Tap size for pilot air screw

[Tucker Moon]

I have messed up the pilot air screw thread hole on a 78 special with bs38 carb.
I need to know the correct tap size with certainty so I don’t make worse.
my guess is m5 .05 but i just don’t know anyone know for sure
thanks

 

[Jan_P]

Perhaps M5x0.5 0.05 is very small
If you have a jet and the tap you can hold the tap against the thread holding them against a light source
se if it is the right pitch.
The outer diam is close to 5 on the jet M5
How did you mess up ? .. worst case the 5 mm is gone

 

[Tucker Moon]

Perhaps M5x0.5 0.05 is very small
If you have a jet and the tap you can hold the tap against the thread holding them against a light source
se if it is the right pitch.
The outer diam is close to 5 on the jet M5
How did you mess up ? .. worst case the 5 mm is gone

I was removing and installing while on bike and I did cock eye the pilot air mixture screw. Should have done this with carbs off bike.
Now I want to be 100% sure of the needed tap size to ciorrect. I have a tap set and nothing matches using light to compare. My kit does not have some of the sizes so I want to be sure before I buy more taps and be sure the one I use to clean threads is 100% correct. I would love someone who knows size without a doubt before I proceed.

 

[Adamc]

Someone here may know the details you require.
Or you could get a cheap thread gauge from Amazon:

 

[Tucker Moon]

Perhaps M5x0.5 0.05 is very small
If you have a jet and the tap you can hold the tap against the thread holding them against a light source
se if it is the right pitch.
The outer diam is close to 5 on the jet M5
How did you mess up ? .. worst case the 5 mm is gone

I was removing and installing while on bike and I did cock eye the pilot air mixture screw. Should have done this with carbs off bike.
Now I want to be 100% sure of the needed tap size to ciorrect. I have a tap set and nothing matches using light to compare. My kit does not have some of the sizes so I want to be sure before I buy more taps and be sure the one I use to clean threads is 100% correct. I would love someone who knows size without a doubt before I proceed.

Someone here may know the details you require.
Or you could get a cheap thread gauge from Amazon:

View attachment 208732

my tap set has gauges. The threads are so small it’s impossible to match up. just hoping someone knows since my set does not have this weird small size

 

[hovel]

weird small size

Nothing weird about it. Measure the diameter of your pilot screw. A millimeter is approx. .039" so you can't miss by much. Small metric threads 3mm in diameter and larger are sized in even mm, there are no half sizes but there are fine or coarse deviations. M5 for instance is available in both .5mm and .8mm pitches. Another option would be to take your pilot screw to a good hardware store that has a nice assortment of itty-bitty metric screws and match one up. Remember that when you chase the hole (use a plug tap), you're gonna be creating some tiny swarf so put some grease in your tap flutes to try to catch it. Good luck.

 

[Tucker Moon]

Nothing weird about it. Measure the diameter of your pilot screw. A millimeter is approx. .039" so you can't miss by much. Small metric threads 3mm in diameter and larger are sized in even mm, there are no half sizes but there are fine or coarse deviations. M5 for instance is available in both .5mm and .8mm pitches. Another option would be to take your pilot screw to a good hardware store that has a nice assortment of itty-bitty metric screws and match one up. Remember that when you chase the hole (use a plug tap), you're gonna be creating some tiny swarf so put some grease in your tap flutes to try to catch it. Good luck.

Thanks for the insight and help.

 

[Jim]

I don't have a -38 carb to compare. This is the needle screw from a -34. It's 6 X 0.5mm
Yes, using your pitch gauge is hard to judge when you're dealing with this small a pitch. Try this...
A 0.5 gauge looks like it fits, but it's hard to tell for sure.
So, go down to the 0.45mm. It's obvious that it doesn't fit... not even close.
Now, go up to the 0.6mm. Again, it's obvious it doesn't fit.
By eliminating the smaller and larger gauges, I'm 100% certain the pitch is 0.5mm

 

[Tucker Moon]

I agree. Thanks