How To: Restoration of Ford M8 armoured car US Ordnance number 7373

[Big D]

Hi all,

This weekend, a friend and I fired up the M8 engine for the first time. Ed is a good engine man and he worked on British Army military vehicles in his younger days. He did most of the internal assembly and set up on the block.

I had completed the assembly of all the accessories on the engine on a tubular steel stand and I set up a wooden platform in front for the radiator so I could run that at the same time and make sure that was working okay as well. I don’t have an original upper radiator pipe so I cut a bit of pipe to temporarily use. This bit of tube is a bit longer than what the parts manual specifies but I needed the extra length so I could give the fans enough clearance from the makeshift radiator platform.

I remote wired the engine into the hull of the M8. I thought this was a good idea initially but upon reflection introducing a whole lot of other variables into the start-up equation was maybe not the best move, when all the wiring in the hull was untested. It worked out alright in the end though.

The first switch on produced nothing and we found the starter solenoid wasn’t engaging. As I learnt on another forum, the M8 is wired as such that when the ignition is switched on, 12 volts is supplied to the control terminal on the starter solenoid. When the starter button is pushed, it grounds that 12 volts which activates the solenoid and sends 12 volts to the starter motor post. I wasn’t completely sure the solenoid I had needed the 12 volts on that post grounded, or needed 12 volts sent to that post, to actually activate the solenoid.

After checking with another M8 expert (thanks Willy) I confirmed the solenoid I had fitted was the correct type. After a bit more tracing I found that the cause of that issue was a bad earth on the instrument panel, so the grounding through the starter button was not taking place. After working through the circuit diagram again I couldn’t see how the instrument panel was actually grounded, as it is mounted on rubber insulating blocks. Anyway, I ran an extra length of wire from the instrument panel body to the panel mounting bracket on the hull. This gave the ground required.

Once I had the 12 volts coming off the solenoid to the starter motor, I found the starter motor wasn’t working. After a look at that I found that one of the brush springs was shorting out against one of the nuts holding the brush pigtails to the starter casing. Those wee springs don’t like high current through them and it damaged the spring. I had to search a bit to find a spring locally but a retired auto electrician I know managed to find one in his box of spares that would do the job. We put it all together and it worked but the nut on the little screw holding the brush pigtail to the case was still very close to the brush springs. I have wrapped the nut with a strip of insulation tape in the meantime but both of us agreed that the nuts are probably not standard. I have a new set of brush springs coming from the US and when I fit those I will find a nut with a lower profile.

I rigged up a fuel supply to the newly built fuel pump and first touch of the starter button on the instrument panel, the engine ran. It sounded very nice and seemed very responsive. The NOS fuel pump which I had put new neoprene diaphragms into worked well.

There were a couple of minor issues during our various tests, like one of the water pipes coming off and spilling water everywhere. I have been advised that I will receive the ‘Golden Screwdriver Award’ for not tightening the screw on the clamp enough. There was also a slight leak in the gasket in the thermostat housing, which I’ll sort out when I actually fit the thermostat.

After a number of starts we found the battery was starting to slow. I did a few voltage checks and it didn’t look like the generator was feeding the battery enough so I will check that and the regulator out.

I have a new calcium type N150 battery but I don’t have a compatible battery charger here and I wasn’t super confident it was being charged fully prior to us starting the engine. It certainly looks like that is the case now so I will order a new calcium compatible battery charger tomorrow.

It’s a great feeling seeing and hearing the engine run and knowing most of what is attached is operating correctly.

I will fit the clutch assembly and gearbox next and then fit the correct ignition lead terminals now that they have arrived (thanks Brian). They need a little ferrule at the end to stop the wire coming out which I have not seen available so but I might have to come up with an alternative for that.

After that, my next step is to fit the newly rebuilt brake hydrovac into the hull and complete all the plumbing on that. I will then pressure bleed the brake system and that should get the hull pretty much prepared for fitting the engine and gearbox assembly.

Here are a few videos of us running the engine, testing the generator and regulator etc:

https://youtu.be/sOCfMGhI3mU

https://youtu.be/VUlI76GAZBE

https://youtu.be/If3TuDe7TIQ

https://youtu.be/dMuRJgVTYV0