FS550 destroying pistons -- non-OEM parts the problem?

hdykstra

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  • / FS550 destroying pistons -- non-OEM parts the problem?
Hi everyone:

I'm not a small engine mechanic, but a handy home-owner. I have an FS550 that I bought used and that I'm using to try regain control of an overgrown acreage. After a couple of summers of great use, the compression was so low that it was really hard to start. The quote from my local dealer to replace the cylinder and piston seemed really high to me, so I did some research and found non-OEM parts online for pretty cheap and replaced them myself using a Stihl repair manual.

The saw ran perfectly for about 10 minutes, and I heard a small 'clunk', and it quit. It wouldn't start again, and when I tested the compression it was about 50. I ordered another piston and cylinder set, and installed them. The old piston had major gouges in it. It again ran perfectly for a few minutes, and then gave the same clunk and died. Same story again -- trashed piston. I've done this three times now, and I'm frustrated.

Here's my question: is it because the non-OEM (probably made in China) parts are so poor that I need to bite the bullet and buy the Stihl parts (which I'm happy to do), or is there something I'm missing when I do the replacement which would destroy the much more expensive Stihl piston and cylinder just as quickly as it has destroyed my non-OEM parts.

Any thoughts? I'm happy to buy the Stihl parts, but I'd hate for the same thing to happen again.

Thanks very much for any thoughts.

Harold
 

bertsmobile1

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  • / FS550 destroying pistons -- non-OEM parts the problem?
Please post the photos of the pistons and the bore.
We buy Stihl parts for Hurtzl in HK and have neve had any problems with them except for the bad Chinese copy of the cheap Italian copy of the quality Bosh part ( which was actually made in Spain ).
Having said that there are a lot of internet vendors selling similar looking parts for less than our wholesale parts and tossing in free postage,
A lot of these tha customers have asked me to fit were faulty so I am guessing they are buying reject parts for recycling and flogging them off to cheapskates as good parts.

I see this happen a lot with British bike parts.
Avendor will inform me that they have rejected a big batch of a part and within a week they are every where on the web, really cheap
 

hdykstra

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  • / FS550 destroying pistons -- non-OEM parts the problem?
Thanks for the reply, Bertsmobile1.

I'll post a couple of photos of the piston and cylinder below for what it's worth.

Do you think these repeated failures are likely due to defective or poor quality parts?

What I'm suspecting now is that it's something like debris getting sucked out of the crankcase area and into the cylinder. Is that likely or even possible? I'm suspecting that now because of how suddenly it stops each time, and how the piston has a small number of really deep gouges in it.

Any further thoughts are appreciated!

Harold

piston1.png
 

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bertsmobile1

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  • / FS550 destroying pistons -- non-OEM parts the problem?
First rest your phone on something and zoom in tight on the objects.
Shoot the barrel down the exhaust port, inlet port and from the head.

That sort of damage says screws fallen into engine.
Also check the transfer ports for big chunks of carbon.

I seriously doubt the barrels will be in a serviceable condition if that sort of damage is being done to the piston.
 
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