I drilled the exhaust manifold with a 21/64 drill bit as the directions state but there is NO way a 1/8-27 tap is going to come close to fitting in that hole. Every chart I have says you should use a 21/32 drill...which of course I don't have.

So before I really make a mess of things, who has done this install before and what size bit did you use?

 

my chart says Drill size "R" for a 1/8 NPT

but you probably don't have that size ether

 

Nope... the thing is that I don't see anyone complaining about this in posts here or on other forums. I've checked my tap about a dozen times...it's stamped 1/8-27...unless it's wrong. Of course I choose to do this on a Sunday...

 

1/8" NPT -------------- 21/32"

http://members.aol.com/pullingtractor/taps.htm

Unless I'm reading something wrong.

 

have you ever taped anything before???

 

I prefer duct tape ...

About a million times... ran an engine shop for 10 years. But I will confess to not memorizing drill size to tap combos.

A 1/8-27 won't even come close to starting in the hole.

 

I just check it with a micrometer you need drill size 21/64.....21/32 is way to big!!
21/64 is just a hair smaller then drill size "R"
remember do not tap it all the way through you need the tapper of the NPT to make it thread in there tight and if you go all the way in it may never tighten up

 

here is a tap chart that is correct ....
http://www.engineersedge.com/tap_drill_chart.htm

 

I used a 5/16 Drill Bit..??? 1/8 Tap worked great!!!

Shane

 

I used whatever Quads directions said and it worked perfectly. Double check your tap size.

 

This tap MUST be mis-marked... guess it will have to wait until tomorrow.

Thanks guys...

 

Instructions from Quadzilla Commander say 21/64ths drill size for 1/8-27 or 1/8 NPT tap. It worked for me.

 

I followed Quad's instructions and it worked fine. Pretty sure I wore that bit out 'cause I'm a ham-fisted idiot and didn't use any oil or such basic machinist-type techniques!

 

Did you ever get this thing tapped? I used 21/64" also, and it worked perfectly.

 

Yeah, I was wondering if he got it also. Just goes to show you what kind of mis-info you can find on the web. Quads instructions were correct (21/64, NOT 21/32).
Did I interpret correctly that he actually drilled it to 21/32? If so, he needs to drill it out to a 23/32, buy a 1/2-14 NPT Tap and tap it to that. And go to HomeDepot and get a 1/2-14 to 1/8-27 (or 1/4-18) adapter.