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Faunsdale, AL | The U series D6 was introduced in 1948 and produced until 1959. All have a 4-1/2” bore 6 cylinder D318 engine and a 5 speed manual transmission with a reverser lever next to the gearshift. No reverse in 5th no matter where you shift the reverser lever. That can bite you if you forget when you’re trying to get out of a cloud of bees in a hurry!
All CAT tractors in those days had hand clutches.
At serial number 13263-up, the oil clutch was introduced. You may find some that were retrofitted to earlier machines as there was a kit offered at that time. Look for a dipstick and a breather on the right side of the clutch housing of the oil clutch. Oil clutch is practically indestructible compared to the dry clutches.
Direct electric start was an option but pony start was most common by far. Pony USUALLY had 6 volt starter and the diesel had a generator to charge the battery if it had one in a box on the fender. Lots of them are jump started using a 12 volt vehicle and jumper cables. The plain Jane model had no pony electric starter, you used a pull rope wound around the flywheel. And it worked!
The factory direct electric start system was 24 volt but most folks use 12 volt starters these days when replacing the pony motor. It’s far cheaper to buy a 12 volt DELCO 40MT and the special drive for the bevel cut flywheel gear than to overhaul a blown up pony motor. The problem is the older ones aren’t machined to bolt the starter on. The flywheel housing has the boss cast in and the hole can be cut with a hole saw and the holes drilled and tapped without removing the engine from the tractor.
Around 1960 the 8/9U were replaced by the D6b 36/37a I think it was. Engine was a different 4-1/2” bore 6 cylinder called a D333. Grease type track tensioners became standard but the rest of the tractor wasn’t much changed. Hydraulic system was a little more integrated into the engine compartment instead of being a self contained unit hung on the front of the hard nose like the earlier ones.
A pony motor was still an option on the D333. If the old horizontally opposed 2 cylinder ponies on the U series tractors were troublesome, the vertical two cylinder inline design used after 1960 was doubly so! Just as a comparison, the older engines were governed at about 3500 rpm while the later ones revved to 6000! They quickly earned the “buzz bomb” nickname and probably hastened the end of pony motor start as an option on Caterpillar equipment.
When the D6C came out in the mid 1960’s it was a larger more capable machine still with the D333 and ultimately the 4-3/4” bore D333C which was basically the 3306 until around 1973 when the engine was designated the 3306. Sometime around there the tractor became the D6D.
Tracks for the U series were 6-3/4” pitch and are replaced by the D5 (6.91” pitch I think) tracks and sprockets these days.
A large part of the value of an old tractor is the undercarriage!
Edited by ccjersey 10/25/2021 20:43
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Cat d6 dozer history
