I tried one more belt today, a 1/2x52" belt which is the smallest I can put on it and still be able to release the clutch.
Its exactly 1/2" wide. Its better but it still slips in 4th against a hard pull. However, with the 52" belt, I can't seem to stop the creep in forward when the lever is in the center. The clutch/brake operation is short, maybe 4" travel total vs. 8" with the 53" belt.
I tried a 5/8" belt but that won't clear the guards or stay on the idler pulley The 52" belt actually measured 52 3/8", while the 53" belts all measured 53 5/8" total or more after being used.
I'm up against two issues, first the belt is narrow in the pulleys and bottoming out, second, with the 53" belt, there's not enough wrap around the BGB pulley for good grip. I think the only reason the 52" belt works is that its got another inch or so of wrap around the BGB pulley.
With the 52" belt, and the clutch adjusted to about 1/2" clearance on the clutch rod, 1/8" on the detent rod spring stop, and 1/8" at the brake shoe, with the brake band tightened up one full turn past an easy fit it creeps forward almost immediately after unlocking from the reverse stop on the lever. Sometimes when I toggle the shuttle lever back and forth while holding the brake I can lessen how much it creeps forward.
I get this same problem on three of the machines I've got here. It seems like the adjustment on the spring and collar on the rod with the neutral detent keeps changing.
The original 416S has the newest parts on it now, there is no play in any rods or shafts and all the friction parts are in good shape.
What I get now with the 52" belt is belt chatter and squealing as I engage the clutch with the pedal, or when first moving forward after shifting into high gear. With the 53" belts, it just slipped and made no noise, now with the rubber faced, wider belt the belt jumps and chatters on the BGB pulley. If I try to pull anything in high gear, even a wagon, the BGB pulley slips, but with the shorter belt, it screams and chirps loudly.
The 1/2" x 52" belt fits on tight, its not loose like the 53" belts were. The idler is at a better angle as well.
I also tried a 50" belt, which had to be rimmed on without the idler on at all, the belt fit like a piano string yet still slipped in high gear the same way. Once the machine is moving, high gear seems fine but its getting rolling but any resistance and the belt will slip and scream. The kevlar belts didn't make noise they just slipped. This belt is made by Mitsuboshi and came from a local industrial supplier. It was the only belt I could find that wasn't under 1/2" wide.
Its still not has wide as the one original belt I had tried and broke.
Something I notice on this tractor is that low gear is way, way low, and high gear is way too fast, fast enough to worry about being safe, even at half throttle.
When the belt slips, it'll slip at idle, it don't need any power to slip. With the machine chained to a stump by the rear hitch, with no rider on it, it'll slip the belt in 3rd and 4th gear before spinning the back tires on a gravel driveway. The rear tires are filled with about 50lbs of washer fluid and its still got the snow plow mounted up. No other weight on the tractor. In my opinion it should with no doubt just sit there and spin its tires, not slip the belt. I can watch the belt compress and get sucked down into the BGB pulley when it slips, its not like its not up against some force, the belt just don't handle it. The BGB pulley is off super clean 410S I bought for parts. I see no signs of wear on any of these pulleys.
If I were using it to cut grass, I'd probably never notice it slip, but since all this does is push snow and plow a garden, its working all out all the time.