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Go by the water pump on the engine. I've read all the responses to your query, and most of them are correct as far as they go. I've added a little history below to help you out as much as I can. The 420 was a performance improvement to the 40 by increasing the bore 1/4 inch (4.00 to 4.25). The early 420's are no different than the 40's other than the engine. However, the increase in bore required the addition of a water pump. On the early 420's the block is the same as the 40, but the water pump was added by machining the block to accept it. This, of course, is time-consuming during production. The block casting was modified accordingly, but the basic block is the same as the 40. The casting number didn't change at this point, so you can't always go by that. Later, the block was altered in other areas, and the casting number was changed accordingly at that time. The early block has the dipstick on the left side while the oil fill can only be accomplished from the filler tube attached to the top of the valve cover. The later block has the dipstick on the right side and it has a large bayonet cap that covers a large hole that is also an oil fill port. The upper tube on the valve cover is still there if you want to fill it that way. This later block is also used in the 430 series, with the removal of the upper oil fill tube. As for crawler tracks, rollers and frames; this area was under constant development during the 40 and 420 series productions. The three roller option ended in the middle of the 40 production and the 5-roller was also added somewhere around the same time (I've never been able to determine the length of time the 3 & 5 roller undercarriages were co-produced. I think the 5 superseded the 3, but there were lots of 3-roller parts in house and the model continued on if it were ordered....). There were several different style lower rollers, rock guards, upper idlers, etc. Also, the 61 (light) and 62 (heavy) bulldozers were used on both models, so you can't even go by that. As far as the paint goes, both the 40 and 420 crawlers were available as industrial units. That really means JD painted the machine yellow instead of green. The early 420's were all green, with the usual yellow stripe appearing several months into production. This is not a good way to tell them apart, as many 40's have a yellow stripe on them. Owners and dealers had a tendency to add that stripe. If you look at my tractor-town picture of my 'M' you can see that I've commited the sin of adding that yellow stripe, using a John Deere font from the 420, but keeping the 'M' designation (Hey! I like that spiffy yellow stripe!!) Many other 420's are all green, because the owner was repainting and didn't want to fuss with that two-tone stuff. Now, you can go by the presence of the water pump to tell for sure. Of course, someone could have transplanted a 420 engine into a 40 at some point. That will still get you a 420, because that's the real difference from JD's engineering (4 = 40, 20 = 20% more power gets you 420). On other point, though, no 40's were made with the 5-speed gearbox. All 40's have a 4-speed, and many 420's have as well. The 5-speed came out in the second year of the 420's production, so if you have that AND a water pump you either have a 420 or someone went to a lot of work when they fixed up the 40!
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