Not for the weak minded.

John Deere 3hp, Episode 6: Lower assembly

January 16th, 2008 Josh

For this article, I’m going to call all the parts inside the crank case and below the “lower assembly.”


This was an easy, but at the same time frustrating, part of the project. The frustrating part was using the wrong gasket sealer. I used some black tar-based sealer called “Form-a-gasket” that my grandpa likes to use. The problem that I had was this stuff seemed to liquefy and run right out the sides when I bolted the parts together. This wasn’t too bad since it meant that the parts had a good fit and didn’t need much of the sealant, but there were two places that kept leaking. The parts in this assembly are the main crankcase, then a gasket, then the oil pan, then another gasket, then the gas tank, then a support casting that bolts the engine to the runners. These 6 parts formed a sandwich, and the problem is that the oil pan must seal to 2 passages on the bottom of the crank case, but there is no support on the other side of the oil pan to press the metal against the gasket. So every time I bolted the whole assembly together, there was a huge leak.


My solution was to weld 2 plates on to the bottom of the oil pan, so that the pan will resist deflection. These plates were also bowed slightly upward so that they must be compressed by the gasket and passages above. This worked, and while pressurizing the gas tank to check for leaks, there was a funny sound followed by a rush of air and the gas tank lost pressure. I thought for a few seconds that the tank gasket had failed, however, I noticed that I could feel the air rushing out next to the gas filler neck, where there was no gasket. There I found a vent hole that I had never seen before, and may have been the cause of the gas tank not holding pressure the whole time! Either way, all the gaskets held and the lower assembly was bolted to a set of wooden runners.


The other parts of the lower assembly (shown from the bottom of the engine before putting the gas tank and runners on) are the governor and cam gear. These were installed and tested for bearing play (using the old-fashioned method of wiggling with the hand) and appeared to be in excellent condition.
Once all the parts in the lower assembly were together, the biggest hassle was cleaning the black tar form-a-gasket left on the side of the engine. It was resistant to gas, solvents, water… The only solvent I found that was effective enough to remove it was acetone, and even that took liberal amounts. Once the lower assembly was complete, the piston and rod were installed and the parts for the governor and cam were installed in the box on the side of the crank case.
The only thing left is the upper assembly and final detail!