Nelson Bros. 'Jumbo Line' High Speed Engines

With the Introduction of its Line of Air-Cooled Singles Nelson Bros. Moved in a New Direction

Air-cooled vertical VA

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Some years ago while shopping for a sump pump at an old, local business, I noticed an International LA engine sitting in a pile of junk. The company had been one of the first manufacturers of chain saws, and their site was a sprawling maze of old buildings. I asked if they were interested in selling the IHC (they were glad to be rid of it), and if there were any more engines on the property. 'Probably,' one of the owners said, 'but you'll have to ask Ollie,' a retired, former employee of the company who knew everything about what was in the old buildings.

Ollie often stopped by the company, and the next week he took me on a tour of the company grounds. What a bonanza. The old patterns and jigs for making chain-saws were still in an attic space, and I saw three pallets of new-old-stock (NOS) Delco generator parts (they had also been a Delco generator dealer) being palletized for shipment to a collector in Florida.

In an old barn I found some old Kohler generators, along with a few air-cooled engines that were so dirty I couldn't tell what they were at first. They turned out to be Nelsons, and I learned the company used 2 HP Nelson engines to run milk coolers they manufactured for local farmers. They switched to Fairbanks-Morse engines after Nelson Bros, went out of business.

I also found various NOS parts for the Nelson and F-M engines they had used, and I went there many times, filling my car trunk with old equipment, odd piston pumps, a magneto charger, NOS spark plugs, NOS small engine parts, some old literature I found in a box labeled 'antique engine literature,' and two Nelson engines. That box is where I found most of my rather limited source of material covering the Nelson Jumbo Line of air-cooled engines.

Nelson's 1/2 HP air-cooled vertical VA featured an open Maytag-style flywheel with integral cooling fins. Cylinder heads on these engines tended to suffer from ineffective cooling.

Nelson's 1/2 HP air-cooled vertical VA featured an open Maytag-style flywheel with integral cooling fins. Cylinder heads on these engines tended to suffer from ineffective cooling.

The VBG was almost identical to the VAG, but was rated at 3/4 HP instead of 1/2 HP. Its higher rating evidently came from the use of a higher compression ratio and higher running speed.

Air-Cooled Jumbo Line

Along with their better-known hit-and-miss engines, Nelson Bros. Co., Saginaw, Mich., built a line of highspeed, enclosed-crankcase air-cooled engines ranging from 1/2 HP to 3 HP. Judging from dates in various ads and catalogs, the air-cooled Jumbo Line was built from the early 1930s until Nelson Bros, closed its doors in 1940. There were two distinct types of Nelson air-cooled engines; over-head valve (OHV) and L-head (flat head).

Deciphering Models

The nameplates of air-cooled Jumbo Line engines are stamped with either two or three letters designating the engine style and horsepower, followed by a serial number. The vertical engine models start with a V and the horizontal engines with an H. The second letter refers to the horsepower rating, and the horsepower rating increases (with one exception) in alphabetical order.

The A, B and S models were all OHV design, while the C, D and F models were all L-head design. The presence of a third letter in the model designation, 'G,' references a gravity-type fuel system. If the engine was equipped with a base mounted tank and a fuel pump, the third letter was omitted. The basic engine model designation is also evident in the casting number prefix on many major engine castings. If there are any other engine models between the F and S models, I haven't run across them.

OHV Vertical and Horizontal

The OHV engines had a common 2-1/4-inch by 2-1/4-inch bore and stroke. Operating speed varied according to horsepower and ranged from 1,450 to 2,400 rpm. The various horsepower ratings within the same cylinder displacement appear to have been derived from a combination of operating speeds, compression ratios and carburetor throat diameters.

At the low end of the vertical line was the model VA. It had an open Maytag-style flywheel and open push rods driven by a single lobe camshaft. The VA was rated at 1/2 HP at 1,450-1,700 rpm and had a modest compression ratio, making for very smooth running.

The VB engine was nearly identical to the VA. It was rated at 3/4 HP at 1,500-1,750 rpm. The VB appears to have had a slightly higher compression ratio. It used the same open Maytag fan flywheel as the VA, but is more commonly found with a conventional finned flywheel and flywheel air shroud. The shrouded version was an improved cooling system providing more air flow across the cylinder head. The air shroud was cast in either iron or aluminum.









5/8, 3/4














Rated at 1 HP, the VS series had a cast flywheel air shroud with a sheet metal extension running over the top of the valve train to improve cylinder head cooling.

Rated at 1 HP, the VS series had a cast flywheel air shroud with a sheet metal extension running over the top of the valve train to improve cylinder head cooling.

The third OHV engine in this series was the Model VS. The VS was rated at 1 HP at 1,800-2,400 rpm. It was characterized by a slightly different shaped cast flywheel air shroud and by the addition of a sheet metal extension over top of the valve rocker arms. These changes evidently directed even more cooling air to the head area. The VS used slightly larger valve head diameter in later models, and a larger carburetor throat diameter than the VA and VB models.

The horizontal OHV series is most commonly found with the open Maytag-style flywheel, but there was an HB model with a sheet metal shrouded flywheel. I assume this to be a later version with improved cooling. A 1 HP Model HBG with a Tillotson carburetor was offered in the latter configuration. If model logic followed, it should have been labeled an HSG, but it wasn't. The horizontal cylinder engines are uniquely attractive and were obviously designed for washing machine applications. Some literature indicates they were also sold for lawn mower use.

The horizontal 1/2 HP HA series had the same open flywheel as the vertical VA and VB. These were likely intended for washing machine duty.

There were two different types of push rods used on the OHV models. One was a solid push rod with an adjusting screw and locking nut in the rocker arm for valve adjustment, while the other type had an adjustable length push rod for valve adjustment. This was evidently an across-the-line design change, as one early brochure shows all OHV engines with the adjustable length push rods. I can only assume the adjustable length rod gave way later to the more conventional adjustable rocker arm.

All of the horizontal cylinder OHV engines I have seen have kick-type starting, but literature indicates rope and hand lever starting were also offered. Hand lever starting is very common on the shrouded vertical OHV engines. A 3-inch diameter 'A' section V-belt pulley was built into the kick or hand lever starting clutch. A 2-inch by 2-inch flat belt pulley and a V-belt pulley were available for the flywheel side. Chain sprockets and flexible couplings are also mentioned in literature.

The horizontal models disappeared from the Jumbo Line of air-cooled engines about 1937. Evidently not as popular as their vertical cousins, they are not listed in 1938 and 1939 engine catalogs I have seen.