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

[Big D]

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Hi all,

Progress on the M8 the past couple of weeks is still a little slow with most of my time going into working on the Scout Car, so I’ve just been tinkering away on the M8 when I can.

We got the M8 crankshaft assembled into the block last week. The journals on this crank were at 0.060” so near its limits for wear but it looked to be in nice shape. All appeared good until we measured the end-float which was over 0.011”.

After some searching for the problem we identified the problem as being the crankshaft itself. The gap between the rear main bearing shell thrust washer face and the crankshaft journal was too great. It looks like that journal where it meets the thrust washer has been ground at some point, possibly in the process of its last refurbishment?

Why that would be is a mystery but we were wondering why the rear main bearing shell had punch marks around the circumference of the thrust washer face of the shell and this would explain why. Whoever did the last work on it was trying to take up the excessive clearance by putting the punch marks in the shell and dimpling the thrust washer surface.

After some discussion with the engine reconditioners I decided to go with the option of resurfacing the thrust washer face of the bearing shells. This has the effect of building up the surface and taking up the excessive clearance. Apart from the end-float the crankshaft is in very good condition and I felt that it was a shame not to use it especially when we had gone so far with it. Now that the crankshaft is back in the block with the resurfaced shell, the end float measures at an acceptable 0.003-0.004”.

I do have a spare crankshaft which is at standard size along with some 0.020” shells but I will save that combination for a future rebuild on either the scout car or the M8.

I have a number of other parts on the way including the water pump parts I needed (thanks Brian). I am in the process now of gathering up all the loose parts and taking them to the workshop where I will assemble all the bits and pieces onto the engine prior to lifting it into the hull. The gearbox synchro rings are due here anytime so I’ll be able to reassemble the gearbox and get that fitted to the engine block before the assembly goes into the hull.

I had the generator that I rebuilt tested at the local auto electricians and it tested fine. I can’t say the same for the voltage regulator. I bought that believing it would work but it is a no-go. Fortunately I have another which looks like it has never been used so I will test that one out. Despite it appearing to be NOS, I had to solder a wire back in place. It had broken away as one of the coil mounts was loose. I still need to find another pulley wheel for the generator, or else repair the one I have.

I am making up an ignition lead harness. I had the tubing but it came without cabling in it. Complete NOS ignition lead sets are available but I figured I might as well use what I had and the leads don’t take long to make up.

I have the later thermostat housing and pipe outlet. I tested the NOS thermostat I had in the thermostat housing and I see the fit isn’t right. This thermostat is a French one – part number G136 73 47630. The length of the thermostat means that it seems to be pushed hard against the inner surface of the housing so I don’t see how it could work. The 3” diameter at its widest point also makes it a little big for the water pipe outlet to fit against the thermostat housing so that the two pieces close.

I am aware that there are different types of thermostats and housings but does anyone have any photos of the variations of these so I can confirm things?

That’s it for this week. Merry Xmas.

[Big D]

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[Big D]

Hi all,

The gearbox assembly was an interesting exercise. You will recall I had to replace all the synchro rings and third gear so I had to completely dissemble the mainshaft. The mainshaft is reassembled on the spline in this order: Second gear, thrust washer, 2nd gear synchro ring, synchro hub assembly, 1st gear synchro ring, 1st gear, reverse gear.

I had a retired mechanical engineer give me some assistance which was great as he had rebuilt countless gearboxes in his career and at times during this rebuild I found two heads were better than one with a couple of little hurdles that I struck.

The main problem I had was that the new 1st and 2nd gear synchro hub assembly was a very tight fit on the mainshaft spline. The manual refers to ‘sliding the synchro hub assembly’ onto the splined shaft during reassembly. These hubs were never going to slide on. The old hub had to be pressed off and the new one had to be pressed on and we tried various methods in doing that, with the first gear down on the press and then first gear upwards on the press.

The difficulty with pressing the assembly on was that you had little control over the thrust washer and 2nd gear synchro ring positions once the process of pressing the assembly on started. The slightest movement of the first gear or the shaft would change the position of the thrust washer and then the teeth on the synchro hub assembly would not align properly with the thrust washer meaning the hub had to be pressed off again. The other problem was that if you pressed the hub assembly on with first gear down (shaft into hub), you couldn’t control the position of the synchro ring and this had a tendency to cant on the shoulder of second gear, at the last stage, meaning the hub would not go on any further. One also needs to be careful to keep the synchro hub together as an assembly during that pressing to ensure the locking rings inside the hub didn’t come away from their position in the process.

We repeated this process about 5 times trying different things to get the synchro hub aligned into the correct place. Finally, we decided to put a smidgeon of Loctite on the top surface of the thrustwasher to hold it at its precise location on the shaft splines long enough to get the teeth in the top of the hub assembly to align with it. As soon as torque was applied the Loctite broke away but it was enough to do the trick. We pressed the hub on with second gear down (hub onto shaft) and also used a little tool to hold the synchro ring up tight against the hub assembly as the hub was pressed down.

I’m not sure if all the synchro hubs are this tight, or whether it was this particular shaft and hub(s), but if the hub was a sliding fit as per the manual, then I wouldn’t have had these issues.

The rest of the gearbox assembly was relatively straightforward. I replaced all the bearings in the housing, most obtained off EBay at very reasonable prices. One of those bearings appeared to be just slightly different in thickness to the original meaning the end circlip was too tight a fit in the groove on the shaft. In the end, I cleaned up the circlip on a very fine bit of wet and dry paper and it worked fine. There would have only been maybe 0.001” in it, but it was enough.

I fitted the bell housing and set up all the clutch release bearing assembly. I need to find a spring now for the clutch release bearing sleeve. The gearbox is now painted and just about ready to fit.

The brushes for the starter motor arrived. These weren’t originals but they do fit and they should work fine.

That is all for this week.

[Big D]

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[Big D]

Hi all,

Things are progressing with the engine assembly and my plan is to get this running before it goes into the hull.

I initially had the engine on some wooden blocks but now have it on a stand which we’ll run it on for a test before it goes in the hull. Yes, I know the engine mounts are upside down in the early photos…it was just easier…

I ran out of time to remove some broken manifold studs before I sent the block in for machining so I have now done that. What a pain in the butt job it was removing them. I’ve also drilled and tapped the new holes required at the ends of the block for the longer manifold used on the M8 versus the Scout Car.

I’ve fitted the water pump, oil filter, fan pulley mounts, coil, manifold and starter motor. I had to fabricate the generator mount stay and the bracket that holds the coil.

I was about to fit the fuel pump I had but realised the mount was wrong. I rebuilt this pump several years ago thinking it was for an M8 and had it stored away until now! I have since been told it is actually for a White halftrack. The correct fuel pump is now on the way to me….

The timing cover on this engine didn’t have a plug in it for blanking off what would be the oil filler hole on the scout car engine. What I did was to use a 1.5” frost plug, clean up the circumference a bit as it was fractionally too big, and tap that into place in the timing cover.

The dipsticks for the M8 and scout car are obviously different in length. Is the length of the handle the only difference? I know the sumps are slightly different but I wondered if it was just the handle being a bit longer for accessibility in the hull.

I have a NOS distributor on the way along with some water pump parts I still need. I actually wasn’t aware until now that the distributor cap for the M8/M20 uses an acorn type connection for the leads. The thread diameter is ¼” UNF. Has anyone seen these connectors commercially available?

I fabricated the small adjustable rod for the automatic choke. I modified the design a bit and included a locknut on the end of each leg to hold them onto the centre piece. I understand the original had grub screws securing the legs. I will need to go back to the forum thread where I got the dimensions and photos for this though to confirm the weight of the rod joining the two legs. I imagine that will have an effect on the operation of the auto choke….

That is all for this week…

[Big D]

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