Mighty Mac Knife Replacement

garystpaul(4)July 14, 2013

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

Thank you for reporting this comment. Undo
tom_nwnj(z6 NJ)

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

    Bookmark   July 14, 2013 at 1:21PM
Thank you for reporting this comment. Undo
garystpaul(4)

Thanks, Tom. Will give that a try.

    Bookmark   July 14, 2013 at 1:24PM
Thank you for reporting this comment. Undo
tom_nwnj(z6 NJ)

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.

    Bookmark   July 14, 2013 at 6:06PM
Thank you for reporting this comment. Undo
zen_man

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

    Bookmark   July 15, 2013 at 11:59PM
Thank you for reporting this comment. Undo
garystpaul(4)

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

    Bookmark   July 17, 2013 at 10:52PM
Thank you for reporting this comment. Undo
garystpaul(4)

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

    Bookmark   July 20, 2013 at 9:56AM
Thank you for reporting this comment. Undo
zen_man

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

    Bookmark   July 20, 2013 at 11:10PM
Thank you for reporting this comment. Undo
garystpaul(4)

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

    Bookmark   July 21, 2013 at 9:37AM
Thank you for reporting this comment. Undo
zen_man

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

    Bookmark   July 24, 2013 at 11:09PM
Thank you for reporting this comment. Undo
garystpaul(4)

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

    Bookmark   July 28, 2013 at 9:09AM
Sign Up to comment
More Discussions
New Ryobi S430 engine dies on full throttle
Just bought a new Ryobi straight shaft 4 Cycle S430...
losrubios
MTD 8HP SnowBlower
I have an MTD 8hp SnowBlower Mod # 317E640F352. The...
Jeffery Smith
Harbor Freight small utility tralier question
I just purchased this harbor freight trailer. http://www.harborfreight.com/1090-lb-capacity-40-1-2-half-inch-x-48-inch-mini-utility-trailer-with-12-inch-four-lug-wheels-and-tires-90153.html I...
rlmunsey
What locked my Neighbor’s 6.5 Briggs Quantum Edge Engine?
I wanted to see what locked my Neighbor’s 6.5 Briggs...
loger_gw
General craftsman tiller gearbox question.
Ages ago in the early 90's, I bought a Craftsman 5hp...
bliackbeard
Sponsored Products
Landscape Lighting: Hampton Bay Alternative Energy Solar Powered LED Starburst
$19.97 | Home Depot
Tech Lighting | Cabo Pendant
$173.60 | YLighting
Stila Paper Knife by Alessi
$42.00 | Lumens
Super Elliptical Extension Table
Design Within Reach
Set of Two Comete Martini Glasses
$129.50 | FRONTGATE
PAR20 LED Bulb, 36 LED
Super Bright LEDs
© 2015 Houzz Inc. Houzz® The new way to design your home™