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Mighty Mac Knife Replacement

Posted by garystpaul z4 MN (My Page) on
Sun, Jul 14, 13 at 11:44

Anyone have experience removing three bolts holding chipper knife in place? I've used heat (propane) with the 3/16" allen, per directions, but can't get any of the three bolts to budge. Actually managed to strip the inner one a bit. Advice here?

Thanks,
Gary


Follow-Up Postings:

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RE: Mighty Mac Knife Replacement

I'd try penetrating oil before trying heat, give it several days to work in.


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RE: Mighty Mac Knife Replacement

Thanks, Tom. Will give that a try.


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RE: Mighty Mac Knife Replacement

Hi Gary,

When using heat, remember it makes metal get bigger. If you are heating the bolts, they will get tighter. If you are heating the shredder drum, the bolts will get looser.

My FIL (RIP) was a retired machinist at Bendix. He forgot more than I know.


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RE: Mighty Mac Knife Replacement

Hi Gary,

I had the exact same problem you had. I used a propane torch to heat the bolts, and broke a hex socket piece trying to loosen a bolt.

The secret is the Loctite that is used to keep the bolts from loosening under all that repeated battering the blades get. It has to be melted to loosen the bolts, and a propane torch doesn't get the bolts hot enough to melt the Loctite because they are in a heavy flywheel, which serves as a heat sink, keeping the propane torch from melting the Loctite.

The solution is a MAPP torch. MAPP burns much hotter than propane. MAPP gives you three times faster heat transfer than propane. I heated the bolt heads red hot with the MAPP torch, and after I let them cool off, the bolts backed out easily with very little torque needed.

When you replace the bolts, be sure to use the right grade of LocTite to re-lock them in. It could be catastrophic if the bolts loosened and released the blade while you were chipping with it.

ZM


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RE: Mighty Mac Knife Replacement

Zenman, thanks. This was substantially what the tech guy at Mackissic told me. But I discovered MAPP torches are not easy to come by. I picked up a Benzomatic Map-Pro cylinder today and will give it a shot tomorrow. I hope that will get it hot enough.

Also, in the knife replacement kit they included what I would assume to be the correct grade of LocTite.

I'm actually replacing the entire weldment (new shaft, hammers, new bushing), so this is a minor stop on the way. I thought it would be easier to replace the knife with the enhanced access (and that's true). Will see how it goes.

Thanks again for the tips; very helpful.

Garyt


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RE: Mighty Mac Knife Replacement

I hope folks don't mind if I bump this up. I could use a bit more advice on heating the bolts (see previous post). About how long should this take on each one? Is the "red hot" meant literally? I ask because about two minutes on each one got me nowhere; they still don't budge.

Thanks.

Gary


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RE: Mighty Mac Knife Replacement

Hi Gary,

"But I discovered MAPP torches are not easy to come by. I picked up a Benzomatic Map-Pro cylinder today and will give it a shot tomorrow."

It should say right on your MAPP cylinder, "Use only with torches designed for MAPP gas." Red hot is meant literally. You need to use a MAPP torch.

ZM


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RE: Mighty Mac Knife Replacement

Thanks, ZM. Well, I tried it again, using the Map-Pro, six minutes per bolt. After cooling, got them out with a torque wrench. For the stripped one I hammered in a slightly larger hex key, then used the torque on that as well. It worked!

Now on to the main event. Hope that goes without problems (yeah, right....ha)

Appreciate your help.

Gary


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RE: Mighty Mac Knife Replacement

Hi Gary,

"I'm actually replacing the entire weldment (new shaft, hammers, new bushing), so this is a minor stop on the way."

If you still have it, you might want to save the old rotary assembly to cannibalize for spare parts at some later time. I plan to rotate my hammers this Winter, and in case something happens to one of the groove pins, I looked for them at several local hardware stores. My groove pins are probably badly rusted. The groove pins were not available anywhere locally. I will get a few spares from an online source before I start driving them out.

Anyhow, if you have the old assembly and keep it, you will have backup groove pins, swinging hammers, and such.

ZM


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RE: Mighty Mac Knife Replacement

Thanks again, ZM. This turned out to be quite a project. I now know just about every inch of that amazing machine and learned a lot along the way.

Appreciate the tip about the pins. I was going to donate the rotary assembly to a scrap metal collector, but will keep it instead.

Gary


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