Restoring ‘TheTwin’ Jacobsen Engine

Disassembly continues and restoration begins with the rods and wrist pins on a pair of rare Jacobsen twin cylinder engines -part 2 of 3.

| August/September 2020

The cleaned-up lower crank case with center split bearing in place.

In part one, collector Andrew Mackey described the roots behind his love of Jacobsen engines. After a 20-year search, Andrew was the owner of not one, but two, Jacobsen Twins. While evaluating the near-running engine’s condition, he began the tear-down. He removed the magneto, starter, tinwork, flywheels and ended with the lower shroud. 

The next step was to split the crankcase. To do this, I had to remove the engine base from the wooden mount. I then found where the mystery nut came from. Two of the four engine mount bolts were missing, and one of the other two, still in place, was missing a nut. How it got inside the flywheel housing the world may never know. Next came the removal of the upper block from the lower crankcase mounting bolts. There are four bolts that are threaded into the engine block lower section, at the center bearing location. There are also 12 nut and bolt assemblies that go around the perimeter of the block. Once all the bolts and nuts were removed, the exhaust stacks were removed and the engine was placed upside down, resting on the heads after the broken spark plugs were removed. I split the case by gently tapping on the crank with a plastic hammer. There seemed to be no gasket between the upper and lower case halves, just a tightly machined fit. After reading a manual for “The Twin,” it was explained. No gasket exists. Expert machining of the crankcase halves and the bearings make for an air-tight, metal-to-metal fit. The bearings are clamped between the case halves. The center bearing is split and is anchored by a 1/4-inch dowel placed in the upper case half. The two outer bearing mains are solid and must be slipped onto the crank. They are pinned in place by 1/4-inch dowel pins, located in the upper block as well. All bearings are grooved internally to help circulate oil. The crankshaft seals needed repaired.

Once I had the case split, I found quite a few things that needed attention. The lower case half was full of crud. Gummed up oil and rusty debris filled the engine sumps. Next, the connection rod big end caps were reversed. Stamped in location marks did not line up. Someone had been inside before and sullied the alignment. Besides that, the conrod big end retaining bolts were not anchored. The conrod bolts are drilled, and castle nuts are supposed to be secured with cotter pins. The cotter pins were missing.

I removed all the retaining nuts and the rod caps, and installed the caps correctly, tightening the cap bolts finger tight. The crank was bound up tightly. I noticed not only were the rod caps reversed, they had been swapped side to side as well. I removed the caps and retaining bolts and installed the caps with their respective rod alignments, and now the crank turned over easily. The problem was the caps were facing the wrong direction. I disassembled again, lifted the crank out of the upper block, and removed the piston and conrod assemblies.

Connecting Rods and Wrist Pins

Now that I had the pistons out, I found the issue of wear and the source of the clanking noise. The wrist pin clearances were bad. The next thing to do was the remove the wrist pins and conrods from the pistons. I found another screw up there. The wrist pins are hollow and the pin goes through a 1/8-inch hole drilled about 1/4 of an inch from one end of the pin and piston wrist pin support casting. A wedge is driven through the halves of the pin, within the hollow wrist pin, anchoring the corer pin in place, thus holding the wrist pin to the piston. In this engine’s case, long cotter pins were driven into the pistons until they bent against the inside of the top of the pistons. As I was disassembling them, one pulled out easily, the other fell out. As the conrod rotates on the crankshaft, the top of the rod acts as a bearing, allowing movement between the rod and the piston. There are no separate bearings in the conrod. Removing the wrist pins was not easy. On the Jacobsen twin engine, besides the cotter pins, there are also bronze buttons pressed into the wrist pin bores, and up against the pins themselves.


Gas Engine Magazine A_M 16Gas Engine Magazine is your best source for tractor and stationary gas engine information.  Subscribe and connect with more than 23,000 other gas engine collectors and build your knowledge, share your passion and search for parts, in the publication written by and for gas engine enthusiasts! Gas Engine Magazine brings you: restoration stories, company histories, and technical advice. Plus our Flywheel Forum column helps answer your engine inquiries!

Facebook YouTube