Refurbishing the early pneumatic throttle pedal

[John A. Ross]

Not something people will ever see never mind have to deal with but as I love all things early I thought I would share this in advance of project Doris

Early cars has a pneumatic throttle system which in my own opinion have a far superior feel on a road car than I have yet to come across in a standard Imp.

The modified throttle cable (modern cables) and modified quadrant come close but not a patch on the early air power




Fairly complex , or is it ???? the full linkage on the cable based system from where your fit rests to the abutment and cable counts up a fair few failure spots on its own.

The other end is also fairly simple, but unbfortunately these cannot be serviced and an NOS ones I have picked up, the innards have suffered badly with poor storage so the future is not looking great for these.

Saying that, small linear actuators ae 10-a-penny so I could modify one using the original can if I ever need to.



I have quite a few of these pedals but as I do, the worst of the bunch was picked to see how they fair. It always surprised me how bad things can look yet still be of use with a little TLC.




Biggest issue I found was getting the pivot pin out, a pair of pliers and a loose nut manage to get past the lip to bend down the retaining lug and out it came with a small tap




A closer look, its not a clip, it is a bent retaining tang, not a particularly elegant solution

[John A. Ross]

And some of the pedal in some states of tear down.





The metal work was in a terrible state and the coatings falling off on the inside

And finally as I do not have any blast capability, off they went for vapour blasting, local company starting out and did a great job.

[John A. Ross]

Some NOS parts (diagphram and seating cup)

Steel parts although vapour blasted also give a small bake/heat up and then washed down with some zinc phospate solution to kill anything in the pits and crevices then a light coat of zinc primer applied to some lifghtly warmed metal.



Then a coat of matt black enamel paint out a can.

Usually I would do a single coat 2K epoxy but this is pretty low exposure where it is and I prefer to keep the coatings thinner.






Now thats the parts all lined up, cleaned the other parts sorted out, ready for re-assembly

I just need to find some straight blind rivets to match the old ones, call me particular but I would like it to have the same crappy rivet finish as factory even if some nice stainless ones would do the job a bit better.

[John A. Ross]

Parts ready for re-assembly, it is truly shocking how long all these little detail parts take to do



A quick scuff up and final coat of satin black, leave for a few days in warm just to make sure its well flashed off.

[John A. Ross]

And as they say, onwards and well onwards

No show on the blind rivets yet so M3 countersunk screws and nuts with a little thread lock will have to do for now.

Not an easy thing to re-assemble, the parts manual, like many other drawings, does not actually show the parts in assembly order adding to confusion. The woerkshop manual sectional drawing is correct and we have a pair of these assemblies that use the spring ring rather than the rubber seating ring

First base correct way up, there are 3 stepped notches on the base, these locate the bottom plastic section of the bellows and the single one in line with the end is the exit for the bellows





Next up assembly of the upper section of the bellows which contains several parts

There is the rubber section, a locating plastic ring and the metal flange that sets its primary position.

Note the plastic locating ring has a ridge, this faces UP and the upper metal ring floars around the recess with the ridge keeping it away from the rubber









Next is assembly of the piston which is easy enough and in order below and with this step you see how it works a little better








Before you tighten that screw it MUST be tight BUT BUT BUT do NOT put too much force in it as it is seated in the plastic plunger and its not a metal insert, its just plastic threaded

[John A. Ross]

Now the next bit is quite important, the plastic base of the bellows has a very small lip for the upper rubber part to seat onto and the metal cover holds that in place

So its really important that it does seat correctly and be held in place

I find it arkward but better to hold the plunger down while seat it as the upper bellows stay in place.






Now in all the times I have done this, if the rubber lip and top cover are seated properly they will stay put when the plunger comes up to rest

The spring ring sits on top of the cover so add a little smear of grease




Then a small smear on the matching surface on the top part of the assembly




Then add the sponge washer



Then the top cover can be seated and secured with several screws of riveted

[John A. Ross]

Time for a little testing before adding the pedal assembly to it

Air pushed when pressed
Press actuator and while down block air connector, should stay put until unblocked
With actuator returned to home position block air connection and apply light pressure to the actuator, it should present a lot of restance and stay put until blockage (thumb ) removed

[John A. Ross]

OK, Youtube links not rendering any more, my apologies it will be fixed soon.

Onwards fitting the pedal assembly

Bit of grease in the pivot apertures




Add the bushes



The pivot pin as one end that is "D" shaped so make sure you fit it correct way round, mine was a little tight as the paint coating had filled up the recess




Again a small bit of grease on the faces for the return spring which is assembvled like below including the pivot pin area






Now next bit is fiddly, apply some grease to the pplastic piston and part insert the small pivit pin

Push pedal down to locate the piston and push the pin into place, then both tabs either side of the pivot point on the pedal are bent to act as stops to prevent the pin dropping out, you will sratch something, so a little grease again to keep the roost away wont do any harm, it will get damp down there.

[John A. Ross]

And finally repeat x 2 and the pedals are done