I use one that IS factory.(and stealth!)

1. Take an old fuel/temp/ oil pressure/economy gauge section from a GASSER 126 and transplant it into the 126 diesel instrument panel!
2. Remove needle from economy gauge (remove STRAIGHT up).
3. Replace needle on the ultra thin gauge shaft from where it came, but place it with the needle pointing at ~10:00 o'clock position.
4. Rotate the needle CC and lift/ bend it over the left side stop pin.
5. What used to be the economy gauge (Bourbon type vacuum gauge) is now a boost gauge (Bourbon type pressure gauge).
6. Attach with appropriate hose to "T" in boost line or to the empty intake manifold fitting.
7. I attached the hose to an aquarium gang valve in the boost line to the overboost protection switch so that I could rig a parallel hose and shop gauge temporarily into the cabin in order to calibrate (by moving the indicating needle on the shaft of the former economy gauge).
8. Full travel (left to right) is ~16 psi after calibrating. (I run ~14psi full out uphill). FWIW

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)