Restoring ‘TheTwin’ Jacobsen Engine

Author Photo
By Andrew Mackey | Jul 7, 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.

The buttons are .626 inches in diameter, and about 1/16 inch thick in cross-section and have a 3/16-inch hole in the center. They are dish shaped in profile. After the cotter pins were removed, a thin, long punch had to be placed through the wrist pin and the pin caps punched out from the inside. The bronze caps are concave in section and are placed in the wrist pin bores, and are hit with a punch or dowel to make them swage themselves in place. They do not float in the wrist pin bores in the pistons. They are there, should the cotter pins fail. They would keep the hardened steel pins from scoring the cylinder walls. After the wrist pin caps were punched out, the pins themselves were driven out of the pistons. They were a snug fit in the pistons.

Damaged wrist pins — showing excessive wear — had to be special ordered.

I then took a micrometer to the pins and found several things. The wrist pin is .626 inches in diameter. One thousandth over 5/8 inch – must be a Jacobsen thing! They are also about 2-1/2 inches long. The pins are hollow, having a bore of about 1/4 inch inside. They also have a 1/8 inch hole drilled through one end, about 1/4 inch from the end of the pin. The magneto side pin was worn .002 inches out of round near the center, and the starter side, over .004 inches out. The conrod small ends seemed not to be worn at all.

I needed new wrist pins. I did some research and ran up against a problem. You could not get new pins with the dimensions needed. No matter where I looked, no one could look up a new pin by size. I was repeatedly told I needed a manufacturers name and the serial number from the engine. Only one problem: Jacobsen did not serialize the engines. The only way to get the needed number is to have the original equipment the engine is mounted on. Since I had only the engine, the number is no longer available. Also, due to the fact that the engine is at least 77 years old, I do not think a new one is available anyway. I was put in contact with machinist Ellis Konkle, through the SmokStak forum, who could make one for me.

I sent Konkle an email about my problem and asked if he could make a new set of pins for me. He sent a message back that he could make a set out of hardened stainless steel, but he wanted me to send one of the used pins for measurement. I mailed him the lesser-worn pin and two days later I had my reply. $25.00 plus shipping, and I gave him the go-ahead and mailed a check the next day. Believe it or not, I had the pins within two days of my order. They were a tight fit in the pistons, as were the originals. They were polished mirror bright, too. He even returned my original wrist pin as well.

The pistons and conrods on “The Twin” seem to be the same as on the “4-Acre,” Sturdex and old Estate upright single-cylinder engines. The 2-5/8-inch bore with 2-1/2-inch stroke made the engine slightly oversquare. The new Estate was fitted with a horizontal engine — totally different from the uprights. The older engines use a cast iron piston, a hardened wrist pin and a solid phosphor bronze conrod. On the near-running engine, there were no shims on the conrod’s big end. The rods have an I-beam profile, with a divot on the intake side to deflect the incoming charge around the interior of the crankcase. The caps and rods were milled smooth at the mating surfaces. Then, they were assembled, reamed to fit the crank. Crank diameter is 3/4 inch and in the area of 1.5 to 2 thousands of an inch of clearance was specified and present. On the donor engine, the rods were shaped differently. There is no divot in the body to deflect incoming air flow, and the lower end was given the appropriate clearance with several brass shims, about .002-inch thick each. The machined surfaces were smooth, like they were polished, where the shims were mounted. On the near-running engine, the cap and rod mating surfaces looked like they had been milled. The machine turnings were still evident. On both engines, a pin punch was used to locate the front of the rod and cap, so the caps would not be reversed. You could also tell that both the conrod itself and end caps were cast of the same piece of metal — the casting seams all matched when the caps were assembled to their respective rods.


The near-running engine has a divot that faces the intake port on the cylinder. This helps divert flow of the fuel/air mixture around the interior of the crankcase. As mentioned earlier, the donor engine does not have this feature. I installed the new wrist pin in each piston, part of the way. I set it in so that the end of the 1/8-inch retaining hole was only extended into the piston interior by about 1/8 inch on the plain side of the piston. I then slipped the small end of the conrod onto the end of wrist pin that was sticking out inside the piston, making sure the conrod was facing the right direction. I continued to drive the pin through the rod and piston until it was in place, with the holes aligned both in the piston and the wrist pin. It took three or four tries to get the holes lined up. Once all the holes were aligned, cotter pins were installed. With the wrist pin now properly anchored, I reinstalled the pin retainer buttons. They were installed with the belled side out, then gently tapped in place with the peen of a ball-peen hammer. Once set, the pistons were ready to be reinstalled in their proper cylinders. Each piston has three rings of an overlap design. The overlaps are about 1/4 inch. The end gaps were set 120 degrees apart, and an effort was made that they did not overlap the ports in the cylinders as well. The bottom of the cylinders have a 45-degree taper machined into them, so no ring compressor is needed. I lined up the pistons and gently installed them in their bores. This was accomplished by liberally soaking the rings and piston as well as the cylinders with a good coating of SAE 30 non-detergent oil, and then gently rocking the pistons side to side while pushing them into the bores. Each piston went in steps, as each ring compressed and settled into their respective grooves. You cannot force them in quickly. After the third ring settled into its groove, the pistons were set in the cylinders halfway into the bore.

The arrow denotes an oil dipper on the conrod end cap.

One rod was set leaning toward the intake, the other, toward the exhaust side of the engine. I inspected the center bronze bearing on the upper block, and decided to replace it as well as the bottom shell half. The one in the donor engine was in better shape. The outer bearings were okay as for fit on the crank, so they were re-used.

Getting the bearing pins and the crank installed took a bit of time. You have to juggle getting the conrod big ends to align with the crank throws, as well as the two bearing location pins within the outer bearings. After the crank is settled into the upper half of the block, you need to tap the outer bearings toward the center, to take out the excessive end play on the crank itself. I left about .002 of an inch of play side to side. After the crankshaft was set in place, I installed the rod caps. Do you remember the crank was bound up when I first tried to install them? Well, I discovered why. The caps had been switched from their parent rods as well, when the engine had originally been torn down. It’s a good thing this engine never started after someone else had been inside. As another note, the rod caps themselves are directional.


Unlike today’s modern 2-stroke engines, the Jacobsen upright engines depended on liquid oil in the engine sumps for lubrication. There is a D-shaped dipper on the conrod cap that extends into the sump on the lower block half at the bottom of the power stroke. At the base of the leading flat side of the D, on the conrod cap, is an 1/8-inch chamfered hole through the cap that leads to a machined-in gallery for oil within the conrod crank pin bore. This gallery is both on the rod and cap. As the engine turns, the dipper splashes oil around the crankcase interior and into the rod itself. Some falls into these gallery notches in both the end bearings and the center bearing. Oil weeps out of the bearings by means of clearances, so oil is retained in the galleries for a while. The action is the same as on a 4-stroke engine. The Jacobsen engines of the 1920s, ‘30s and early ‘40s needed this liquid oil for best lubrication. Jacobsen found that the 16:1 oil-fuel mixture worked to supply oil to the sump without the external drip oiler by the mid-1930s. It still depended on the liquid oil in the crankcases in the 1940s, when the horizontal type engines came out. By that time, Jacobsen realized that oil in the fuel was enough to lubricate the engine without the dipper in the engine sump.

The Jacobsen Twin features sump drain levers on the lower crankcase. These levers are used to drain off excessive oil from the unit when it shows signs of being overly lubricated.

The low speed of the engine allowed enough oil to come out of the solution in the oil-fuel mix to settle in the crankcase, where the dipper could pick it up. The clearances on the big end were checked and turned out okay. The conrod big end cap retaining bolts were tightened a bit more until the holes in the bolts aligned with the castle nuts. Cotter pins were then installed and crimped in place, making sure the conrod conrod bolts never loosen and come apart.

There is a drain in the bottom of the sumps. A lever on the outside allows you to drain off excess oil. You knew to drain off some oil when liquid began shooting out of the exhaust, the engine smoked heavily, or you found oil-fouled spark plugs. As most old engines go, these need 2-stroke oil. No synthetic lubricants or one-mix lubricants. Your engine life depends on it. The modern lubricants and synthetics are too thin to be retained in the soft bronze bearing material.

Replacing the gaskets and seals

Next, I took some Indian Head gasket sealer and coated the entire mating surface of the lower block. I slightly lifted the crank, and using a cotton swab, coated the outer bearing sleeves and their block bores as well. The lower crankcase was then settled onto the upper, then tapped inward until the desired .002-inch end play was achieved. Then, the bolts and nuts were tightened. There are no specifications for torque. The gasket sealer oozed out and told me that a tight seal was made. There won’t be any air or oil leaks through the block.

Now I had to see to the bad crankshaft bearing seals. The seal is made by the crankshaft bearing, a steel shell crimped onto the bronze bearing, a felt washer installed into the steel shell, and a Babbitt outer seal bearing surface. The Babbitt outer seal section is held in place by the flywheel, by two dogs that engage the grooves in the flywheel mounting surface. The flywheel goes against the Babbitt which in turn goes against both the bronze bearing and the felt at the same time. The felt is a crush fit. It absorbs any leakage of oil from the crankshaft and uses it to seal against air and also lubricates the seal. In the case of this engine, excess end play of the crankshaft had pushed both steel shells off the bearings. On one of the shells, the felt was also missing. The backside of the steel shell was caved in, allowing the shell to ride up on the bearing itself. For the magneto side, the fix was relatively simple. The steel shell was peened outward with a ball-peen hammer. It was then placed over the step on the engine bearing, and pushed onto the bearing until the shell swaged itself back onto the bearing. It took two tries to get it right, both on the bearing and perpendicular to the bearing and crank. Once fitted, the felt was installed back into the steel shell, after soaking lightly in oil. The outer Babbitt was then installed onto the crank and then the flywheel, making sure the ears engaged the flywheel hub. After the assembly was complete, the retaining bolt in the flywheel was tightened. A Woodruff key holds the flywheel in place. One side done!

The starter side was a real pain though. The felt was missing and the Babbitt seal was actually worn into the interior of the steel shell. The shell itself was severely deformed. I managed to salvage a Babbitt seal from the donor engine. I got the steel shell off the bearing on that engine too, but it deformed badly. After five attempts at remounting it on the engine, I decided to get it in a close fit and epoxy it into place. I cleaned the seal to the bearing surface with some carburetor cleaner, as well as the back and inside surface of the steel shell. I mixed up a thumb nail sized pile of JB weld and loaded up the step on the bearing and then mounted the steel shell in place. The epoxy was spread around the back of the steel, to make for about a 1/4-inch bead. Hopefully, this will hold the shell in place. The felt and outer Babbitt was then installed along with the flywheel, to make sure the seal face was parallel to the Babbitt face. The flywheel assembly was left about .005 inch from seated until the epoxy had hardened overnight.

Tinwork Reinstallation

Next, I removed both flywheels and went to install both lower tin covers. There was no damage to the block where the sheet metal screws had been installed in place of the threaded screws, but when I tried to install new screws, I found the threads in the block did not match with modern screws. Even metric threads in the block did not match. Must be another Jacobsen thing. I did find, however, that between the original proper screws and those on the donor engine, I had enough to mount the lower inner covers. There are a lot of holes in the covers so it was a real pain getting the alignment right. Three screws hold each cover in place, two on the base of the block, and one toward the top on the cylinder. The screws were installed just snug, and the flywheels were placed on the crankshaft. I had to jockey the covers a bit to get the needed 1/16-inch clearance to allow the flywheels to clear and turn freely. The flywheels were again removed and the screws tightened fully. I had wanted to add a flat washer behind the screws and lock washers in order to better hold the tinwork in place, but the upper screw hit the flywheel. On that screw, I left the flat washer off.

There is a clearance groove on the back of the flywheels that allows for clearance for the washers on the lower two screws. Once the covers were tightened up, the flywheels were reinstalled snug against the seals making sure the out Babbitt face was engaged in the flywheels. The flywheel mount bolts were tightened up good and tight.

Vintage Jacobsen advertisements highlight the brands power, easier-starting and durable construction.

Now, to install the upper shroud work. The center cover was placed between the cylinders. It rests on the cylinder heads by two tabs, and is held to the lower tinwork backing, sandwiched between the upper and lower covers by two screws. Most of the mounting bolts were stripped out. Originally, the holes were stamped in and machine screws were used to anchor the pieces together. Most of the machined threads were wiped out by use of self-tapping screws in place of the original machine screws and stripped out small self-tapped screws that were in the shroud. Aligning the three upper shroud sections was a pain. I ended up removing the fuel tank to complete the shroud installation and while I was at it, I replaced the upper tinwork from the magneto side with one from the donor engine.

Some clean up with a scouring pad and fine sandpaper had it pretty smooth, and a quick coating of Rust-Oleum silver spray paint had it looking like the rest of the engine. I can only hope that oil and gas will not take it off. At this point, the basic engine block assembly was finished but not complete.

Wooden Engine Mount

The next item to be worked on was the wooden engine mount. The mount was loose on the rails of the cart, and I decided to shore them up with longer lag bolts. As it turned out, it needed a bit more work. The 2-inch-thick wooden base for the engine was split partially, in two places. Instead of replacing it, I decided to cross drill it and install 5-1/4-inch x 6-inch-long lag bolts through the block of wood. These cross bolts will prevent the block from splitting apart entirely. I then drilled out the holes that originally mounted the block to the cart, and installed four carriage bolts, clean through the mounting block and cart rails. No chance of a lag bolt ripping out now. After making sure the mount block was good and secure, I placed the engine block assembly on the wooden mount and installed one bolt into the engine. I took a long 3/8-inch drill and made sure the three holes lined up with the crankshaft parallel to the cart rails. As it turned out, the original holes were about 1/8-inch out of square with the cart rails. This explained why the starter and magneto mounts split and ripped out — misalignment. Everything beyond this point had to be either remade or re-aligned in order to get things together.

Lubrication Recommendations

SAE 30 non-detergent oil

This  is comparable to oils made when these engines were built. You do not want to use detergent oils as they leave an abrasive material ash in the combustion chamber after the fuel/air mixture is burnt. The ash will increase wear to the engine rings, pistons and cylinders. Detergent oil may also loosen crud in the crankcase, and get into the bearings and on the cylinder walls. This can lead to excessive wear and possible seizure.

Uncut TCW type-3 oil

tUncut TCW-3 oil is a good oil, used on marine water-cooled and heavy-duty air-cooled 2-cycle engines. You do not want to use synthetic lubricants nor premix lubes in these old, plain-bearing engines. The synthetics are pre-lubes as well as the cut TCW-3 oils are diluted with hydrocarbons and/or mineral spirits in order to make them mix with gasoline easier. They are thinned out to the point where they do not lubricate the bearing in an undiluted state, as regular SAE 30 non-detergent or uncut TCW-3 oils do.

A good test is to smell the material you intend to use in your engines. If it smells like oil, you are okay. If it smells like solvent, do not use it in an engine with plain-bronze bearings. This includes Maytag and other plain-bronze-equipped engines.

Jacobsen never put serial numbers on the engines, only on the carts. If the engine was removed, there was no sure way to place when the engine was built unless the equipment tag was taken with it. You could come close by comparing an engine to a known-dated unit. You can see certain production changes over the years, but without the original equipment tag, you can only guess. As for my two twins, I do not have the mower ID plates. I can only guess that the near running unit was made early on in production, as evidenced by the advertisements.

Jacobsen used quality materials and their machine work was exacting.  There were six different sized pistons. Standard, .010, .020, .030, .040, and .057 oversized rings in 2946. Although, there were no overbore conrods, there were oversized wrist pins. Besides the standard, there were .002, .003, .005, and .010-inch sizes. It seems you had to bore the rod small end and the pistons to accommodate the oversized pins. The crankshaft bearing was not undersized. If the crank was out of spec, the crank and bearings had to be replaced.

Contact Andrew Mackey at PO Box 347, Rockaway, NJ 07866 • (862) 432-1552

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