engine damage autopsy

[hector]

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?

 

[frank_]

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

 

[LegDeLimber]

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.

 

[banditt007]

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..

 

[Manic84]

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.

 

[Huskybill]

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.

 

[mexicanyella]

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.

 

[mexicanyella]

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.

 

[hector]

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

 

[hector]

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.

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

 

[mexicanyella]

You’re welcome and happy trimming

 

[MacAttack]

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.

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.

 

[mexicanyella]

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.

 

[Manic84]

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.

 

[MacAttack]

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.

 

[mexicanyella]

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.

 

[MacAttack]

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.

 

[hector]

nice trimmer

 

[mexicanyella]

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.

 

[MacAttack]

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.