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Edit: I labelled these gearboxes wrong. The drive pulley direction is reversed, not the input from the driveshaft.

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.
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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 think a 6.5" pulley is the max diameter that will fit. I have a 6 incher on it now that has enough clearance and it looks like a 6.5" might work but I haven't verified that yet. I know a 7" pulley won't work because I tried one off a 400 deck. Another issue is clearance with the belt where it crosses the deck under the gearbox mount along the back.

Watch the proximity of the gearbox pulley to the deck spindle pulley when you go to a larger one. They are very close as it is and the larger pulley will increase the probability and a degree of damage that will be caused by larger debris getting between them. Also, any slack in either pulley may cause inadvertent contact of the pulleys.

No part number. It's a generic mix and match type pulley. The type with different sized hubs that can be welded to different sized pulleys. The groove isn't exactly the same as the stock pulley. Not quite as wide so it's only using the sides of the grooves for grip. So far so good though, It's been on there since last fall with at least 50 hours use this year and the belt still looks fine. I'm not using a non-standard pulley by choice. I'd put the correct one on it if one was available new in the larger diameter I need to compensate for the slower gearbox on it now. The groove profile (on the generic replacement pulley) will undoubtedly affect the belt longevity. But.. I bought one 400 with a deck belt that had no V shape left at all. Looked like a black 3/8" rope but amazingly still worked.

I actually put another drive pulley on my 420 deck before I started trying to speed it up. The speed was actually excellent till I replaced the gearbox with one that's only a 1:1 ratio (vs the 1:1.5 ratio) on the original 420 gearboxes. I initially replaced stock pulley because the bore was wallowed out badly. The keyway on the shaft was buggered up too.

This is the pulley mounted to the gearbox, correct? If so, what part number is it?
 

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The gearbox is reverse rotation on a 420 deck with compared to a 400 deck. Gearbox itself can't be reversed unless the PTO rotation is the opposite to normal. My 400's deck gearboxes have 1" shafts and 1/4" keyways. The 420 uses half moon keys.

I wish I knew how to ask questions first. :D

I have the earlier 60" deck. I haven't checked the gearbox ratio yet, but reversing the gearbox and/or adding a larger pulley are feasible options? The pulley is 1" bore with 1/4" keyway?

.....just asking questions before I remove my deck.
 

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Edit: typo. This is the deck belt that looked like a nylon rope. It was worn round. Didn't matter which way it went on. They don't make tough belts like that in China. I mowed with the deck it was on to test the gearbox and spindles. No deck cover. It was inspirational watching the belt refuse to give up. Reminded me of Rocky Balboa. Didn't have the heart to toss it out so I put it in the old shed. That was almost 2 years ago but it's so tough it's still hanging in there.(y):D
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Oh ok.. The gearbox doesn't care which way it's facing. Doesn't matter whether it's upside down or sideways either. If the input is the same the output is the same.
Not the direction, the orientation mounted on the deck. From what Im reading it can be mounted either way.
 
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