54 Heald Street ~ Pepperell, MA 01463




Baby High Butterflies

(not for sale)

We recently received a set of the elusive 2.5 liter ST High Butterfly throttle bodies, a.k.a. the Baby High Butterflies, that were originally manufactured for the '72 911 ST. These Baby High Butterflies were the first castings produced and are different in some ways from the later 2.8/3.0 liter RSR type. Here are some detail photos along with a description of the difference between the ST and the RSR type High Butterfly throttle bodies.

The casting number for the ST is 911.110.107.2R
The casting number for the RSR is 911.110.107.5R

The ST has no 6mm threaded grease fitting hole on the housing for the bellcrank axle, unlike the RSR.

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The ST bellcrank lever (pictured on right) is flat with no strengthening rib. This particular bellcrank was replaced with an RSR type (left) because of wear on the axle itself (very heavy usage).

The housing on the left is the ST housing which shows the holes for the fuel line brackets  (on the lower part of the casting between the barrels). Notice the holes are located differently on the RSR (empty RSR housing on right).

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This picture shows the factory 6mm air screws which are common to both. However the air screw hole for the ST is 3.3mm vs 3.9mm for the RSR.

The throttle shaft return springs on the ST (left) are made from thicker wire than on the RSR (right). It is difficult to see in the picture but the diameter of spring wire is approximately 10% thicker. On this particular ST housing the two spring retainer bosses on the throttle lever end were worn down and had to be replaced. The heavier ST springs likely contributed to the aggressive wear and all four return springs were replaced during the rebuild.

Appears from the side to be very similar to the RSR. Throttle shafts and couplers are the same.

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Same 50mm top opening on ST and RSR, shown here with new bushing and the required overbore ( < 1mm in this case). Stud spacing is the same.

This set had the port side enlarged to 42.5mm at some point. The exact port size on a factory set should be about 41mm but we do not have an unmodified set to measure. We will post this measurement when we find one. The port side on an RSR measures a little bigger than 43mm on the very bottom (about 43.5 or so  depending on how it is measured).

The finished 2.5 ST set, note the modified throttle arms. This set showed extremely heavy wear. In addition to the worn spring retainer bosses which were replaced, this set required six new throttle shafts, one shaft coupler, the bell crank axle, and the four return springs.


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