Pitfalls of Tractor Engine Rebuilds
Rebuilding the engine of a project or working tractor is one of the more rewarding processes you can undertake. The first startup after you have put the machine together with your own hands is exciting and pleasing. The whole experience can be marred if one moves too fast and makes too many assumptions that they can use "as is" some parts that should be closely scrutinized and possibly attended to. In such cases, rework makes what could have been a fun project turn into an irritant or even a nightmare.
To give you an example of a minor but irritating problem, picture this; You are standing there reveling in the pride of your newly rebuilt engine, listening to the purr of those bearings, pistons, and rings all working in smooth harmony and you look down to a drip of oil coming from the rear seal or possibly a puddle of anti-freeze forming from a small but steady drip from the water pump. I call this an irritant because even though you will have to pull things apart, this type of problem does not impact the primary work of the rebuild and likely won't mean repurchasing parts such as bearings or rings.
You won't even get to the point of hearing the nightmare engine run. With a nightmare engine, you may have tolerances set "by the book" on the ring gap, the main bearings and rod bearings, but every time you torque the bolts, you must use a four-foot pipe wrench on the crank pulley to turn it over. How does a normal rebuild become a nightmare? You may know or have heard of the scenario. You waste three gasket sets putting it together... tearing it down... putting it together... tearing it down etc., each time waiting for the UPS truck and new parts that you should've put in right off or replacements for new parts you destroyed on your first attempt. (Note: there's nothing I dislike more than cleaning gasket surfaces that I cleaned the weekend before). What should have been a weekend project has taken months, much of the time spent trying to avoid the shop so you can forget this perplexing failure. Many times, you are not at fault for a rebuild working out this way. It can happen frequently with engines that seized up in a hurry. Fortunately, most rebuilds are undertaken because the tractor was just getting tired and nothing catastrophic happened to force the rebuild. By catastrophic, I mean something happened that made the engine stick hard and stick fast...while it was spinning at 1500 RPM.
Since tractors don't always get the best maintenance but still must work hard, these catastrophes do happen. If you are the one that has put it back together, you are called upon to do a much far more detailed diagnosis than the normal rebuild would require. In these situations, metal will distort, crack, break, twist, and bend.
If you try to just get by with problems like those described above ("Well ... it’s just a nice tight engine") the results will be disastrous. If you proceed with putting it together ("I'll just pull start it with the 4-by the first time to loosen her up a bit") You will likely destroy your new rebuild and probably end up junking the engine or tractor out. If you want to make sure that when you put it back together, it stays together and works right, you must check tolerances and alignments that you could just take for granted on a normal rebuild.
The things you don't have to worry about
One nice thing to remember is that your serious rebuild is probably going to have a complete engine kit so this eliminates many items from scrutiny. You will have new pistons, rings, pins, sleeves (or a bore job), valves, guides, bearings, and gaskets. For these items, someone else was responsible to make sure that everything was the way it should be, either the manufacturer, or in the case of the bore, your machine shop. Your crankshaft will also be "taken care of" since it will have been turned or welded and turned (more on that later) and by default must be straight or they wouldn't have been able to turn it.
What you do have to worry about
There is certainly more to rebuilding than just stuffing in the new parts, especially when you have a machine that froze on the move. I will list a few that I have found through great pain and many gasket sets. Note that some of these items may bear looking at on your "normal" rebuild also since:
“Assembling an engine once is fun. Assembling that same engine twice is a serious bother. Assembling it a third time is a downright pain in the ...”
Connecting rods may give the appearance of being very strong and rigid but not so. Just to realize how flexible they are, lock a solid shaft in a vice, slip the piston pin over the shaft with the rod attached and torque the rod bearing caps on to manufacturers specs (you will have to do this anyway to determine if the surfaces that hold the bearings are perfectly circular). You will see as you are torquing that you could twist and damage the rod quite easily. If you are working on a machine that seized while running, you will likely find at least one of the rods is visibly bent or twisted. If you can see the bend, you cannot use the rod. Even if you can't see the bend, rods from such an engine should be checked for straightness. I replace the rods or have them reconditioned (during the reconditioning process, they will tell you if you have a rod that is unusable).
Another critical thing to look for on the rod is the out-of-roundness of the surfaces that hold the bearings. To do this, use the inside micrometer to measure the circle at right angles. As described above, the bearing cap must be torqued on to do this. More than .001 is probably not acceptable and may cause your bearing to lock up tight even though the Plastigage measurements check out during assembly.
A symptom of either of these problems will be that the crankshaft will not turn easily when you have all bearings and pistons assembled.
Push rods bend easily if the valve timing was off or even a valve was adjusted to be open all the time (you may end up with a tractor that was someone's mechanical learning experience). Visibly bent push rods should be tossed, and others should be checked. Checking them can be done with a very flat surface and feeler gauges. I have heard people say they can straighten push rods, but I have always just replaced them from a tractor junk yard.
Camshafts can bend but it is unlikely since the push rods tend to sacrifice themselves much sooner. The real problem on the camshaft is the bushings that they ride on. If the surface went too long without oil or had a foreign object lodged in between, the scoring may be too severe for quiet operation. While it is possible to get the shaft turned you need to check first if oversize bearings are available since normally only standard size are sold for old tractors. If no oversize is available, you can still have a machine shop make new bushings and as an alternative a used or reconditioned camshaft may be purchased.
While a bent crankshaft is unlikely, it is possible. If you aren't getting a rebuilt but rather having it turned or welded and turned, the straightness will be determined for you. Nonetheless, when you get your freshly welded and turned crankshaft back, examine it very carefully for imperfections in the surface. These do happen and will cause you to re-replace your bearings before you ever start the engine.
A symptom of this problem is that the crankshaft will not turn easily when you have all bearings and pistons assembled and you will have horrible scoring on your new bearings.
There are a few things that can go wrong with the basic block. The first and most obvious is the lack of flatness of the head and block mating surface. These are checked with a straight edge and feeler gauges. Your manual will provide the tolerance that is allowed. If the engine is sleeved, this tolerance takes on an extra importance because the stand-up (or how far they stick out) of the sleeves must be considered. If there are radical differences between the cylinders, you have probably assembled the sleeves either with dirt under the flange and down where the O-rings fit, or the lower mating surface is distorted for some reason. This can cause leaking at the head gasket, seepage of oil into the coolant at base of the sleeve, and distortion of the sleeve that hampers free movement of the piston. This is easily recognized before you install the sleeves by slipping the sleeves in without the O-rings to be sure they fit with the required stand-up tolerance. Don't depend on a "bigger hammer" or the head bolts to fix this problem since neither will work. Fix it before assembling the sleeves.
Another problem with the block can be hairline cracks. Such cracks must be repaired before any new parts go in or the welding and surfacing will surely damage them. Though there are probably many weak points on several machines, a common example is the lower right corner on a Farmall Cub engine. This flange commonly cracks and will be a persistent oil leak once your engine is assembled (One that can't be corrected without complete disassembly).
Rocker arms can be distorted which can make adjustment difficult and sometimes even cause the push rod to slip by (makes an enormous racket) when combined with a slightly bent push rod. Check these for straightness and smooth profile.
If the oil pump won't pump sufficiently, your new parts are at risk. Usually, you can either resurface the pump facing to bring it into tolerance or take it to a machine shop and let them do it. It is a simple job while the engine is off but, on most tractors, requires removal of the engine to perform.
The seal of the water pump may be unwilling to hold the anti-freeze in after sitting in one position for many years (as happens with many of the machines we rebuild). It's not too expensive or difficult to take care of while the tractor is disassembled. Fortunately, the bearings in tractor water pumps are frequently still good and reusable so the fix is cheap.
While you are reassembling the engine, check to see if you have too much slop in the governor shaft. Replace this simple bushing so your governor will work correctly. Slop in this bearing will change the centrifugal force action of the governor causing it to compensate too slowly when fast compensation is required.
I find this to be one of the most irritating mistakes to make when rebuilding. The mating surfaces on each end of the crankshaft must be smooth and perfectly round or you end up with a nagging oil leak. You don't see the leak till you start the tractor and get it hot, but fixing it requires complete disassembly.