Mine has the shear bolt connecting the PTO shaft to the gearbox. The roll pin is there so that when the shear bolt breaks the PTO shaft remains in place on the gearbox input shaft and doesn't drop down.
If I'd seen this thread earlier I would have advised you to save some money and use the 9 inch auger for the 4x4 posts. It gives you some room for error, and if you tamp around the post when you refill the hole it'll be pretty solid.
I've got some 6x6 posts to set later this year and plan on getting a 12 inch auger for that job. I currently have a 9 inch.
If the owner's manual doesn't specify what to use for shear bolts I'd be surprised. Mine are 3/8 x 2.5, grade 2 bolts.
Sean
Thanks to both Soundguy and Chilly807 for the advice and what the roll pin is for; I believe you're right but I don't fully understand how it works. I need to look at the point of attachment again very closely. The gearbox drive shaft has a deep groove in it that would appear to be for a ring to hold the PTO shaft to the gearbox when the shear pin breaks; but the roll pin wouldn't seem to serve that purpose because it is listed as 3/16 x 2 which is long enough to pass all the way through from side to side of the connection. In addition I didn't think the hole for this roll pin aligns with the groove.
And now for a new question. The first time I used the 6" auger it snapped the shear pin very quickly and I had a hard time getting the auger bit out of the ground. I had tried drilling a smaller hole at the bottom of a hole my 9" auger bit had made. I understand the logic of using a larger bit to make the hole so you have room for adjustment - and that is what I will be doing it seems. However when I manage to drill a very vertical hole in this clay it is like a concrete tube in the ground that the post can drop into - I have only successfully made 9" holes. I am using 8' long 4x4s and only need 4' above the ground so I was hoping to drill about 3' deep and have a post almost completely solid just by its depth, minimal packing of rock around it. I am not using concrete, would take a lot of concrete per hole and I will ultimately have about 250 posts.
So I chalked up snapping the shear pin to redrilling a hole, probably a bad practice anyway. Today I was able to work on this project again and had put in that 1/4" shear bolt with the 6" auger bit and tried a new hole. As soon as the auger bit into the earth it spiraled down and I could not raise it. I clutched the PTO to stop the driving action but the 3PT would not raise the bit at all. So since I had read about others getting their bits stuck and using a large pipe wrench to get it out and I had done this the first time, I did that again. I did use the 3PT to help power up the bit while I unscrewed it with the pipe wrench, a dangerous practice at best but turning the bit without that upward pull resulted in nothing but a lot work on my part. I got the bit out and very carefully tried drilling the same hole again, feathering the 3PT as best as I could. As soon as the auger bit into the earth or maybe it's tree roots, it buried itself again. Repeated removal process.
I usually operate the auger at a very low speed, probably 1000 tractor rpm and the 540 setting is about 2200. So I thought maybe the speed is too slow and is allowing it to grab the earth like a wood screw rather than like a drill bit. I sped it up to about 1500 RPM - same problem but faster. Now with this new auger bit the cutting bits are sharp and square and they actually seem to extend outside of the radius of the spiral flutes. So is it possible that with new blades the action of the auger is harder to control? The 9" bit has worn blades but they still work fine for me. I am going to give up on the 6" auger and just do a lot of packing around my posts since I know that works even if it is extra work. Is there a secret to drilling a smaller hole such as higher PTO speed? Is it a result of my tractor being too small for this size bit in terms of shaft length, the 9" is only 24" of but the 6" has at least 36" of drilling depth. My 3PT has two holes where the top link (boom in this case) can connect, I usually use the higher one but for this longer bit I discovered that I gained some auger height by using the lower hole. This may come at a cost of leverage for the 3PT but I don't think my tractor has enough power to stop this bit from burying itself no matter what. My tractor has 29hp and is a cat 1 3PT from what I understand. A new bigger tractor is not going to be my solution, now I am just curious why a smaller hole is harder to drill than a larger hole.
I spent the rest of the afternoon getting the 6" bit off the auger, the auger tube bolt got bent, 1/2" and nice snug fit, and I couldn't get it out. I ended up cutting off both ends and then drilling the bolt out. I used a spare 1/4" shear pin bolt as a drift punch but that only moved the bolt about 1/2". I will take a look at my "undersized" shear pin wear on the connection but if it is a problem I really seem to be in a quandry since 1/4" is loose and 5/16" won't fit, not many bolts inbetween those two sizes. Thanks for all your advice and tips.
