What do you think is causing this piston damage? (372 clone)

[BenC]

I've got a Baumr Ag SX92 which turns out to be a Husky 372 clone. It's done plenty of work over the past few years, but I'm now stuck scratching my head over a particular problem.

4-5 times it's died on me and on pulling it down it looks like it has eaten a small piece of hard metal, which has bounced around making indentations in the piston and sometimes scoring the piston (see pics below) and causing the ring to seize. Thus the saw loses compression and stops. Once or twice I've found something embedded in the piston and it looks like a small section of piston ring, or circlip. But after checking everything over I never find any missing screw or broken parts. It's an absolute mystery to me what the foreign material is. And yes, the air filter is still there.

If I free up the ring and smooth things out it'll run again fine. Sometimes for a few hours, sometimes for a few months. This time I split the cases to check the crank bearings and they looked fine (replaced them anyway). But I am still stumped as to what is causing this.

Does anyone have any suggestions as to what the cause might be?
Thanks

 

[wcorey]

Is the ring pin still in the piston? Or maybe wrist pin bearing roller chunks...

 

[BenC]

Good thought, but yes, it's still safely in place.

 

[Mad Professor]

My first guess would have been bearing parts, lower rod bearing, if it did not ingest something through the intake.

 

[Wizard]

Ring pin?

 

[Cantdog]

Have you checked the muffler for missing bits of baffles?

 

[alexcagle]

I've got a Baumr Ag SX92 which turns out to be a Husky 372 clone. It's done plenty of work over the past few years, but I'm now stuck scratching my head over a particular problem.

4-5 times it's died on me and on pulling it down it looks like it has eaten a small piece of hard metal, which has bounced around making indentations in the piston and sometimes scoring the piston (see pics below) and causing the ring to seize. Thus the saw loses compression and stops. Once or twice I've found something embedded in the piston and it looks like a small section of piston ring, or circlip. But after checking everything over I never find any missing screw or broken parts. It's an absolute mystery to me what the foreign material is. And yes, the air filter is still there.

If I free up the ring and smooth things out it'll run again fine. Sometimes for a few hours, sometimes for a few months. This time I split the cases to check the crank bearings and they looked fine (replaced them anyway). But I am still stumped as to what is causing this.

Does anyone have any suggestions as to what the cause might be?
Thanks




View attachment 1201693

View attachment 1201695

Clip tang from wrist pin or spark plug insulator chip, or a rivet off bearing cage.

 

[Bob Hedgecutter]

Going off of the photographic evidence and the shape transfers into the piston sides and top- short sections of harder than piston metal in a cylindrical profile- exactly like the needles in a bearing one might find between wrist pin and upper rod eye, or crank and lower rod eye.
The remains- if not squished into the piston contact areas- will often be lodged in the squish band of the top of the cylinder.
Goes in the uneconomic repair section of copycat failures.

 

[thompsoncustom]

Going off of the photographic evidence and the shape transfers into the piston sides and top- short sections of harder than piston metal in a cylindrical profile- exactly like the needles in a bearing one might find between wrist pin and upper rod eye, or crank and lower rod eye.
The remains- if not squished into the piston contact areas- will often be lodged in the squish band of the top of the cylinder.
Goes in the uneconomic repair section of copycat failures.

I agree with everything said except the uneconomic repair part.

Fixed a saw like this which was a 660 clone for someone it had a lower rod bearing failure and destroyed everything but was only about a 100 bucks for a brand new motor (case,crank,cylinder,everything) made for a pretty easy/cheap swap.

 

[ballisticdoughnut]

I’d be taking a thorough look at that rod bearing. I’m also wondering, what’s the cylinder look like?

 

[Bob Hedgecutter]

I agree with everything said except the uneconomic repair part.

Fixed a saw like this which was a 660 clone for someone it had a lower rod bearing failure and destroyed everything but was only about a 100 bucks for a brand new motor (case,crank,cylinder,everything) made for a pretty easy/cheap swap.

So it can do it all again with the same quality parts- perfect.

 

[thompsoncustom]

So it can do it all again with the same quality parts- perfect.

sure could but that saw was done 3 years ago and I know it's still running.

I personally have a clone 660 that's going on over 5 years now, I actually bought 3 of them but never had to use the other 2 and ended up selling them off but this ones been perfectly reliable and from a economic standpoint the clone saws always gonna win at this point. Those 3 660's cost me $600 bucks so if they only lasted 5 years when you compare that to the price of a brand new 660/661 I'll be dead before I would have saved money from buying a real one if it never broke once.

 

[BenC]

Thanks for all the replies. To answer them I can confirm:
- Ring pin is still in place in the piston.
- Ring is unbroken and looks generally OK.
- Piston pin circlips were in place and undamaged when I disassembled them.
- Rod upper bearing is complete and undamaged.
- Rod lower bearing, while tricky to inspect while the crank is assembled, looks fine with no missing rollers or damage to the cage.
- Crank bearings are undamaged, including the cages holding the balls and rivets in these cages.
- Spark plug looked fine. Just had another look and nothing is missing or suspicious.
- Muffler looks fine. It has no internal baffles (because cheap) and nothing inside looks loose/damaged/etc.

The culprit looks to be a hard, curved section of metal. A chunk of piston ring or large circlip maybe. But this has happened 4-5 times now and I'm absolutely stumped as to what's causing it.

I'm not too concerned about the piston, I've got an 064 piston here ready to be turned into a popup if it's terminally damaged. And the cylinder has some marks, but doesn't look beyond repair. I've fixed this damage several times before and the saw has run fine until it happens again. It's by no means perfect, but it is just a cheap, educational saw that has both cut alot of wood and taught me heaps.

 

[thompsoncustom]

Thanks for all the replies. To answer them I can confirm:
- Ring pin is still in place in the piston.
- Ring is unbroken and looks generally OK.
- Piston pin circlips were in place and undamaged when I disassembled them.
- Rod upper bearing is complete and undamaged.
- Rod lower bearing, while tricky to inspect while the crank is assembled, looks fine with no missing rollers or damage to the cage.
- Crank bearings are undamaged, including the cages holding the balls and rivets in these cages.
- Spark plug looked fine. Just had another look and nothing is missing or suspicious.
- Muffler looks fine. It has no internal baffles (because cheap) and nothing inside looks loose/damaged/etc.

The culprit looks to be a hard, curved section of metal. A chunk of piston ring or large circlip maybe. But this has happened 4-5 times now and I'm absolutely stumped as to what's causing it.

I'm not too concerned about the piston, I've got an 064 piston here ready to be turned into a popup if it's terminally damaged. And the cylinder has some marks, but doesn't look beyond repair. I've fixed this damage several times before and the saw has run fine until it happens again. It's by no means perfect, but it is just a cheap, educational saw that has both cut alot of wood and taught me heaps.

so you've had this exact thing happen 4 or 5 times? cleaned up the cylinder and put a new piston rings in it?

how's the bevel on the ports? could be hanging a ring in the same spot everytime.

 

[BenC]

Sorry, there's one more detail. I had it happen twice, then installed a big bore kit (new cylinder and piston) and it has since happened 2-3 times.

When I installed the big bore kit I checked and touched up the bevel on the ports. Damage definitely looks to be foreign object rather than ring catching though.

 

[Bob Hedgecutter]

So one copy cat inexpensive saw has taken out 5 top ends?
Starting to become an expensive inexpensive saw that still is not running.

IF indeed all lower engine parts are still intact, my guess is whatever is causing the damage is remaining in the crankcase each time you slap a new top end on it and it is just a case of wash, rinse, repeat.
How about some detailed photos of the engine internals?

 

[thompsoncustom]

Sorry, there's one more detail. I had it happen twice, then installed a big bore kit (new cylinder and piston) and it has since happened 2-3 times.

When I installed the big bore kit I checked and touched up the bevel on the ports. Damage definitely looks to be foreign object rather than ring catching though.

I would say it's time to just replace it all as more than likely you have pieces in the lower end/bearings that are working their way out from time to time. Looks to be $128 bucks now for a complete setup but if you want you can purchase an amazon 3 years warranty for 20 bucks more.

Here's an example as it's for the 660 and you would need one for the 372.



https://www.amazon.com/Farmertec-Crankcase-Cylinder-Crankshaft-Chainsaw/dp/B07CPM6C9G/ref=sr_1_1?crid=3G8U0534XRA42&dib=eyJ2IjoiMSJ9.Qb8HR2f9d7trTcMQY5-Ak890v7Z4H76yqeJl5kYVNoBO0MUoZuQRFyb8Y-fEKrhj6_gsFZSPx8UZMB5kwc_oVGjMhbf8yNkUVmFfzbgNGh7tSp0K9NLUAwWLlNn5-fu25-X7KLnlCnkUVJJQPNzqv8ithESVpEJyf0iNx8dBCV5GZcYn0Zou430NajObLqhJ_jpID1hJc7iGk0ES2l2ZIywKNsmGDZk05Zm69361-TA.cbrDSohqHCmdN8e5uJ2d-3D5AnGE_zdRSdlItnzZUgc&dib_tag=se&keywords=ms660+engine&qid=1725356297&sprefix=,aps,176&sr=8-1