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Its been a bad year, and my lyrics would make a great county song…in the midst of it, I overtightened my lawnmower blade adapter bolt and could not get it out no matter what. So, I decided to drill and cut it out. Much to my dismay, I cut out the threads in the shaft making it too big for the new 3/8” bolt. This is a Predator engine on a 1991 Snapper 21” self propelled. I do have taps, so I think I can go up a bolt size. What would you recommend? I imagine I will need to enlarge the blade hole too. Will the crankshaft hold up to this and still work? I am having major surgery in a few weeks, have another mower, so this is will be a project for probably March 2024. Any advice, words of wisdom, etc, would be greatly appreciated.

Take a couple good pictures of project & show them to your local machine shop Foreman for a $ quote. Crankshafts are brittle & require TLC, let us know how it goes, thanks Mark

Your shaft can take the next size , get a Drill bit and tap to go one size up and have at it, ( it's not working now so there is no down side) I'm pretty sure it's a fine thread bolt If it was 3/8 - go to 7/16" the same depth , use a bottom tap for the final depth

Bertrrr said:

Your shaft can take the next size , get a Drill bit and tap to go one size up and have at it, ( it's not working now so there is no down side) I'm pretty sure it's a fine thread bolt If it was 3/8 - go to 7/16" the same depth , use a bottom tap for the final depth

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Thanks for the replies…since I’ve had trouble in the past finding a machine shop, and since it is already semi-toast, I will try going up a size. Will let you know next year!

Drilling and tapping a crankshaft can be difficult, due to its metal structure after heat treatment. You will find it very hard and brittle. HHS drills and taps will not cut it. If you try to do this lots of time and patience will be needed, as well as the proper tools. Most times the taps break off ruining your attempt.

With the price of new Honda knockoff engines it's hardly worth taking to a machine shop. I'd either find a used crankshaft on eBay or pick up another cheap engine.

The problem I see with attempting to drill and tap the crank is that you could weaken the metal without realizing it and then if your blade hits anything it could become a spinning weapon. But maybe I'm catastrophizing.

Going a sixteenth larger is not going to weaken the crankshaft a huge amount considering it is a 7/8" shaft. It will still be at over 50% of the solid shaft strength. But yes it will be weaker but the OP shouldn't be mowing rocks in the first place; unless, he is down under in Aussie country where they use swing back blades.

Will need to be very patient both drilling and tapping this out. Use plenty of lube during both , no down side really because it's basically junk now anyway.

Drilling a hole slightly bigger with a standard twist drill can be tricky. Easy to burn and dull the ends of the drill lips. Slow revs and good amount of pressure is needed. You don't want to let the drill spin and not cut. Drill at least 1/4" deeper than you want to to run the tap to. Mark the tap so you don't bottom it out. Best to use a starting tap and finish with bottom tap.

Hammermechanicman said:

Drilling a hole slightly bigger with a standard twist drill can be tricky. Easy to burn and dull the ends of the drill lips. Slow revs and good amount of pressure is needed. You don't want to let the drill spin and not cut. Drill at least 1/4" deeper than you want to to run the tap to. Mark the tap so you don't bottom it out. Best to use a starting tap and finish with bottom tap.

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Well the depth spec is 25mm so with a bottom tap you should be able to get to the 25mm spec'd depth.

But as you said most handheld drills are way too fast for most bits. Many of the cheap bit are only rated for 1500 rpm max, if even that. And really you need to be around 500 rpms or less to prevent bit burn.

Most machinists drill and tap cast iron dry, no lubricant. Basis for this is that cast iron contains graphite which acts like a cutting fluid. Those who use a lubricant, use a very light out one, ex. kerosene.

Pretty sure cranks are cast steel not cast iron. I use tap magic cutting fluid for drilling and tapping.

Wouldn't a Predator engine have metric bolts?

Forged steel, not cast, started dying out in the 50’s and was replaced by iron. Most of the time they were made of grey iron, which is much easier to machine than the other types of iron.

Fish said:

Wouldn't a Predator engine have metric bolts?

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