Stihl HL KM trimmer PSA

Bart_

Active Member
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My angle adjustable hedge trimmer head just committed suicide while still operable. Let me explain. It was working but had a new intermittent metallic squeak which I thought I remedied with that super duper Stihl lube/cleaner - but it persisted. I assumed the lube didn't get into the casting to blades junction area so I disassembled it. That's when I found plenty of grease but with what looked like dirt on its surface. Couldn't figure how or why dirt could get inside and it was certainly never operated near dirt anyway. S I degreased each piece I pulled out. First connecting rod had needle bearings that laid two by two in skidding, not rolling direction. So I thought has this been misassembled at the factory and skidding the rollers in the grease like bushings for two years? Second connecting rod has no rollers and a broken shell edge but I'm not seeing the rollers nearby. I did find less than half the rollers in the grease eventually. Then I see one of the crank gear needle bearings basically completely missing. On further work about 3/4 of the shell remained in the hole. The other needle bearing in the housing "cap" was just starting the break up. So how the heck was this thing still operating? Small miracle.

The question: what happened? My previous trimmer got beaten for 8 years until a drive gear cracked. When I was cleaning the housing I found corrosion, perhaps more than ought to be and noticed scraping the metal that it was hard. Bingo, the damn thing is magnesium not aluminum. This hearkens to a relatively recently learned lesson about dirt bikes where the water pump housing would self destruct with time because it was magnesium. I found the exact same corrosion pattern in the trimmer. CSI of the failure is the housing around the needle bearing went away, failed the needle bearing, put wobble forces into the crank assembly that messed with the con rod bearings and the last hold out needle bearing was on life support just keeping things moving. Research showed another guy encountered the switched direction needle rollers during his failure too. My old head just had bushings in the con rods, no rollers, totally reliable. Rained on plenty, put away wet.

Just over a hundred in parts, I cheaped out, could've dropped another hundred on the crank gear. I was fortunate enough to have all the bore in the cap and 3/4 of the bore was left in the destroyed needle bearing casting so the bearing could be replaced and the missing magnesium replaced with JB Weld.

To prevent further corrosion death: I'm going to paint all the internal casting surfaces with grease, not just load surfaces and I'm going to be hyper sensitive about the unit getting rained on etc. Also likely disassemble periodically to check for moisture.

I recalled seeing another sad sack at he service counter with a suicided $400 trimmer head about a year earlier but I paid it no heed. Now I know what happened to him.

Noted that the newest $300 on sale replacement heads look to be painted. Word must have got back about the corrosion. That's my theory anyway.

take care
 

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