2016 July 23     Tech   Metal & Shop


Recently I picked up two push mowers for a few dollars each.  Ah, but did they work? 

"I'm only getting rid of them because I got a riding mower," the lady assured me.

She said the mowers worked. 

Once home, I found there was heavy white mold underneath both mower decks.  Neither would start.  And one of them.....

This article will talk about what to do if your lawnmower has a bent crankshaft.

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In This Article

A Curious Clanking Noise

Bent Crankshaft!

Applying Heat

Hydraulic Press

Best Method

For Everything Else (a.k.a. "The Basic Lawnmower Repair Toolkit")


A Curious Clanking Noise

After a couple parts and a few hours of work, I got the one mower running.  It still needed some more work, though.  Actually, a lot;  a bolt broke off in the aluminum engine casing... whole 'nother article on the way.

The other mower, which she said was the better one... was in worse shape.

It made a clanking sound when I tried starting it.  At first I thought it was because I'd turned it over and gotten oil in the carb and cylinder head.  So I took the cylinder head off and cleaned out some extremely heavy carbon deposits.  Then I did the usual maintenance:  new air filter, cleaned the carburetor, new spark plug.  Oh, and I installed a new cylinder head gasket.

I started the mower, and more clanking.  Blade was loose!  So I turned off the mower and tightened the bolt.

After it was started up again, it ran for about two seconds and the blade started clanking again.

That's odd, I thought.

Then I realized the blade adapter key was sheared.  And the blade adapter was cracked.

"Hmm.  Okay, yeah, that happens sometimes."

Which meant the mower had probably hit something. 

Which meant the flywheel key was probably sheared off, too.

"No problem, I can still fix this..."

But if someone had hit a stump, and it was enough to crack the blade adapter, and the blade was loosening that quickly...

...there might also be...


"Uhh, Houston...."

Bent Crankshaft!

The "redneck try-square" verified it.  (Note the deluxe pressure-treated version;  for real pros only.)  Yep, the crankshaft was bent.

Some guys on the Internet were able to straighten bent crankshafts by hammering on the end while the mower was tilted on its side, carburetor-side up.  (Spark plug disconnected, for safety.)  A couple good hits with a four-pound or eight-pound hammer, et voila, they had usable lawnmowers again. 

Problem is, that can go very wrong very quickly.  It could crack the oil sump cover.  It could damage the gear teeth or the crankshaft bearing surfaces inside the case.  It could damage the oil seal;  it might even damage the flywheel because of the leverage it would probably put on that.  That's very bad, potentially even deadly.

So I figured the smart way was to remove the crankshaft, where I could then work on it without wrecking anything else.

The next evening I managed to get the crankshaft out of the engine case without removing the whole piston assembly.  So now I had the crankshaft, by itself, where I could hopefully straighten it out.  (WARNING:  read through this entire article before you even think of trying such a thing.)

Applying Heat

This is potentially a bad idea;  it can remove the heat-treatment.  Then the crankshaft could bend easily next time, when you don't want it to.  Then again, some types of steel (and also cast iron) aren't much affected even when you heat them to 650 or 700 C. 

I embarked on this adventure not even knowing what the crankshaft was made of.  Forged steel?  (Nah, probably not this one.)  Cast iron?  Cast steel?  I'm sure I could have looked up the casting number, which was on the crankshaft itself, but it was getting late and I just wanted to get this finished.  (Not a good way to do engine repairs, yeah?)

If I understand correctly, cast iron is not necessarily heat-treated;  Sometimes the hardness is from the composition, not heat treatments.  I'm not going to pretend to understand all iron and steel alloys, or even many of them.  I do know from years of research in Redneck Engineering that cast iron is not that bendable.  So, if you have a crankshaft that's bent but not cracked, it might not be cast iron.  Or it could have adopted a set of freakish physics for just an instant in time, such that a cast iron crankshaft was able to bend to an extent that you couldn't possibly have duplicated if you tried.

But enough of this fancy metallurgy talk... did the hammering work?

Long story short:  Nope.  I brought the metal to dull orange-hot, hammered right where it should have counteracted the bend perfectly.  And it wouldn't budge! 

I mean I really wailed on it, with a 3-lb hammer.  For what seemed like minutes, that bent crankshaft would not straighten. 

After way more hammering than it should have required, the crankshaft was just a little bit straighter.  But now the end of the crankshaft was dented a lot.  It's so bad that I might save it for blacksmithing experiments;  I don't think she's a gonna work as a crankshaft a-no-more. 

Overall:  heat bending of the crankshaft was mostly Fail. 

Hydraulic Press

Generally you would press the bend out without heating the crankshaft.  Not good for a severely-bent crankshaft, because it might crack the metal.  But it seems to work great for ones that are bent slightly.  (Just keep in mind that you're probably much safer in just buying a whole new crankshaft, which we'll get to... so I present this information for your entertainment.  I'm not saying you should actually do some of the things I'm describing in this article.  It might be safe to straighten a crankshaft if you can get a machine shop to check it for runout, and balance it if necessary.  You want to make sure it doesn't cause the flywheel or mower blade to wobble.)

To accomplish such a straightening would require a decent press, at least twelve-ton.  It would also need the right jig, or the right set of plates. 

A proper press should have a pair of heavy, movable square plates with a notch in each of the four sides.  No thin sheet metal here;  these are probably three-quarters thick, maybe even an inch.  Many of the presses today cheap-out on this useful accessory.  They just give you a couple of generic, featureless rectangular plates.  I like the presses that include the notched plates

Normally you would lay these flat so the notches match up.  Two semicircles of equal size, and you've got a circle through which you can press out a bearing or something.  Well, for straightening a crankshaft, it's entirely possible to stand these plates on edge, set the crankshaft across the pair of correctly-shaped notches, and press it straight like that.  Safe?  Maybe, maybe not;  do that at your own risk.  (And wear a face shield.)

Safer:  Get a good pair of steel bed plates to replace the cast iron plates that come with your press (or maybe didn't.)  The cast iron plates are useful, but they might be cheap castings and they might break.

Safest:  Weld up a jig out of steel plates, such that the plates can't go crooked and suddenly fall over under the tremendous force.  Because if they do, it's possible they could propel a piece of steel across the room, or onto your toes or something.

So anyway, once again... I wouldn't try pressing a crankshaft that's severely bent.  (Okay, I went ahead and did that anyway... but I seriously Don't Recommend that you do;  and you can be sure that when I tried that, it was Face-Shield Time.)

Shop presses are awesome, and when you need one, it is the only thing that will work.  Get a twelve-ton at the very minimum, or better yet a twenty-ton.  (See Guide to Shop Presses).


Best Method

Trying to straighten the crankshaft was possible, but not really the best way.  The alloy was unknown to me.  It didn't want to bend back, even when heated and mini-sledged. 

It took someone two seconds to mow over a rock and bend the crankshaft.  You had to spend years learning mechanics and metalworking, so that you can spend hours and days trying to straighten that very same crankshaft.  This can drive men to invent new cuss words and speak with improbable accents.

For the amount of time you could spend trying to straighten a crank, there are two alternatives that could make more sense. 

Alternative one, if you're not a DIY-type person in the first place, a brand-new push mower would be the smart choice.  And that's probably what the guy at the lawnmower repair shop told that lady, before she ditched it in a yard sale.  (Along with the other used mower that also had some problems.)

Buy new and you won't need to deal with that.  You can even have your brand-new lawn mower delivered right to you, without ever having to drive to a store. 

Alternative two:  Fix it yourself, but fix it right and order a new crankshaft.  Find the correct one for your model # of engine, and just order one. If you spend hours messing with a bent crankshaft and it's still not fixed, getting a new crankshaft (or even a good used one) is a much better deal.

Don't forget to order a sump gasket and a new oil seal, while you're at it. 

For Everything Else

We've looked at one of the tougher repair jobs on a mower. 

A lot of other lawnmower fix-em-ups are easier.  But you'll still at least need the right tools.  And you'll need to have them assembled together before you start work.  Obvious, right?  Ha!  Somehow I end up using the same motley and half-scattered assortment of yard-sale wrenches and sockets.

Here is a short list of needed tools for working on a typical Briggs & Stratton-powered lawn mower.  On a Briggs you should find SAE bolts only, but some manufacturers appear to be mixing metric and SAE (Argh.  John Deere LT-series.)

1. Socket setThis set by SK Tool is definitely on my want list.  This is not some re-branded import by some "used-to-be" brand that's been hollowed out by a band of corporate raiders.  These sockets and wrenches are made in USA by a company that still seems to care about quality.  For the typical push mower, you will use sockets much more than you would use regular wrenches;  it's just the way the bolt heads are situated.  I think I used an open-end wrench maybe once;  the rest was sockets and...

2. Nut Drivers. These greatly speed up the process of taking apart a lawnmower.  You will need 5/16", 3/8", 7/16", and 1/2" nut drivers.  I would get this set style=, which I believe are still made in USA. 

For this job I had a 5/16", and that was it.  Many times I wished I had the rest of the set.

3. Something To Remove The Flywheel.  Read this new article for a discussion of the simple tools you'll need, and how to use them.

4.  Needlenose pliers.  Use 'em for removing small clips, etc, when getting everything out of the way to work on the engine.  Just can't do without these in a toolbox.  I've always liked Channelock brand, made in USA.  The ones with side cutters are extra-handy.

5.  Ball pein hammer.  Eight or twelve ounce.  Useful for tapping punches... or that piece of wood scrap that seems uniquely suited to removing carbon deposits from the piston and cylinder head.  This hammer by Craftsman would be a good choice. 

6.  Vise Grips.  Actually I managed to do the whole job without once requiring these.  However, there were many times when they would have helped.  And sometimes, something goes wrong and you absolutely need a pair of these.  Most useful size and type:  this two-pack.  Actually I really want these even more.  The 10" curved jaw ones are the best all-around ones, in my opinion.


This little adventure reminds me not to mow over rocks. 

And if you buy a used lawnmower, assume that the seller tried to mow over rocks. 

If you have to mow where there's probably rocks or stumps, do yourself a favor and get one of those brush mowers with the plastic trimmer cord.  Totally worth it.  It sure beats wasting four or five days and a bunch of secondary parts messing around with a torn-apart push mower because you bent the crankshaft. 

As I said, fixing a bent crankshaft is kind of a big job.  Many other things on a lawnmower are easily fixable, but it requires a basic set of tools.  It really helps if they're well-organized so you don't have to spend half the day looking for a 3/8" socket.

This has been a look at what to do if your lawnmower has a bent crankshaft.  I hope this article was helpful or at least entertaining;  if so, please help me out by using the links on here to purchase any of your stuff.  It helps me keep this website on-line and bringing you helpful articles like this one. Thank you in advance.


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