1941 John Deere Model HN - Replacing a Cracked Head and Block

Fair warning, this is going to be a long one.... I purchased a Model H "parts" tractor with hopefully a good head and engine block to put on my JD HN that has a freeze-cracked head and block. I won't know if the "new" head and block are usable until they are removed from the tractor and thoroughly inspected, but fair warning, I did notice a welded-up freeze crack in the parts tractor's engine block before I bought the tractor. I had driven my truck and trailer 2 hours to buy the tractor and when I first got there, I immediately looked on the side of the block for a crack or repairs. Yep, there was a repair. I told the seller and he said he didn't know about it. Before the trip, we had agreed on $700 for the tractor, but after seeing the welded crack, I offered $500. He countered with $550, which I accepted.

It was a gamble buying the parts tractor but I'm hopeful things will work out. I'm starting to write this blog 3/4 of the way through the project. I have both heads off and both engine blocks out of the tractor's frames. Let's start at the beginning though.

(For context, here are two links showing what I did initially to the HN's freeze-cracked head and block to try and repair them: (part 1 and part 2))

I'll start with the HN and its cracked head and block. This first photo shows both items out of the frame. The process would have been a lot easier if the hood and gas tank were removed first but that is a big job I'd rather not tackle.

Below is the head, and on top of the head is the crankcase cover with coagulated oil and water mix. It's a mess in the engine due to water getting into the crankcase.

Below shows the mess a job like this produces. It's maybe half the tools needed. And all these tools then need to migrate to the parts tractor to do the same thing.

Here's the offending engine block. Cute little thing, all 99.7 cubic inches of it.

I happened to notice on this piston that the top (left) piston ring is stuck. You can tell by the gap at the ends of the ring, and by the fact that the ring has not sprung out from the piston groove. That didn't help at all with compression.

9/4/2026: On to the parts tractor. Let me say up front, I don't think this tractor's engine has been messed with much, if any. This photo below shows that the inside of the head is in decent shape but the water passage at the bottom is half full of rusty crud.

9/5/2026: The head took a bit of effort to get off but wasn't all that terrible. The exhaust manifold shown in the photo below has had a crack welded up, so I tried to get the stud nuts off but was unsuccessful, even with an acetylene torch. Not sure what to do about that. I'd really like to use the non-cracked manifold from the HN if I can get this one off.

Here are the valves. The exhausts are rusty and stuck in the guides. Them being stuck caused the push rods to bend when the previous owner checked to see if the engine was free to rotate. The rods can probably be straightened but I am just going to use the good ones from the HN.

Here is one exhaust valve out. It is very pitted, but I'll try to refurbish it.

The exhaust manifold passages and head's exhaust passages were completely full of junk and nuts that small critters had stored in there. This photo below is of the first exhaust port.

This is a small portion of junk that initially fell out of the manifold.

This next photo below shows the gizmo I welded up to compress the valve spring to get the keepers off the valve stem. It was a bit too sketchy.

This next photo shows my new and improved method. The flat pry bar has a hole drilled in it that the bolt goes through, so it's less likely that things will 'ping' into oblivion.

Here is the second valve's port that is full of junk.

And here's the second valve. It looks worse than the first one.

I'd bought a valve seat grinding kit from Amazon when I worked on a Ferguson TO-35 several years ago. I didn't expect much and the kit lived up to my expectations. It is the absolute worst in terms of quality but surprisingly it works well enough to be used on an old tractor.

The kit came with 5/16" and 11/32" pilot rods to fit into a cylinder head's valve guides. Problem is, the valve guides in this JD head are 3/8"..... The one good thing is, the pilot rods have a larger top end to fit the grinding wheel holder. I was able to turn down a small section of one pilot rod in the lathe to snugly fit the 3/8" valve guide.

While not spectacular, the valve seat cleaned up sufficiently for this old tractor.

As I've done before, I tried cleaning up the faces of the valves on the lathe using a standard cutting tool. On these valves it didn't work that great. I'm guessing the valve faces are hardened. You can still see a bunch of pitting.

I resorted to using an angle grinder on the faces of the valves while they were still chucked up and turning in the lathe, which seemed to work much better.

Once the valves were ground and lapped to the freshly ground valve seats, it was time to reinstall them. I was able to finagle the first valve spring under my makeshift spring compressor but I just couldn't get the second one. It was then that I remembered that zip ties work wonders when holding compressed springs. I put the spring in the vise and put on two zip ties.

It worked great.

9/7/2026: Now that the parts tractor's head was done, it was time to tackle the engine block removal. What a headache that was. I'm guessing this block has never been off this tractor. Suffice it to say, the Facebook Model H forum came to the rescue. Other people have had the same problem and one or two knowledgeable folks spoke up. The solution, while still a bear to accomplish, was to loosen two frame bolts under the block (see two photos below) and jack up the transmission case so the whole front end of the tractor would slump down, break things free, and pulling the block loose.

This photo shows the two frame bolts. Yes, doing this did start things moving but it was still a colossal struggle to get the block out. Jack up the transmission case, pry, brace things, let the jack down, pry some more, jack up the engine block, pry more, brace more, let the jack down. Do this 100 times and the block will come free. Even after the block was loose, it still kept getting hung up on various things.

The reason the block that's been sitting in the frame for decades gets stuck is that dirt and silt get deposited between the frame rails and block, things solidify, and pretty much make the two parts one solid, inseparable entity.

Unfortunately the pistons conceal all the debris.

Once both blocks were removed, I measured bores. The HN's severely cracked block has been bored 0.030 over and has oversize pistons. The parts H's minimally cracked block is still its original bore size with original pistons, so the pistons from each block need to go with their respective block.

Standard bore on the H is 3-9/16, or 3.563 inches.

The parts H's bores measured at about 3.568, so 0.005 over the nominal standard bore. It's a small enough difference that I will just reuse the same rings after honing the cylinders.

The HN's cracked block bores measured at about 3.597, so also about 0.005 over the nominal 0.030 (3.593) overbore. I don't know if this block can ever be reused due to the internal cracks.

9/9/2026: The parts tractor's pistons have interesting stampings on them. I'm guessing inspection marks. There must be five or six different stamps on each piston.

You can't tell by this photo but above the top piston ring is supposed to be a groove to collect carbon. Well apparently it has done its job, as it is full of carbon.

After half an hour of chipping away, the groove is clear. Now on to the second piston....

I decided to braze the crack on the parts tractor block. I ground out a groove in which to fill with braze.

This is after brazing and grinding the excess. In hindsight, I probably should have left the high spots alone, as you'll find out later.

The block is ready to be put back on the HN. I installed the parts tractor's pistons on the HN rods, then proceeded to put in the block. What a royal hassle that was. I'm guessing most folks unbolt the rods from the crankshaft, put the pistons/rods into the engine block, then slide the whole mess into the frame. I didn't do it that way. I think I did save time in the long run but it was still a lot of work finagling the pistons into the cylinder bores while sliding the block rearward, also while trying to keep the rods from binding on the transmission case.

9/12/2026: All the required bitsgot put back on the tractor today and things got buttoned up. I filled the fuel line with gas (the gas tank or sediment bowl appears to leak so I need to address that at some point). I choked the carburetor, set the throttle, and gave the flywheel some tugs. Nothing. More tugs. More nothing. This went on for a couple/three hours over a couple of days. I rechecked timing, spark, carburetor float. Nothing wrong.

One thing in the back of my head was that I never did look at the intake valves. Was the intake manifold full of nuts also? It didn't appear so but I didn't check very closely. I pulled out my borescope and tried as best I could to look into the manifold. I didn't see anything, which was great. So more flywheel tugging and pretty much nothing but a very occasional very weak pop. I even went so far as to unhook and dig out my air compressor that had a 2hp electric motor on it, so I could try using a V-belt from the tractor belt pulley to the motor. I turned on the motor and realized it spun the wrong direction. Sigh.... So it was back to pulling on the flywheel.

After what seemed an eternity, the engine finally started spinning over a couple of revolutions at a time, then it wasn't long until the engine finally took off. Yay! Finally! Lots of little black bits came out of the exhaust, I'm presuming debris from the intake manifold. Once the engine ran for a while, the piston rings and valves got broken in and the tractor is now very easy to start. Here's a 

I then put water in the cooling system. Hmm, is that tiny streams of water coming from several pinholes in my brazing? Yep. I guess I need to address those at some point.

Once the engine ran for a while, the piston rings and valves got seated and the tractor is now very easy to start. Here's a short video of starting and running the engine:

9/13/2026: There was a lot of gasoline staining on the engine block under the sediment bowl, and the bowl itself was pretty crusty looking, so I unthreaded it and the three-way valve from the tank. The shutoff for the sediment bowl was worn out so I ended up buying a new one on Amazon for a Ford 8N. It was $16 versus one that was a JD clone for over $70. The 8N bowl had a flare type outlet so I rummaged around in my brass fitting bucket and came up with a suitable fitting to adapt the three-way valve to the sediment bowl.

This next photo is the three-way valve, which selects either Off, Gasoline, or Distillate, although in this day and age, everyone uses only gasoline in these tractors. I was able to find a couple of O-rings that sealed off the end of the valve so it doesn't leak.

Once the sediment bowl and three-way valve were reinstalled, I put some gas in the tank and nothing leaked. That's a big win.

Now on to the radiator leak. This being a John Deere, nothing is simple. The front grilles need to come off, but to get them off, all the screws holding the hood on need to come off. Then the radiator shutters need to come off.

It was readily apparent where the radiator had sprung a leak. Just follow the rusty water stains. The photo below kind of shows two vertical tubes that are bulged out, but the photo under this one shows it better.

The two tubes are four to five times wider than they should be. It looks like only the left one is leaking though. I'll try to open up the fins enough to get a soldering iron in there and solder up the hole, but these kinds of repairs are never easy, and there's usually a 50% chance they won't work the first time.

I haven't yet gotten to the radiator repair but I thought I'd mess with getting the radiator shutters freed up. After maybe 1/2 an hour of fiddling, they finally started moving, but it's going to take a lot more back and forth movement before they are ready to be reinstalled.

9/16/2026: Today is "plumber day". I'm going to fix the pesky, drippy leaks. The radiator is first. This photo below shows the ballooned-out tube with what appears to be a crack down the center.

I've moved the fin material out of the way to get access to the brass tube. The crack is there but it doesn't show up very well.

Initially I tried using a propane torch to do the soldering since I had to heat up a lot of surface. In the past I've never been very successful doing radiator repairs using propane. This time was no exception. I then switched to my 260 watt soldering gun, which worked a lot better. After this was soldered, I filled the radiator and it didn't leak. It's not pretty but it is functional.

Next was to grind out the pinhole leaks and fill them with JB Weld. I noticed there was one crack (the one furthest up) that I had missed when I did the brazing.

Once the JB Weld was hard, I filed down the high spots and put a coat of paint on, then filled the system with water. I then noticed a small bubble forming under one spot.... Hmm, that's probably a hole. Out came the carbide burr one more time to grind out a divot and fill with another dab of JB Weld. Hopefully this is the last of it.

Smoothing things out yet again.

And paint for the third time.

I again filled the cooling system with water and this time I found no leaks. It's about time! Next it was on to the process of getting all the tin screwed back down.

I dug the LA out of the garage so I could put the HN in the garage. The LA will probably be getting sold sometime soon. I think I'll end this blog post here, as it is quite long enough.