Corwin Groth has several friends who are antique tractor pullers. So when the Eldridge, Iowa man known as “Cork” to his friends saw the amount of horsepower they were getting from what were originally 5-½ by 6-½ inch bore and stroke unstyled “A” tractors, he wondered what one of his favorite tractors (a “GP”) could produce. So with a pile of extra, but not inexpensive, “GP” parts he had accumulated over the years, he set about to find out.
A 1931 “GP” standard main case, a 1930 wide tread belly pan and front pedestal were the starting point. The short rear axle quills from a 1930 “GPO” were added, which would eventually lead to the tractor’s “Nar-O-Tread” designation. An operator’s station and seat assembly from an over the top steer wide tread was added to the back.
Power for this chassis started with a late model six inch “GP” block that was bored out to 6.17 inches. A small bore head was bolted on, but not before 5/16ths of an inch was milled off. To get even more power, Cork did what his tractor puller friends do to their engines and modified the crank to add 2.5 inches of stroke to make the total 8.5 inches. The result is 501 cubic inches with a compression ration of 8.1 to 1. Compare that to 339 cubic inches and a compression ratio of less than 4 to 1 on a standard large bore “GP.” The camshaft was rebuilt to give the valves an extra 1/8th inch lift.
All those cubic inches won’t do much good, though, if the engine can’t breath and to take care of that problem, two “GP” manifolds were cut in half and turned to face opposite sides of the tractor. This made it possible for the engine to utilize dual exhaust and dual carburetors. A second arm off the governor from a 1947 “D” was added to run the second carburetor. Both carbs are Marvel Schebler DLTX 5s, standard equipment on a late “GP” tractors.
Atop the engine sits an unstyled “B” fuel tank, covered by a hood, custom made by Dave Geyer. The Nar-O-Tread decals were made by Travis Jorde. The steering gear box was made by combining parts from a 1937 “D” and 1929 “GP.” The proper steering shaft support was found on a 1943 “D.” Cork machined and added bronze inserts to a steering universal joint from a 1928 to use in the tractor.
New final drive gears were made, much heavier and geared lower than the originals. A factory “GP” has a final drive ratio of 4:11 to 1, but Cork’s new tractor has a 6.66 to 1 ratio.
After getting the tractor together, it didn’t take long to realize that turning an engine with all those cubic inches and compression by hand was going to be difficult. A 12 volt starter from a “G” now turns the electric start “A” flywheel. The starter bracket was fabricated from steel. A model “M” generator tucks neatly in front of the head, while the battery sits below the seat.
By now you’re asking yourself how many horsepower? Cork doesn’t know for sure, but it is no coincidence that he named the tractor “Sampson” after the biblical strong man. Shortly after completion, the tractor, then fitted with 13.9 by 38 inch rear wheels, easily pulled a five bottom plow in high gear. It has been belted up to a dyno a couple of times but with the narrow “GP” type pulley, the belt would start slipping at about 60 horsepower. Cork doesn’t dare try channeling the tractor’s horsepower through a “GP” power take-off shaft which, he says, “would be turned into shrapnel” adding that the original PTO was “barely adequate for 23 horsepower.”
Cork Groth is always glad to answer questions about what is likely the world’s strongest “GP.” So if you see the tractor at a show, look for Cork and ask away.
