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

[Big D]

Hi all,

Today I spent some time looking over the various Hercules JXD engines I have for both the M8 and the White Scout car I am restoring.

My engine reconditioner has had two Hercules engines at his workshop for me for several years (that’s another story). Well, he has now started work on the job of getting one engine rebuilt for my Scout Car so it was a good chance to take stock of what I have and what engines I could put together.

As I say, I have two engines for the scout car with the engine reconditioner. I also have three partly complete engines for the M8 at the workshop. I’d like to know exactly what parts on these engines are interchangeable between the M8 and the Scout Car.

From what I understand, the short blocks on these are the same. That is, the short block on an M8 will be fine for a Scout Car and vice versa. I know there are differences with the clutches, waterpump, gearbox and oil pump where these parts are not interchangeable. What else is there that is different?

Of the two engines at the reconditioners; one has a good block and had .40 thou pistons in it. This block has now been crack tested and it appears fine. It also looks like the bores and pistons will be okay with new rings.

The other block had the pistons completely stuck in the bores. Several pistons got broken in the process of getting them out. The bores look bad but the walls themselves appear fine and have not collapsed at all that I can see so far.

The three engines for the M8 in the workshop all have the pistons stuck in the bores, as per the photos. All have a wall in the bore that has collapsed. I understand that sleeving these bores is an option providing that there is an inch at the bottom and an inch at the top that is sound which will support the sleeve. Has anyone else had experience with sleeving these with such a scenario?

So, can I mix and match the blocks between the M8 and the scout car? What other parts can I mix and match and still end up with a correct M8 engine/Scout car engine. What other differences do I need to consider?

I have spare valves, STD pistons, 0.20 thou pistons and 0.60 pistons, plus spare valves and as you can see three spare crankshafts. I also have mains and big end and camshaft bearings. These bearings are all readily available anyway.

My ideal scenario is to build a good engine for the scout car, a good engine for the M8 and have a spare engine that I could potentially use in a future project.

Any thoughts or experiences with the Hercules JXD engines would be appreciated.

[Big D]

More photos.

[Lynn Eades]

Hi Darryl, I have rebuild info, if you don't already have it.

[Big D]

Hi Lynn

Thanks for that. I have TM 9-1706 which is the manual for the Hercules JXD gas engine for the scout car. It is a 1942 manual so doesn’t go into any detail aside from what is pertinent to the scout car engine. I’m not aware of a similar manual for the M8.

What data do you have?

[Lynn Eades]

I have a "Motors Truck and Tractor Repair Manual" from 1950.
It covers 50 truck makes, tractors ,and 28 models of Hercules engines, including the JXD.

[Big D]

Thanks Lynn. Will keep that in mind.

[Big D]

Hi all,

Thanks to Willy, who is a fountain of knowledge on M8’s, Scout Cars and anything armour, I’ve worked out the differences in the engines on the M8/M20 and the Scout Car. The basic engine blocks are identical and have interchangeable crankshafts, camshafts, valves etc. The parts that are different are all the extras:

• Oil pan and oil pump
• Bell housing and timing cover
• Flywheel and clutch assembly
• Starter motor, water pipe and distributor

The reason I was keen to know this was to make best use of all the Hercules engine parts I have. I have a spare engine block from my scour car restoration which now has all the pistons out of it. The bores on this are worn, but are sound with no damage to the walls.

The three engine blocks I had for the M8 all have stuck pistons with damage to the bore walls. After comparing the spare scout car engine block with the three M8 engine blocks, I’ve decided that the scout car block is the better candidate for a rebuild for the M8, so I will use that one. I’ll remove the timing cover, bellhousing, sump etc from the M8 blocks and use those on it. I will work out what combination of pistons and sleeves etc I will use at a later time, but it was just good timing to be able to check this while the scout car engine was getting rebuilt. By the looks of things, I should have enough bits and pieces to rebuild at least two more Hercules engines on top of these two, but that will be something I will sort out later. The Hercules block and head being machined in the photos are for the scout car.

Thanks also to Willy for his recommendation on trying the rubber shock bushes for the Dodge M43. I’ve found these are a better fit than what I had. The ends still need cutting down a bit though so that they fit in the 1.75” gap in the shock absorber mounts. I used a hacksaw to do this and then smoothed the ends with a flap disc on a grinder. This gives the correct width for the rubber bushes to fit into the shock mount.

I got the turret mantle and recoil system back from the engineers yesterday. They had several attempts at removing the screws holding the recoil system in place. They couldn’t spark remove them and so they mig-welded bolts on the screws to see if they could wind them out. They kept breaking off so they arc welded them instead and that worked. As you can see, months of penetrating oil on these screws appears to have made no difference at all.

Has anyone pulled these recoil systems apart? Is the spring under load in the housing? Both recoil systems are a bit knocked about and as one is sold, I want to keep the best one for myself (sorry Lynn). It looks to me like stripping one down will be a challenge as well. There are NOS recoil systems still available so I will see how we go with this one.

I think I found out why my 37mm barrel was not straight! As per the photos, it looks like something has struck the barrel in two places, which was covered by the incorrect sleigh. I see also that the barrel has previously been cut near the breech and welded up. There is a bit of work to do there to tidy the barrel up before I put the correct sleigh that I have on there. I will also need to rebuild the mounts for the lever arm shaft and straighten the lugs that secure the recoil system coupler on the breech ring.

The intermediate and rear axles are progressing. Getting the diff housing bearing cones out was a problem but I understand a gas torch has now encouraged them to come out. I spoke to the engineering guys about the cracked differential housing and they say it won’t mig weld. We are going to look at whether it can be brazed but they tell me there is no guarantee that this will work.

Is there anyone out there with a worn M8/M20 differential who would be prepared to part with that part of the diff assembly? That would save me the trouble of trying to repair this piece which mightn’t be successful anyway. Thanks to Andrew, I might have some ready made gaskets coming to, so we are getting there.

Most of the hydraulic lines are cut and shaped now but I’ve still got to do the ½” tube.

That’s all for this week.