How To Decide Which Taps Are Needed For My 3/4" IH Carb

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How To Decide Which Taps Are Needed For My 3/4" IH Carb

Postby Rudi » Tue Sep 25, 2007 11:22 pm

Taps seem to be an item that is required for almost any Carb Rebuilding or Repair work. We might require them simply to chase threads and remove a bunch of gunk that has deposited itself over the last 50 years, or we may have to get in deeper and repair stripped or missing threads. Then comes the question of whether we should build up the body with JB Weld or drill and repair with a Helicoil type insert. If we chose the former, then we will definitely need the proper tap to recreate the original threads.

A lot of us who are not mechanics either of the professional or shade tree variety, most likely do not have a good quality tap and die set. Even many who do may not have ALL the taps that might be required. This requires us to start on another "got to find the right tool trip to the local home improvement store" nightmare.

Before we run off to the tool store, maybe it would be beneficial to understand a bit more about these critters. The Virtual Machine Shop has a useful page on Hand Tapping and might be a interesting read.

There have been a number of threads as of late in regards to which taps are required for a specific application, and in trying to help out, I would search and see what I could come up with. As is usual when responding to posts, I sometimes mess up on the url's, or even on the type of item I am looking for, which can cause a bit of embarassment. Soooo, I thought that I might put this together as an aide to getting the right taps for the job at hand. I am trying to figure out which taps one would need for a Carb Rebuild Project and so far I have been able to glean from previous threads that the following are the most commonly required.

For the Fuel Inlet:
    Inch Thread Size 1/2"-20
    Thread Type NF (National Fine)
    Thread Direction Right-Hand Thread
    Surface Coating/Treatment Uncoated (Bright Finish)

For the Idle Tube:
    Inch Thread Size 4-48
    Thread Type NF (National Fine)
    Thread Direction Right-Hand Thread
    Surface Coating/Treatment Uncoated (Bright Finish)

These taps should be available at most of the Big Box Home Improvement stores, but so far stores such as:
in the USA and in Canada
proved to be a serious wast of time. After almost 2 hours of searching every Home Improvement Store I could think of they seem to only carry 1/2-20 Plug Style Carbon Steel Hand Taps and do not carry any Bottom Taps as far as I can find.

Quality For Trade Vendors such as McMaster-Carr, Snap On, Mac Tools as well as a number of on-line merchants however, have yielded some pretty good results. Quality of the tools and the price brackets seem to be pretty uniform across most of the sites I have visited. The key here will be to choose the brand that you are most comfortable with and is within your personal budget.

Here are a few that I have found so far.

JC's Online ToolShed carries D.H. Hanson Taps and Dies and has the 1/2 Inch - 20 NF Bottoming Tap and is available for online ordering.

KTec Equipment and Supplies also carries No. 101-7327 - Tap Hand Bottom 1/2-20 NF and is available for online ordering.

JTS Machinery & Supply Companyhas the 4-48 NF Bottoming Tap and is available for onlline ordering.

McMaster-Carr is a Quality For Trade vendor and has a very complete catalog of tools. However, I find the site to be a bit intimidating to someone like me who even though I am well versed in tools and tool terminology for woodworking and such, I find searching for metal-working tools to be very un-nerving. One must know the difference between a Bright Finish High-Speed Steel Spiral-Flute Tap , a Carbon Steel Fractional Size Hand Tap, or one of a number of different types along with other arcane terms which I do not profess to understand :shock: :? :roll: Someday, I will understand how to find these items, I guess practice will make perfect. I would take the time to read the different links that you will see pop up on the McMaster-Carr site as you search for the correct item. Very useful site to say the least.

After a bit of searching I was able to pin-point the required critter. It is Stock #2521A725 Fractional Size High-Speed Steel Hand Tap Bottoming, 1/2"-20, H3 Pitch Diameter, 4 Flute and as of 2002hrs on 10 July 05 it is in stock at $9.27 Each :!:

There are a number of other taps that may be required depending on what is wrong with your Carb, or if you just want to chase all the threads so prevent further damage down the road. These Carbs are getting hard to find, so Preventive Maintenance would seem to be a prudent exercise. This is what I have found so far that may be required.

    4-40NF for the Shutter Assembly Starting Screw Part # 132 622
    5-40 NC for the Butterfly, Throttle Screw Part # 147 754
    6-32 NC for the Screw C-Z Slt-Fil HD Machine Part # 131 899
    10-24 NC for the Screw Cap, Body 4 req'd Part # 26 668 R1
    10-32 NF for the Screw, Idle Adjusting Part # 351 958 R1
    1/8-27 NC for the Plug Auto Sq-Hd Pipe Part # 444 571
    1/4-20 NC for the Manifold Studs Part # 179 796
    1/4-28 NF for the Main Metering Jet Part # 354 192 R91
Judging from the list of taps that are standard, I am hoping that the above are correct. I have not seen both NF and NC for those listed above. If there is an error can someone please let me know. Thanks... :D

I hope this has been of some help and that it is relatively complete. I have checked and re-checked but as I may have missed one. I would appreciate any input on the Idle Adjusting Screw and the Main Metering Jet thread sizes as I think that will be enough to complete this. Then I can format for the Cub Manual Server.

This article pertains solely to the IH 3/4" Updraft Carburator, however, as I find the time, I will try to do one for the Zenith as well. Of course, it would be nice ifn we could find someone to write a Rebuild Article for the Zenith similar to Lurker Carl's Carb Fixes arrticles on the Cub Manual Server.
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