# engine damage autopsy



## hector (Dec 30, 2019)

the two pieces of the crankshaft came out, they are supposed to be attached and tight, this happened after a little more than a year using the maxima k2. I don't want to think that the oil is responsible for the crankshaft to separate. I recognize that they are economical trimmers and do not have much long-term craftsman 30cc and 25cc. one had 8 years of use once or twice a month and the other as 6 years of casual use. I already bought another trimmer of much better quality Husqvarna 430ls and I am worried about using that oil (maxima k2) and that gives problems to my new trimmer.
What do you think happened here?


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## frank_ (Dec 30, 2019)

just a design flaw imo, crankpin probably worked loose when hot


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## LegDeLimber (Jan 2, 2020)

Stack of sheet stampings and a cantilevered crank pin?
Recipe for a low hour engine life.
The stampings flex with every rotation and put all of the load on the corners of the individual layers.
So eventually the crank pin is going to wallow out the hole.
Your oil wasn't the problem.

A pass with TIG welder (across the plates, on their apex at the pin hole) when the crank assy was brand new, might have given a 2~3 more seasons of run time.

The units are built to a retail price point (low-ish).
Most people end up strait gassing or letting ethanol fuel destroy them in about 3~4 seasons. 
So not many manufacturers seem to build homeowner units for much over that life expectancy nowdays.
I've seen string trimmers that the parts were listed as "obsolete & NLA" (No Longer Available) after a couple of years.


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## banditt007 (Jan 28, 2020)

Just questions out of curiosity. What oil ratio did you use, and did you often spin the trimmer to really high rpm with little to no string attached?

This is in no way hinting towards either of those things being the problem..


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## Manic84 (Jan 29, 2020)

frank_ is right, it's a design flaw. I've worked on these type (Sears/Craftsman/MTD) of trimmers before and it's a common problem, unfortunately the pin likes to worm it's way out and as a result this kind of damage is not uncommon.

I think they tried to "fix" this by adding a bearing to keep it in place and called it the "Jump Start" feature, where you'd take a drill with an attachment to start it instead of the rope. (Correct me if I'm wrong here)

The problem I found with that, was the gasket/o-ring on the rear cover would fail, it would suck in air and score the piston/rings and cylinder, and people only brought them in after it was already a goner, you really couldn't prevent it from happening.

LegDeLimber called it with the common fate and short lived nature of these machines and parts soon being listed as obsolete or NLA, these machines were *not *meant to last more than a few seasons if not less.


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## Huskybill (Feb 7, 2020)

It’s not the oil there’s two different fractures there. The dark color is the first fracture the lighter color is the final fracture. Looks like a alignment problem it flexed and finally cracked n broke.

With two stroke oils I use the equipment manufacturers oil.


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## mexicanyella (Jul 1, 2020)

That’s one of those inverted, cylinder-on-the-bottom engines, with the carb and reed attached to the rear cover, right?

Some of those things break early and others seem to defy physics and metallurgy and run just about forever, or until shaft and trimmer head problems cause owners to trash them. I think I’ve read that the guys who convert those engines for large-scale RC plane use like to keep their revs below 7500, via prop diameter and pitch, to keep them from coming apart. 

I’ve had a few of them that ran pretty well for a long time, but that’s with keeping the string at full cutting diameter and not revving it too high. They do have a lot of lower rpm grunt, as trimmers go.

I have several of those engines in my garage from dumpster diving for trimmers. Need a short block? Happy to share. Show me a pic of the rear cover and muffler and exhaust port...there was some variance on those parts over the years on the various re-badged versions. I’d be happy to send you one if you want to fix it and keep using it.


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## mexicanyella (Jul 1, 2020)

Hey, wow, kind of missed the date on the OP. Sorry for the necro-reply. Well, if you’ve been wanting to make that trimmer run again for seven agonizing months, my offer stands.


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## hector (Jul 1, 2020)

banditt007 said:


> Just questions out of curiosity. What oil ratio did you use, and did you often spin the trimmer to really high rpm with little to no string attached?
> 
> This is in no way hinting towards either of those things being the problem..


 40:1 full throttle most of the time, with modified muffler


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## hector (Jul 1, 2020)

mexicanyella said:


> That’s one of those inverted, cylinder-on-the-bottom engines, with the carb and reed attached to the rear cover, right?
> 
> Some of those things break early and others seem to defy physics and metallurgy and run just about forever, or until shaft and trimmer head problems cause owners to trash them. I think I’ve read that the guys who convert those engines for large-scale RC plane use like to keep their revs below 7500, via prop diameter and pitch, to keep them from coming apart.
> 
> ...


 thank you for response and offer me the short block i really appreciate, but i throw it to the trash. i buy a new husqvarna 430ls a very nice trimmer


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## mexicanyella (Jul 1, 2020)

You’re welcome and happy trimming


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## MacAttack (Jul 1, 2020)

Manic84 said:


> frank_ is right, it's a design flaw. I've worked on these type (Sears/Craftsman/MTD) of trimmers before and it's a common problem, unfortunately the pin likes to worm it's way out and as a result this kind of damage is not uncommon.
> 
> I think they tried to "fix" this by adding a bearing to keep it in place and called it the "Jump Start" feature, where you'd take a drill with an attachment to start it instead of the rope. (Correct me if I'm wrong here)
> 
> ...


Yep ....common problem, usually the pin backs out and grinds through the plastic cover, and they lean out. My friend had one of those pieces of junk.


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## mexicanyella (Jul 1, 2020)

The first time I saw one of those cranks I was kind of horrified. But the more I thought about it, the more I thought it was kind of cool that such a design could work at all.


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## Manic84 (Jul 1, 2020)

MacAttack said:


> Yep ....common problem, usually the pin backs out and grinds through the plastic cover, and they lean out. My friend had one of those pieces of junk.



Yes, it is very common and frustrating to deal with, and repairing them is almost never worth it.


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## MacAttack (Jul 1, 2020)

Manic84 said:


> Yes, it is very common and frustrating to deal with, and repairing them is almost never worth it.


It's a shame because the older Craftsman stuff was well made. I have an older Craftsman trimmer with the interchangeable attachments that my uncle gave me because he couldn't fix it (needed repair to the throttle trigger). I fixed it and it still runs great, I would guess it's from the late 80's or early 90's, red housing and a Walbro carb. Starts on the first pull every time.


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## mexicanyella (Jul 1, 2020)

Did it look like a Craftsman-badged version of this? We had one of those and it held up pretty well for several years, but it lived at my Dad’s house and as I recall, tuning it to run right at 9,000 feet elevation was fiddly.


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## MacAttack (Jul 1, 2020)

mexicanyella said:


> View attachment 839897
> 
> 
> Did it look like a Craftsman-badged version of this? We had one of those and it held up pretty well for several years, but it lived at my Dad’s house and as I recall, tuning it to run right at 9,000 feet elevation was fiddly.


Nope completely different, I'll snap a picture of it tomorrow, I think the one I have was a rebadged Poulan. But I think my dad still has an old Weedeater like that with the round tank.


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## hector (Jul 1, 2020)

nice trimmer


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## mexicanyella (Jul 1, 2020)

I liked the shroud configuration. The muffler exhausted into the cooling air stream, out the front of the shroud. It had a pretty pleasant, soft-edged sound and seemed to have good midrange power, and was easy to start.

When it started to show signs of bad crank seals and needing a new coil, I discovered a lot of parts were NLA and I scrapped it and moved on. Liked the overall design though.


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## MacAttack (Jul 2, 2020)

This is my older Craftsman trimmer with convertible attachments, not sure of the year but it has a Walbro carb and carb adjustments that take a flathead screwdriver. Runs strong.


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## mexicanyella (Jul 2, 2020)

Pretty sure that’s a rebadged Weed Eater. Shroud looks like an early Featherlite model, but red instead of green. Fuel tank looks the same too. I see the displacement is 25cc...the older square shroud Featherlites I’ve “rescued” before were either 18 or 20cc. I have a 25cc one now, but it has a newer, sort of Jellybean-shaped shroud. 

The ones I’ve used seemed like good-running powerheads on really light-duty shaft housings, with tiny single-line bump heads loaded with .065” line. I have a fixed-line head on my current one and it swings .080” just fine. It will work with .095” too, but I worry about that small-diameter flex shaft and tube, and .080” seems like a good compromise for the available power and not wearing or breaking off too fast as you cut.

One of the 20cc ones I found and revived a few years ago had a crank bearing failure after a few months but the others only had the occasional fuel line and diaphragm breakdowns. Parts are available and pretty inexpensive, so worth fixing if you don’t expect to use it as a clearing saw/brush cutter.


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## MacAttack (Jul 3, 2020)

I think you're right about it being an older Weedeater Featherlite 25, here is a picture I found. I actually have 2 of the newer style ones (got both for free) that I am fixing. It seems like the newer ones usually just need fuel lines, diaphragm, or a carb. 

I've actually been using that one I have as a brush cutter, after I got a brush cutter attachement at a Sears going out of business sale, and it handles it great.


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## mexicanyella (Jul 3, 2020)

Cool, can you post a picture of that brush cutter attachment? I’ve never seen one on this type of trimmer.

The one I have now is called a SST25CE, and has a straight shaft housing—still with a flex cable in it—and no clutch. Found it in a scrap pile with missing intake parts and swapped on the air cleaner and working carb off the 20cc one with the crank bearing problem I mentioned earlier. It’s been pretty easy to keep running and is my favorite for careful trimming around fences and flowers. I wish it was several inches longer; the ergonomics aren’t quite right, but it is light and easy to handle and not a problem for maybe one tankful at a time.


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## hector (Jul 3, 2020)

those are classic


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## Huskybill (Jul 3, 2020)

I purchased the poulan pro with all the attachments. The first was a refurb, it runs but stalls when hot, I purchased a second one that runs ok.


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## MacAttack (Jul 3, 2020)

mexicanyella said:


> Cool, can you post a picture of that brush cutter attachment? I’ve never seen one on this type of trimmer.



Yep I will, also that trimmer does have a clutch, which I really like.


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## MacAttack (Jul 6, 2020)

mexicanyella said:


> Cool, can you post a picture of that brush cutter attachment? I’ve never seen one on this type of trimmer.
> 
> The one I have now is called a SST25CE, and has a straight shaft housing—still with a flex cable in it—and no clutch. Found it in a scrap pile with missing intake parts and swapped on the air cleaner and working carb off the 20cc one with the crank bearing problem I mentioned earlier. It’s been pretty easy to keep running and is my favorite for careful trimming around fences and flowers. I wish it was several inches longer; the ergonomics aren’t quite right, but it is light and easy to handle and not a problem for maybe one tankful at a time.



Here you go, I'm not sure when the trimmer was made but the brush cutter is only a few years old and fits it, I also have the original string trimmer attachment:


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## mexicanyella (Jul 6, 2020)

Thanks for posting those. Looks cool. Do you notice the drive cable winding/unwinding from the inertia of the metal brush blade?

I installed an 8-tooth Stihl grass blade on a Stihl FS74 trimmer years ago (early 90s model with straight shaft and a cable inside) and it was pretty much unusable. You could feel the cable winding and unwinding with every rpm or load change. Later I installed that same blade on an older Tanaka with a splined driveshaft in the tube and it worked much better. But maybe that Stihl cable was extra boingy. Also, the 8-tooth blade was a bigger disc of metal than that 4-tooth in your photo...possibly making it more of an issue.


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## MacAttack (Jul 6, 2020)

mexicanyella said:


> Thanks for posting those. Looks cool. Do you notice the drive cable winding/unwinding from the inertia of the metal brush blade?
> 
> I installed an 8-tooth Stihl grass blade on a Stihl FS74 trimmer years ago (early 90s model with straight shaft and a cable inside) and it was pretty much unusable. You could feel the cable winding and unwinding with every rpm or load change. Later I installed that same blade on an older Tanaka with a splined driveshaft in the tube and it worked much better. But maybe that Stihl cable was extra boingy. Also, the 8-tooth blade was a bigger disc of metal than that 4-tooth in your photo...possibly making it more of an issue.


I have not noticed the cable doing anything funny, like winding up, bouncing, or anything. And I've hit some pretty rough stuff with it. Does your Stihl trimmer have a clutch?


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## mexicanyella (Jul 6, 2020)

Yes, the FS74 has a clutch. It has been great for bump head use; it’s been used hard since 1992 and only needed the lower gearbox replaced once and the spark arrestor screen cleaned once (running it on 32:1 premix its whole life, I will add). It is only just now needing fuel line replacement and a carb kit for the first time, and that’s on cheap 87 octane fuel with ethanol in it. I’d say we got our money’s worth out of that machine. But it did not like that grass blade. 

I have one Poulan Pro trimmer, a PP033, with a straight-shaft Craftsman “lower unit” swapped on and the same type of 31cc engine as in the OP’s first post...no clutch, fixed-line head. It does a little bit of wind/unwind briefly when I start it, sometimes, but never while cutting. I wonder if it has to do with the spacing of the nylon guide bushing thingies in the drive tube; maybe if they are spaced too far apart the drive cable can whip around in there too much or something.


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## Manic84 (Jul 6, 2020)

MacAttack said:


> This is my older Craftsman trimmer with convertible attachments, not sure of the year but it has a Walbro carb and carb adjustments that take a flathead screwdriver. Runs strong.



If I had to take a guess of when it was manufactured, I'd ballpark it at late 90's - early 2000's. But that EPA certified stamp says 2001, so I would go off that to date it.


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## MacAttack (Jul 6, 2020)

Manic84 said:


> If I had to take a guess of when it was manufactured, I'd ballpark it at late 90's - early 2000's. But that EPA certifed stamp says 2001, so I would go off that to date it.



Yeah I guess if I ever bothered to take a closer look at it... I would have seen the EPA 2001, haha. I thought it was older for some reason.


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