Stihl MS 170 Saw Repair: Faded Piston, Cylinder Kit Replacement, Chain Rotation & Oil Leak Issues

Hello all,
I started repairing the Stihl MS 170 saw, which could not be fired (it was impossible to pull the cord). I took the saw down for the first part. After examination, it turned out that the piston was blurred (stuck rings). I decided to buy a repair kit in the form of a cylinder, piston, bolt, protections, seals and bearings - probably Chinese replacements.
After assembling the saw, it was successfully fired, but the chain on the guide bar did not rotate when adding gas. As it turned out, there was a leak between the chain supplying oil and the oil pump that kept the chain dry. After dealing with the fault, everything seemed to be all right.
At idle the saw worked full fuel tank (with breaks for cooling). On the next tank I decided to do tests - I cut wood with a diameter of ~ 30cm - three cuts. After a long moment the saw went out. Unfortunately I couldn't fire it, I decided to undress it again. A totally scratched piston appeared in my eyes - deep features.
I do not know where I made the mistake or what factors led to such rapid wear of the piston.
The mixture I used was Stihl's oil in a 1:25 ratio.
Below is a photo of both pistons - new piston from above, old piston below:

As you can see, the consumption of the new piston is much higher than for the old one.
I would add that the piston is of the Dolphin brand and was fitted with locks towards the carburettor. The exhaust side is much less scratched. The piston did not have a full arrow indicating the direction to the exhaust but only a dot. The rings were placed with the chamfer upwards.
On both pistons, right next to the locks, there was a jamming of the rings with the piston as seen in the photo below:

I also post pictures of the interior of the new cylinder with significant wear just off the intake duct from the carburetor side and the piston side:


I am asking for help in diagnosing the cause of the damage.
Was it overheating the saw, the wrong mix, poor quality replacements, or maybe a dry chain?

Below photos of the interior of the old cylinder:

Do you think it makes sense to buy a new AIP piston with rings and install in an old cylinder? Will it be more money thrown away?
How do you assess the condition of the old piston (visible lumps are remnants of wipes)?

Thank you in advance for your help and time.

After damaging the piston, I would be afraid to buy and install a new one for this cylinder.
It is best to buy the original and have peace of mind once that it will run without fail and for years and two certainly differences in power are felt but the known price is very high for such a set.
In my opinion, the fault lies with the replacement side, it overheats and swells.

Idle is not very good cooling. You had to chase her at revs but without weighing it down. Then it has good cooling and little heat production because it doesn't do the work.

@rycho T: Security could not be the cause of the failure because they are in their places. As for the piston brand, I met with such a name for the first time. I suppose it is a typical Chinese replacement with a nice sounding name.

@lisekey: Could you look at the pictures of the old cylinder below? I cleaned it roughly and it seems to be in better condition than the replacement. Does it make sense to buy a new piston and mount it to the old cylinder?


I was thinking about the original parts, but their price exceeded the value of the used saw.

Some time ago I did renovation in MS180 - cylinder, piston, sealants was over PLN 400: /
I see a few scratches but if they are not palpable I would try to buy the original piston and it will be a better solution than these "repair" kits for PLN 100-150

@kkknc: after my renovation, the saw burned out one tank at idle outside and then 2 tank with a light cut and works for 2 years very nicely so I doubt it's this wine.

If you as many engines as me and you have some certificates, we'll talk.

This is not wear, just something has clearly come up. Look carefully at the bearings for a piece of wreath.

No touches will help. With such a defect, either the protection was broken where the ring was too worn or the bearing went off.

Lisekey colleague broke off from the Christmas tree.

@kkknc: the question of what your certificates have to describe the situation that I presented with the saw theoretically the same as the topic on substitutes.

@ kamilo23: Who I write here about the cuts, the author of the topic wrote that the protections are in place, the rings are new, as for the bearings the author of the topic should check it before renovation.

Where I fell off the Christmas tree, if I can know, I wrote that if the old cylinder, probably the original one that has small scratches for further checking (if you haven't read) then why you can't try replacing the piston with the original one.

Why do I think that the fault of the replacement because it probably overheated and swollen, or maybe because the piston shows perfectly that after swelling begins to collect the lower part of the intake channel and you can see how it gave up nicely on the part of the channel.

There are a few scratches and they are large. Something fell into the cylinder. You got the advice see still muffler. Is something in it?

Hello friends,
I am not a specialist in this field, but I agree with the colleague's lisekey statement, because the trace on the piston is strangely coincident with the shape of the channels in the cylinder.
And as for measuring skills with the number of certificates and spinning engines, then (after my last year's adventures with one of Stihl's Elblag services) I think that this is not a determinant (this is obviously not a touch to a colleague kkknc or undermining his competence).

Best regards,
Rustic

These are not traces of seizure, obviously a foreign body in the cylinder. If it faded, the tracks would be everywhere, but not where the piston is coolest, i.e. from the intake channel. Something got into the crankcase and made "order."

Robokop and kkknc colleagues,
In that case, can you explain why the features are arranged regularly? If something fell inside, should they not be arranged "stochastically" and not regularly? I ask, not out of spite but from pure curiosity and the desire to broaden horizons.
Best regards,
Rustic

It falls and scratches after the piston stroke. Fall out and do the next. The piston works up and down. Does not turn sideways.

The first cylinder for scrap metal - clearly a foreign body. (Who does two-stroke at first glance knows what it looks like)
The second cylinder also shows that something has hooked on the flushing channels (side ones).

Another thing, why did you put sheet-metal bearings on a shaft?
These bearings at the moment do not have proper lubrication and cooling as well sealants.
Open bearings with C3 clearance must be fitted to the engine.

Remove the shaft from the lower bowl, open the bearings, show more photos.
Look at the connecting rod foot bearing and needle bearing in the crank head.
What does the piston crown look like?

Hello again
I made a second attempt. I ordered the piston with AIP rings.
I looked at all the elements:
- Bearings without mechanical damage, diameter as per original,
- Pin safeguards were in place,
- Needles from the bearing on the connecting rod (where the bolt is installed) all complete,
- Piston height and diameter match the old piston.

In addition, I took apart the entire carburetor, all the elements in place - I compared with the diagrams.

I cleaned all the elements, thoroughly checked them for mechanical damage. I decided to put a new piston into the old cylinder, which I think was in a much better condition than the replacement. I checked carefully for burrs inside. Piston fitted with an exhaust arrow.

After folding, I started the saw. She worked correctly at low speed. Increasing ringing increased disturbing ringing. After several attempts, the saw went out. I tried to fire it again, but unfortunately the crowd stopped.
After dismantling the engine, my hands fell down. Pictures below:


On the photo of the cylinder I marked the area in which there were obvious scratches / burrs - they were not there before.

@kkknc - I also cleaned the muffler - no loose deposits / trash

@ tomason2 - I can only see the bearing in the crank head - it seems to be ok, in the crank foot it is not visible without breaking the shaft, which for my knowledge and skills is not possible.
The bearings that were fitted were complete with a cylinder and piston.

After the piston ruptures, no other components (except the piston and burrs in the cylinder) carry mechanical damage.

On two different pistons and cylinders are the same symptoms, is the shaft or connecting rod not the cause?
It seems that further repair is senseless, however, I would like to discover the source of the problem.

By the way, there was a problem with the oil supply line to the oil pump - the connection was just next to the oil pump. Any pressing or correction of the tube had no effect. I installed O-ring fi 8 (inner diameter) on the part pressed into the housing - the problem stopped, the pump nicely feeds oil into the chain lubrication channel. Zero leakage on connection.

After all, you lack the "meat on the piston" what has cracked fell under the piston and drew it further. Perhaps a piston casting defect.

Hello,
I gave up repairing the saw. Now I know that there was no point in repairing spare parts - a waste of time and money. I'm closing the topic.