If you are a hands-on hot-rodder, like me, you will be forever playing with carb jetting in an attempt to achieve that perfect combination of throttle response and power output. That is why I chose a Holley spread bore carburetor and a two plane manifold when building my street rod. Streetability… Now, I suppose a Barry Grant Demon would score more points at the drive-in but the good ‘ol Holley Double Pumper is a classic and easily obtainable. I prefer a Spread Bore for street use. That is, the primary and secondary throttle bores are not the same. The primary bores are smaller, giving better idle, precise throttle control and better mileage. The mechanically operated secondary throttle bores are huge, passing lots of air. The carb shown in the photos is a list 6212, which came stock on sixties vintage big block Corvettes and Chevelles. It was rated at 800 cfm. The intake noise when at full throttle is oh so fine. It does take some patience to get the primary and secondary jets, nozzles and power valves sorted out but the results are worth the effort.
The problem with all the tinkering with jetting, power valves, nozzels and the like is that you are going to wear out the fine threads of the Holley Carburetor body, removing and re-installing the float bowls. A slight over-tightening and poof, the body’s threaded hole is stripped. It is sure to leak. Now, if you have the money to simply order up a new carb this article is not for you. What the heck, get a chrome plated one while you’re at it.
The smarter solution is to install steel thread inserts into the pot metal body of the Holley. These inserts, known by the brand name HeliCoil, are installed by drilling out the original threaded hole with the properly sized drill, tapping a larger threaded hole in the body and installing a special cylindrical screw that has the proper internal thread for the bowl screws. Believe me, you do not have to be a machinist to do this job and you’ll feel proud of the accomplishment. The carburator will be better than new as it will stand up to repeated float bowl removal.
You will have to disassemble the carburetor. Remove the bowls and the fuel blocks. The fuel blocks may need a little persuading with a rubber mallet but they will pop off. The throttle plate should also be removed.
Then carefully cover the fuel block area of the body with masking tape, leaving the bowl screw holes exposed. One stray metal chip from drilling or tapping can play havoc with air and fuel bleed holes. Special care at this point is definitely advised.
You will want to put inserts in all eight bowl screw holes while you are at it, which works out fine because that is exactly how many HeliCoil inserts come with the kit that is available online from NAPA Auto Parts. You will want to order part number 770-3043, which contains the correct drill, tap, eight inserts and a HeliCoil installation tool. You can also find individual HeliCoils, taps and insertion tools at MSG Industrial Supply. The great guys at Summit Racing have gaskets, jets and the like.
The first step is to drill out the holes to the correct size for the tap. A drill press would make things a lot easier but I sold mine when my youngest son was on the way. Anyway, I wanted to show that you don’t need one to get the job done. You do want to clamp the carb body securely or get someone to hold it down. A fall to the concrete floor will likely damage a milled surface ruining your day. The drill will cut into the hole very quickly so don’t over-do it. The holes are only a little over a half inch deep and you definitely do not want to go all the way through. And don’t let the drill bit wobble around. Straight in and straight out, letting the drill bit do the work.
The second step is to tap the threads to accommodate the HeliCoil. If you have never taped a hole before I suggest that you practice on a piece of scrape aluminum or pot metal. Its easy but you will definitely want to develop a feel for it. Here is the correct way to tap a thread:
Use a drop or two of motor oil on the cutting edges of the tap. Hold the tap, secured in a tap wrench, perpendicular to the surface and gently insert the tapered tip into the hole. Turn the wrench clockwise, applying only slight pressure. The tap will thread itself into the whole. Turn the tap three-quarters of a turn then back off one-quarter turn. This will cause the chip of metal being cut by the tap to break off. Proceed with another three-quarter clockwise revolution and a one quarter turn counter-clockwise. Repeat the process until you feel the tip of the tap reach the bottom of the drilled hole. Don’t force it! When you have reached the bottom simply back the tap out of the hole.
The final step is to thread the HeliCoil inserts into the tapped holes using the special insertion tool included in the set. There is really nothing special about this step, just screw them in. The insertion tool causes the coil to contract as you turn it clockwise. As you remove the insertion tool by turning counter-clockwise the coil expands and causes a small tang at the bottom of the coil to break off. Just be sure that tang ends up in the waste basket. If it doesn't break off its no big deal. Use a blunt ended pin such as a drill bit just small enough to slip into the heliCoil. Tap it gentle and the tang will break off.