How start sheet metal screws?

I need to replace the vent pipes for my hot water heater and furnace- they're 3", 4" and 6"diameter and made of sheet metal. The duct assembly is held together with short sheet metal screws-- and I'm not sure how to start the screws in the new vent system.
I always thought sheet metal screws were self-tapping but doesn't seem like you could push very hard to self-tap without bending or crushing the pipe- especially the large diameter ones. Do I need to drill a pilot hole or what?
Joe
--
=============
"This is my keyboard. There are many like it, but this one is mine."
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
You can drill a pilot hole, but if you get the hex-head screws that have a very sharp point, you can drive them using a cordless drill (or corded, it's just that most people seem to use the battery stuff these days). They will spin against the metal for a short time and then bite in. That is in fact what most people do. If you want to use screws with a phillips head, you will have to drill a pilot hole.
Actually both the drill and the "zip" screws bend the pipe somewhat, but it springs back.
Sheet metal screws are self tapping, in a sense, but most of them are not really self-drilling. These special sharp screws work ok with fairly light material like ductwork.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

You need a pilot hole. Pop rivets work well too.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.