Air Speed Indicator Mk9G 6A/3146

Bit of cheat here in that the Mark 9G instrument was fitted to post war Spitfires and Seafires and not Mk9's but the only difference is that the maximum speed shown on this version is 490 knots as opposed to 480 knots. This was the only one available as non functional so was half the price.

First thing that is not obvious is the presence of a double lock washer, so remove the screw between the two air pipes and then ease up and off the lock washer. You can then remove both pipes with a 5/8th spanner (standard RH thread) then pull up the base plate.

The front glass bezel uses a standard RH thread (unscrew anti-clockwise) that may need the application of some WD40 to loosen the 65 years of dirt in the thread first.

The outer pipe connector marked S for static pressure just holds a brass stud which is loose.

The inner central pipe connector marked P (for pitot or pressure I assume) drives the mechanism holds it all in place as well...

If we flip back to the front and unscrew the glass lens cover we will find that there are two washers 1) a very thin delicate card washer between the glass and the screw lid 2) a thicker cork washer between the glass and the body. You will need a thin blade between the body and the washer to ease this off.

The needle pointer can be eased upwards and off its pin with a pair of angled tweezers, you can see here that I have used some heat shrink to protect myself from loose radium paint on the pointer. Note: the pin is 0.77mm diameter.

Carefully remove the paint from the 3 small screws that attach the dial to the mechanism and undo them. The dial face will then come loose and can be removed.

The mechanism can be pushed out from the rear by applying pressure to the central P tube stud. That is easier said than done with 60 years of stiction. I cheated and used a wood plug cutter to drill around the P stud enlarging the hole and then tapped it with a hammer - but watch you do not damage that pointer pin !

The bellows mechanism is quite fiddly and easy to damage. It converts about 5mm of linear motion (along the axis of the needle pin) to about 630 degrees of circular motion (almost 2 full circles) of the needle. Far too fiddly for me as I can use a stepper motor with the pin attached directly to the stepper motor shaft. So the only pieces of the bellows mechanism that I really needed was the front plate to hold the dial and, subject to the motor I chose, possibly the Y frame to hold it in place.

Again removing these parts without the correct tools was nigh on impossible. So I resorted to brute force in this case drilling (1.5mm) out the core of the 3 mounting plate screws to loosen them from 60 year old locktite. Then using a disc cutter to remove the very thin 3/4 inch nut that held the P tube in place. Without making a jig and a pair of very thin imperial 3/4 spanners you will not get far before you twist the Y frame out of square.

...More to follow - once I get to putting it back together again !