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Carburetor REBUILD (for MGA) -- CB-210C
Installing Bushings in the throttle body

If journals in the throttle body are worn more than 0.010" oversize, then you need to install bushings in the throttle body to restore original bore size. This can be done in your home shop without benefit if a drill press. You will need a 3/8 inch chucking reamer with 5/16" shank to enlarge the journals to exactly 0.3750" to provide for press fit of new bushings.

The trick is to use the reamer in a backward direction with the 5/16 end acting as the pilot and the reamer being driven from the front end. The reamer must be turned counter-clockwise to present the cutting edges of the flutes. A minor modification is to grind cutting edges on the trailing end of the flutes, and to attach a nut on the front end with set screws.

Start with the reamer, 3/8 inch with 5/16 shaft, Enco number 329-1124, Enco Manufacturing Company, 1-800-873-3626). These would be available from any machine tool supply house. You also need one 3/8 inch hex nut, two 8-32 set set screws 3/16 long, bushing and four bronze bushings, 3/8 outside diameter, 5/16 inside diameter. The bushings need to be 3/4 inch long for H or HS type carburetters, or 1/2 inch long for HIF carbs. The shorter bushings will work in H or HS carburetters, but longer is better because there is more surface to absorb wear and control air leaks. You can get them (maybe cheapest) from any bearing supply house. 660 bearing bronze works, but sintered bronze "Oilite" bearings are better. You can also get these from McMaster-Carr; 1/2" long is number 6391K153; 3/4" long is 6391K155.

Drill out the 3/8 nut with a 3/8 drill so it will pass over the reamer. Drill and tap two holes on two flats of the nut for the set screws. Fasten the nut on the front of the flutes of the reamer and tighten the set screws. Grind them flush if they protrude. Grind slightly angled cutting edges on the trailing end of the flutes. This will become the new cutting end. You can use a die grinder, but a hard whet stone may also work.

The assembled reamer, showing one of the set screws.

Clean the carburetter body with solvent. Hold the carburetter body securely in a cushioned vise with brass strips cushioning it. Pass the shaft through the body into the hole on the other side. Turn the reamer “backward” by hand, reaming the near side, withdrawing frequently to remove the swarf. Used plenty of cutting oil to avoid dulling the cutting edges. Check fit a bushing, then remove it, smear with thread-lock adhesive and refit. Then ream the remaining journal hole and fit a bush in the same manner. Oil the new bushings. The reamer is guided by the old bushings for the first pass and by the old and one replacement bushing for the second pass.


Addendum June 24, 2011:
Recent change to cheaper Imported reamer:
Reamer End   -   Reduced Diameter   -   Chucking End


The ENCO reamer 331-1124 (import for $10.58) is no longer machined for the full length of the chucking shank, so it is no longer suitable for this job. ENCO reamer 329-1124 (USA made for $17.42 list or $13.99 web) does have the fully machined shank (shown in prior photos) and will work for this task.


Addendum November 4, 2011:
When the shaft journal is reamed through large enough to install sleeve bushings, the bushings must be carefully installed flush with the inside diameter of the carburetor throat to prevent blow by when the throttle plate is closed. One way to accomplish this is to press the bushing slightly into the throat, then go inside the throat with a porting grinder to grind the exposed end of the bushing back flush with the throat.

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