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Just so we all speak the same Gravely speak, the worm is the little steel guy and the worm gear is the big brass guy, both have lead counts, four, six or eight.

Something to remember when changing out worm gears, the brass component of a differential; early tractor axels had two taper bearings on each axel shaft and the inside ends of the axels never touched the spider pin.

Later axels have a single taper bearing and the inside ends ride against a ‘thrust-block’ on the spider pin inside the differential.

You can change the worm and worm gearing; just make sure you use the right differential internals for the axels on your target tractor.

Of course everone knows, changing the worm and worm shaft in a Gravely is a major tear down and maybe some press work.

Roger,
 

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Gravely bug bit.
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Discussion Starter #22
Couple more pictures.
First one shows from L to R: 4-Lead worm gear, 6-Lead worm gear, 6-Lead SwiftAMatic worm gear, 8-Lead SwiftAMatic worm gear.
Second picture shows Standard 6-Lead worm gear vs 6-Lead SwiftAMatic worm gear.
 

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Gravely bug bit.
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Discussion Starter #23
Additional pic of Swiftys.
Photo 1 has older 400 series on left, newer 2-wheeler on right. Notches on newer one is to allow installation of spider gears while clutch housing end is already on. Spiders will go in if both sides are open without the notches.
Photo 2 shows 1 piece gear from the 400.
Photo 3 shows the pinned gear from the later model 2-wheelers.
 

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RIP Chip, "Gravely Guru, Gone But Not Forgotten"
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Re: For those who have never seen

From Chip Old,

Jack up the tractor so one tire is off the ground...
(much deleted)
Those instructions Roger posted were given to me years ago by a Gravely dealer, but I'm embarrassed to say I never actually tried it myself. Why would I? I know what worms are in all of my Ls. I finally did try it this morning in response to questions some of you raised about it. Here is a rewrite based on what I found:

Jack up the tractor so one tire is off the ground. Make sure the other wheel remains on the ground, or find another way to prevent it from turning. Remove the spark plug and (if electric start) the starter chain so the engine can be turned over by hand. Put the spark plug cable back on the plug and ground the plug on the engine to prevent magneto damage. Lock the high/low clutch lever into low and the forward/reverse clutch lever into forward. If the tractor has a Swiftamatic, it should be in high range. Now rotate the crankshaft and count how many crankshaft revolutions it takes to rotate the wheel 1/4 turn.

L or C
8-lead worm
1/4 wheel rotation requires 6 crankshaft rotations.

LI or CI
6-lead worm
1/4 wheel rotation requires 9 crankshaft rotations.

LS or CS
4-lead worm
1/4 wheel rotation requires 12 crankshaft rotations.

If you do the test with the high/low clutch lever in high instead of low, you should get 4 (L/C), 6 (LI/CI) or 8 (LS/CS) crankshaft rotations per each 1/4 wheel rotation.

I tested this on an L, an LI. I don't have an LS, but due to the gear ratios if an L is 6 turns and an LI is 9 turns, then an LS has to be 12 turns.
 

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RIP Chip, "Gravely Guru, Gone But Not Forgotten"
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My calculations produced 8.1 crank revs per 1/4 wheel rev for an L in high gear. For an L in low, I obtained 11.2 crank revs per 1/4 wheel rev. The 8.1 is closer to the 6 listed in Roger's post for the L, so I'm going to say that it is in high gear. I think my calculation would be closer to the 6 crank revs per 1/4 wheel rev but Gravely's mile per hour numbers may be slightly off, or the circumference of the tire may be slightly different than the ideal 50". I'm sure there's some deformation that would change the actual forward distance traveled per wheel revolution with rubber tires. Anyway, high gear should be correct for that test.
Those calculations don't take the differential action into account. The test is done with one wheel on the ground (or otherwise prevented from rotating), so the differential's internal gear ratio changes the outcome. Your calculations would probably be correct with both wheels off the ground, but if both wheels don't rotate equally (due to greater resistance on one side or the other) then the test won't be accurate.
 
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