classic car forum header
Classic cars forum & vehicle restoration.
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 
How To Register     Posting Photographs     Privacy Policy     F/book facebook.com/oldclassiccar

Piston failure- any idea's?
Post new topic   Reply to topic    Classic cars forum & vehicle restoration. Forum Index -> Mechanical Restoration
Author Message
ukdave2002



Joined: 23 Nov 2007
Posts: 3653
Location: South Cheshire

PostPosted: Sat Jan 23, 2021 3:54 pm    Post subject: Piston failure- any idea's? Reply with quote

I was gifted a non working Stihl TS400 disc saw, pulling the starter there was clearly no compression, whipped the barrel off and this is the state of the piston:





The damage is directly opposite the exhaust port (2 stroke) any thoughts on the cause?

Dave
Back to top
View user's profile Send private message Visit poster's website
Peter_L



Joined: 10 Apr 2008
Posts: 2597
Location: New Brunswick. Canada.

PostPosted: Sat Jan 23, 2021 4:15 pm    Post subject: Reply with quote

Seems you may not be alone. A similar failure is discussed in link below.

You will have to bypass the "dross" but on page two there are some interesting comments. BTW. I run various Stihl garden tools, Chain Saws, Strimmers etc. Have had little to zero problems but know of others who have had problems with "rebuilds" using non stihl parts.

https://www.arboristsite.com/community/threads/piston-failure-on-stihl-046.242657/

https://www.arboristsite.com/community/threads/piston-failure-on-stihl-046.242657/page-2
Back to top
View user's profile Send private message Send e-mail Visit poster's website
Farmer John



Joined: 18 Feb 2010
Posts: 180
Location: Manawatu NZ

PostPosted: Sat Jan 23, 2021 8:22 pm    Post subject: 2-stroke piston Reply with quote

Hi Dave, now this is interesting!
A question first, are the transfer ports at the sides, where the gudgeon ends are? That would rule out something from a transfer port catching.
Some observations instead of diagnosis. It appears from the broken surface that the engine has continued to fire after that broken piece had separated from the piston going by the colour of the break, maybe when there was just a huge crack, but looking at the edge of the break it looks a bit hammered. There are other indentations from foreign objects on the piston top. Be interesting to look at the surface of the broken piece, how did it get out of the cylinder anyway?
The second ring seems to have adequate groove depth, is there any remaining top ring to check? A straight edge across part of the ring will tell you.
Are the rings pegged? There needs to be some way to prevent the rings drifting around.
If rings have insufficient gap this will cause the engine to seize when it gets hot and be difficult to turn over when it stops for ten to fifteen minutes. That will score the bore. When rings get worn they can break but stay in the groove and a short piece will rattle around and chew it's way up to the top of the piston, see lots of those from the era of vehicles we play with you have probably seen a few. 2-strokes do not usually last long enough for that to happen, there will be the warning "tinkling" sound at idle.That top ring land is so stout though that you would not expect that damage. They like the ring as close to the top as possible as that is what defines the port timing, not the piston top, of course it is a compromise. Have seen the top land taken clean off by the ring but the land was hardly any wider than the ring.
From 12,000 miles away there does not look like any detonation, my eyes are not that flash any more so you will be seeing things that I could never see even firsthand which is why I have no answer. There could be a combination of faults. The worst is faulty material, that is usually beyond us to be certain.
Really it is down to the man on the ground and he has probably thought of all the above already.
John
Back to top
View user's profile Send private message
Bitumen Boy



Joined: 26 Jan 2012
Posts: 1495
Location: Above the snow line in old Monmouthshire

PostPosted: Sun Jan 24, 2021 4:06 am    Post subject: Reply with quote

Interesting photos, Dave. When my Herald's (presumably original) engine started making expensive noises on the M50 - no I wasn't thrashing it! - a good many years ago now with, IIRC, about 60k on the clock, I eventually found that #1 piston had failed in a remarkably similar manner, so I wonder if your problem is necessarily something specific to a 2 stroke engine.

I say eventually as I rebuilt another engine fron a scrapper to fit; I guessed that a rebuild would be a protracted job and didn't want the car monopolising the garage for the duration and it was still capable of moving under its own power although useless as a means of transport. With all the paperwork in order I could simply leave it parked on the front street for half the summer... After the engine swap I had to strip the old engine to divide a pretty immovable lump into manageable bits that could be carried to the shed for storage, and found the aforementioned damage to #1 piston though the other 3 were in good order. Next time I went into town I took the damaged piston to show the chap that did the machining work on the replacement engine and he reckoned it was down to a manufacturing fault at the factory, possibly someone didn't compress the rings sufficiently and just walloped the piston into the bore. How it had lasted 40 years or so like that before breaking up is anyone's guess so I offer the story for interest only... Smile
Back to top
View user's profile Send private message
ukdave2002



Joined: 23 Nov 2007
Posts: 3653
Location: South Cheshire

PostPosted: Sun Jan 24, 2021 3:18 pm    Post subject: Reply with quote

Thanks for the feedback

I have done some further investigation:

The link Peter posted seem to show an identical failure, and like that unit I suspect the piston and barrel on this unit are non OEM replacements, both piston and barrel have a logo which appears to be a T and an upside down T?






Italian parts not Chinese, the market is flooded with cheap Stihl replacement parts from China. A complete new barrel/ cylinder, piston, crank conrod and bearings can be picked up for less than 40 Shocked

I recovered about 2/3rd's of the top piston ring, unsurprisingly the missing bit coincides with the missing chunk of piston.


Both 1st & 2nd piston ring and grooves are the same, the broken ring is undistorted. The gudgeon pin is at 90 to the ports. The ring gap on the good ring if anything seems large; about 0.8mm clearly we will never know what the top ring gap was.

I didn't find any large fragments of metal, plenty of small bits in and around the crank case.



I cleaned the piston up, and it is peppered with small indentations, as is the top of the cylinder.



Pic below is looking up to the top of the cylinder (these cylinders don't have a removable head) the hole top center is the spark plug hole, thread to the right of the pic is the decompression valve hole, port in the side is the exhaust.



Interestingly neither the sides of the piston or cylinder have the slightest damage, how did all the fragments get into the crank case without scoring the piston or cylinder?

The exhaust port also appears damage free; if it were a ring catching on the lip of the port I would have thought there would have been some sign of damage?

Nothing appears to have got "hot"



There is zero/negligible wear in the cylinder, which leads me to suspect that someone had replaced the cylinder and piston fairly recently before the latest failure and perhaps just gave up?.

The saw its self was found in the garage by the owner of a newbuild house 3 years ago, he notified the builder at the time but no one claimed it, I saved him a trip to the tip where it was heading!.

I am tempted just to fit another non OEM piston as they are only about 15 from a UK machinery parts supplier that I use (I'm banking that they don't supply the cheapest of cheap Chinese parts !), build it up carefully with new bearings and run it in gently...I have nothing to lose.

Dave
Back to top
View user's profile Send private message Visit poster's website
Bitumen Boy



Joined: 26 Jan 2012
Posts: 1495
Location: Above the snow line in old Monmouthshire

PostPosted: Tue Jan 26, 2021 4:05 am    Post subject: Reply with quote

I would be tempted to try and get it running. Assuming you have time to play with it you have little to lose; and should you succeed then such a tool in working order ought to sell for a modest profit even if you have no use for it yourself... Smile
Back to top
View user's profile Send private message
Clactonguy



Joined: 20 Mar 2018
Posts: 77
Location: clacton on sea

PostPosted: Sat Mar 13, 2021 7:35 am    Post subject: piston fail Reply with quote

need a professional examination to determine real root cause but I suspect it was due to a flaw in piston casting not installation issue of piston. look at lower left side in pic and what appears to have been an 'inclusion' can be seen. these act as stress areas and weak points allowing cracks to occur. thus losing part of piston under severe stress due to detonation ( burning of fuel actually)A crack will progress thus rapidly weakening part under stress. I suspect top part failed fairly early on and was ejected via ports in cylinder. top edge of missing section seems eroded (worn) suggesting long operation without this section occurred. the damage around edge of piston looks likely ot have bene caused by broken ring sections being impacted on piston and possible damage to cyl head might have occurred depending on size of parts being thrown around and clearance between piston and cylinder head. piston rings possibly lasting soem time before eventual failure ..result is what you see.
Back to top
View user's profile Send private message
consul 57



Joined: 09 Nov 2017
Posts: 305
Location: somerset

PostPosted: Sat Mar 13, 2021 1:48 pm    Post subject: Reply with quote

i bought an early 80's honda xr200r field bike many years ago.
it ran and went quite well but needed work, when i took the engine apart it only had the oil ring out of the 3 rings still ok and the inside of the head was pock marked with lots of pyramid shaped indentations where the top ring had broke up and had been bashed into the head.
had it rebored and new piston and rings and it went on to serve me lots of years quite well.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    Classic cars forum & vehicle restoration. Forum Index -> Mechanical Restoration All times are GMT + 1 Hour
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
Forum T&C


php BB powered © php BB Grp.