much neater and easier to start the tap. I would still do this if you drill the tapping holes separately - it's always good practice to take out that first 'thread' before tapping and not after  ;)

Regards - Ramon

smfr:
Or you could clamp the parts together and and drill tapping size through the cover into cylinder, then open out the holes in the cylinder cover to clearance size. Don't forget to mark the cylinder cover if necessary so it goes back in the same orientation. Also, don't make the (easy) mistake of drilling clearance size into the cylinder  :hammerbash:

Simon

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