Thread Maintenance

Thread Maintenance

One of the mandatory steps in maintaining a race car should always be putting a wrench on every bolt before every race, and that includes the carburetor.  All you need is a Phillips screwdriver, a straight blade screwdriver, a 5/16 nut driver, and a 1″ wrench.  The body, bowl, and block of your Holley carburetor are typically cast moldings of zinc alloy (unless you have billet aluminum pieces) and the threads for each screw must be respected.  Torqued is not the goal here, just tight!  Healthy gaskets will make the seal just fine so resist the urge to over tighten.

Stripped or crossed threads do not have to be a part of your racing experience, and we would like to offer some tips to prevent and/or repair this problem.  Below is a list of sizes for every threaded hole in your carburetor; however, the bowl and base plate screws are the only holes that can be helicoiled and all other threads can only be improved by chasing them with the correct tap.  Please be mindful of any shavings created and blow out the area with compressed air.  Follow the footnotes for more information about certain screws.

¹Bowl screw                                    12-24
Base plate screw                             12-24
²Power valve                                 1/2-28
Gas jet                                            1/4-32
Alcohol jet                                   5/16-32
³Pump nozzle screw                      12-28
Needle and seat                           3/8-32
∗Screw-in air bleed                       10-32
Pump housing screw                      8-32
‡Throttle blade screw                     8-32
◊Bowl fuel inlet                           7/8-20
♦Pump arm stud                          1/4-20
Idle mixture screw                          8-32
Primary shaft adj screw               10-32
Sec cam bracket lock screw        10-32
Sight plug in bowl                     5/16-24

¤Float bracket screw               self-tapping and not repairable

¹Bowl screws are the most commonly stripped thread.  They should only  be gently snug with a 5/16 nut driver and can be helicoiled if necessary.  Longer screws are also available to add more thread.

²Now you have permission to bear down on your carburetor!  Use the longest 1″ wrench in the shop to ensure the power valve is VERY tight.  Do not use a screwdriver-handled tool because you will not be able to get it tight enough, and always use a new gasket with a new power valve.  Reusing an old gasket will cause it to split as it delaminates with the twist.  A split gasket will be obvious because it will look like there are two gaskets on the power valve as it has sheared horizontally creating two halves.  A loose power valve or a split gasket will ruin your idle quality.

³Be careful in tightening this screw.  The screw does not go very deep into the threaded hole; and, because of the side slots necessary for fuel flow, the threads do not enclose the screw completely.  Stripped pump nozzle threads can only be repaired with a brass insert that requires installation by a machinist.

∗All air bleeds are 10-32 thread size except for the Holley 80583 / 500 HP which uses 6-32 hi-speed air bleeds.

‡Throttle blade screws are installed with Loctite and typically do not need repeated tightening.  If you need to remove them, use some heat to loosen the Loctite and make sure the tool you use is well-seated to prevent rounding out the Phillips or Torx (T15) head.  Drilling them out is not fun and can ruin the throttle shaft.

◊99 percent of all carburetor leaks are in this area even though it may look like the leak is coming from the accelerator housing below it.  It is not easy to get a good purchase with the wrench on the hex of the fitting, so be mindful of cross threading the hole and always use a good gasket.

♦This stud is installed with Loctite and some heat will be necessary to remove it.  It will break if the Loctite is not loosened.

¤The two holes for the float bracket screws are not threaded.  A drop of light oil will help the self-tapping screws.  The washer on the screws can be removed to get one more thread for bite.