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I did some gearbox testing a while back. The ratio is 1:1.5 (input/output) on a 420 deck gearbox. I don't know if my 420 and/or the deck is an early or later one.

The original 420 pulley on mine was 5.5". The 420 deck is reverse rotation compared to a 400. I installed a brand new replacement gearbox on my 420 deck. Happened to have it on the shelf. Got it at a salvage yard and I don't know what it was made for but the ratio is 1:1, same as a 400 deck but it rotates the same direction as a 420. Not gonna leave the new gearbox on the 420 deck permanently if I can't speed the deck back up to where it was before, The 1:1.5 ratio makes a dramatic difference in cut quality and clipping dispersal.

I put a 6" pulley on the new 1:1 gearbox that's on my 420 deck now to speed it up and that helped but I'm gonna go to a 6.5" pulley. Can't go 7" or I would. Not enough room. The stock belt length should still be ok but if not I'll play with the idler pulley size.

The gearbox on a model 50 snow blower (400/420/430) physically appears to be the exact same as the deck gearbox. Never dug any deeper to see what ratio they are.

The 420 gearbox is on the left in this pic, 400 on the right. Rotation is shown with arrows. Ratios are marked on the gearboxes too.
Gas Motor vehicle Machine Auto part Nut


In JDparts, for a 420/430 decks, there is a early version for deck serial # up to 525,000, and a later version for serial # over 525,000. The gearboxes are different, but the gearbox pulley numbers are the same.
I have a later 420/430 deck in the shed, so I had a look and, turning by hand, the gearbox pulley output definitely turns faster than the driveshaft input. So on that deck, the input shaft would have the larger gear.
On that deck, I agree that the pulley output would be about 3000 rpm. The drive pulley on the gearbox is a little smaller than the deck pulleys, so blade rpm might be around 2500.
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I just finished rebuilding the gearbox on my 430 with 60" deck. There seemed to be a lot of play in the driveline and gearbox. All that was fixed with new woodruff keys.

In doing research, I found out the 60" deck on 420, 430, and 755 all have 18 tooth input and 27 tooth output. In looking at almost all other deck gearboxes (50" and 72" on just about everything including front mowers,) they have 25 and 26 teeth respectively for about 1:1 ratio. Any ideas why only the 60" deck on these machines would have the blade speed reduced? It seems if I used a gearbox from a 50" or 72" deck I could get faster blade speed on my 60" deck.

My first thought is that the 420, 430 and 755 have about the lowest horsepower of machines using these 60" decks and maybe the thought was to not overtax the engines.

By the way, I have sourced all the parts needed to rebuild these gearboxes and will supply the info if anyone is interested.
I would be interested in the sourcing information.
I recently had a sheave failure and had it repaired at a local machine shop for less than the cost of a new one.

BTW, has anyone installed a tach on their 430 and would share the information. I like to have a tach on all of my engines.
 
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