Much as we like working on our old engines, there's a limit to what many of us can handle, and a seized engine is pretty much that limit. But don't throw in the towel if your engine is stuck, because a little insight and the right approach can often get that old iron spinning once again.
Engines seize for any number of reasons; rust (usually from sitting too long), excessive heat (from running the engine without coolant or oil), or for some mechanical problem, such as a foreign object in the cylinder or a crankshaft bearing failure.
If you think your engine is seized, you need to confirm that before doing anything else. The first thing to do is remove the spark plug (s) and try rotating the engine. On the small engines most of us are working on you can bet that if you can't rotate it by hand, it's probably seized. On larger engines you'll want to use a breaker bar to see if it will turn.
Assuming everything is okay with the crankshaft, the easiest and cheapest thing to try is penetrating oil. This works surprisingly well, particularly with engines stuck from years of sitting out in the rain, where water has rusted the piston rings to the cylinder.
Everybody has their favorite penetrating oil, but I like P'Blaster. Start by shooting oil down into the cylinder through the spark plug hole, and don't be shy - use a lot of the stuff. Wear goggles in case the penetrating oil sprays back and hits you in the eye. If you have the patience for it, spray some in every day for about two weeks, followed by gently tapping on the cylinder wall with a hammer handle. This sets up vibrations that help the oil penetrate between the rings and the cylinder wall. After soaking the engine for a while try rotating the crankshaft. If you've been patient, and if the engine wasn't too badly rusted, it will usually break free and rotate. If it doesn't, hit it with more penetrating oil, and make sure you use enough. I've found that about half a can per cylinder is right. If it still doesn't rotate after four weeks of daily treatment, it's time to resort to more serious measures.
Pulling it apartIf penetrating oil doesn't work it's time to remove the cylinder head and, most likely, the crankshaft. On smaller stationary engines this is pretty straightforward, but on larger tractor engines you'll be getting into quite a bit of work. On a multi-cylinder engine it's often possible to discern which cylinder is stuck, sometimes just by looking into each cylinder. If you're lucky and you can figure it out, remove the end cap on the connecting rod of the offending cylinder and then rotate the crankshaft so the crankshaft throw for that cylinder is out of the way. With the cylinder head off take a mallet and try and pound the piston out, placing a block of wood on top of the piston so you don't bang up the piston with the mallet. Make sure the wooden block is clean - you don't want to mar the top of the piston with sand or small rocks. Make sure the piston can slide out the bottom of the cylinder without smashing the crankshaft -you don't want to make things worse than they already are. Most of the time this is the extent of removing a stuck piston, but if it's still stuck it's time to bite the bullet and head to a machine shop to have the piston pressed out or, even worse, bored out.
Assessing damageDepending on what caused the engine to seize, you may have damage to both the piston and cylinder. If you had to beat the piston out with a mallet there is a good chance the rings will be damaged. Piston rings are fairly brittle and will shatter if subjected to too much force. If the rings aren't busted make sure they rotate freely in the piston grooves. Rings often get glued to the piston grooves by varnish or carbon deposits, keeping the rings from expanding against the cylinder wall and doing their job. Stuck rings can usually be freed with penetrating oil, but if they are broken you'll have to replace them.
If you have to replace the rings, make note of where the ring end gaps are positioned, and if the piston uses different types of rings in each groove make sure to mark their type and position. Clean the piston ring grooves with a piece of an old piston ring, making sure to clean out any dirt or varnish from the ring grooves so the new rings won't stick.
Take a good look at the piston itself. Clean it thoroughly and examine it under a light. Is it scored, torn, cracked or broken? Measure the piston at different points and compare the numbers with the specifications in a manual, if you have one. Clean the cylinder wall, oil the piston and try reinserting it in the engine. It should slide the length of the cylinder freely. Rotate the piston as you move it up and down - if it jams at any time you have a problem. Pistons can get distorted when they seize, and how you deal with this depends on how you are going to use the engine and how much money you want to spend.
The most remedial repair is to simply file off any metal that keeps the piston from working freely. Sometimes you can set the piston up and turn it on a lathe. The best bet, and the most expensive, is to simply replace the piston.
That said, be sure to measure and inspect the cylinder for damage before running out and getting a new piston. Clean the cylinder wall and shine a bright light down the cylinder - is there any visible damage to the cylinder wall? Minor scratches can be removed with a cylinder hone, but deep scores will require the services of a machine shop. Depending on how much metal has to be removed to return the cylinder to working order, you may need to put in an oversized piston or resleeve the cylinder. The sleeve is a new cylinder pressed into the hole where the old cylinder was.
You can measure the size of the cylinder with a telescoping bore gauge and a dial caliper or a micrometer. Be sure to take measurements at several locations in the cylinder (generally at the top, middle and bottom) and at 90 degree and 45 degree angles to each other. This will help you determine if the cylinder is out of round. A really worn engine will have an oversized bore, and it's always good to check the manual for the acceptable dimensions for your engine. I realize that if you have an antique engine there may not be a manual, but often you can find a manual for a similar engine of similar vintage and work from there. Working tolerances, by and large, are fairly standardized.
The bottom endThis is also a good time to check the wrist pin and connecting rod bearings, as loose wrist pins or worn bearings will cause a rapping or knocking sound and will accelerate engine wear. Always replace worn or damaged bearings and wrist pins, and check the connecting rod for any damage, as well. Simply lay the rod on a straight edge or the top of a table and check to see if it's warped. And check for cracks - rods can break, so look carefully.
Before installing the rings, make sure you know which way the bevels (if there are any) are orientated, and don't forget that the gaps at the end of the rings need to be properly positioned. If the gaps are lined up in a row top to bottom you'll lose compression as combustion gases escape through the gaps. Generally speaking, you want to space the gaps at 45 degrees to each other, starting at the top, trying to avoid any overlap.
New piston rings are surprisingly sharp, so wear gloves to protect your fingers. The rings have to be expanded to fit over the piston and into the groove, and because piston rings are brittle you need to open them slowly and carefully to keep them from breaking. A piston ring expander is the best option, but you can install piston rings by hand with just a little extra care. If you have any old, unbroken rings, practice with one of those first so you can get a feel for it.
Once the rings are installed, coat the piston and the cylinder with clean engine oil. Double check that there isn't any dirt or grit in the cylinder befores installing the piston. Use a ring compressor (a large hose clamp will do just as well) to compress the rings into their grooves before inserting the piston into the cylinder. Don't force a piston into the cylinder with the rings sticking out of the grooves. This will break the rings. Gently shove the piston into place, and then install the connecting rod bearings and end cap, making sure you didn't get any dirt on the bearings. With everything back together try rotating the engine - it should rotate freely, and it will feel so good you won't stop until your arm falls off.
Contact engine enthusiast Gary W. Grinnell at 9 Laurel Pak, Northampton, MA 01060-1196.