Pondering the fate of my Stihl 025

[H 2 H]

This MS 250 .325 18" b/c

I got this straight gassed saw

Just about every part in this saw is after market parts when I rebuilt it just to see how it would hold up

I couldn't tell ya how many tanks of fuel have been put thru it; it is still running strong

 

[torch]

I agree that quality aftermarket parts can be just as serviceable as OEM parts. From what I read in that other thread, Golf pistons are not very high quality though. In that same thread, VEC pistons were found to be a better casting, but had some dimensional issues causing excessive (0.200") clearance to the con rod.

At the moment, I seem to have 3 options: clean it up and fit new rings to the stock piston, wait for the Greek guy to ship an Episan, or fit a 42.5mm Meteor piston kit.

Is the stock barrel plated or coated? The difference between 42mm and 42.5mm is only about 0.020". I wonder if I can rebore the existing barrel oversize?

 

[7sleeper]

I agree that quality aftermarket parts can be just as serviceable as OEM parts. From what I read in that other thread, Golf pistons are not very high quality though. In that same thread, VEC pistons were found to be a better casting, but had some dimensional issues causing excessive (0.200") clearance to the con rod.

At the moment, I seem to have 3 options: clean it up and fit new rings to the stock piston, wait for the Greek guy to ship an Episan, or fit a 42.5mm Meteor piston kit.

Is the stock barrel plated or coated? The difference between 42mm and 42.5mm is only about 0.020". I wonder if I can rebore the existing barrel oversize?

Not a good idea! The cylinder is nikasil plated. A replate will cost alot. I think someone mentioned ~200$. As I said previously you might only need a new set of rings. And then you have time to search for a new piston.

But as I said previously I would have the carb cleaned in a US cleaner and a complete analysis of the saw parts first. It doesn't sound like your saw has been through a lot that the piston would need replacing. And problems with restarting a saw hot is usually the ignition the culprit(at least in my limited knowledge).

7

 

[torch]

I had to set this aside for a bit due to a death in the family, but got back to it this afternoon. On disassembly, the piston is scored below the bottom ring on the exhaust port side. Fine scratches, the deepest of which is approximately 0.0025" (as best as I can measure with an optical comparator). There is one scratch between the piston rings, and none above. The piston is about 0.0025" out of round, the wider side being in line with the wrist pin.

The cylinder wall is scored below the exhaust port. I have no ready way of measuring the depth of the scratches, but two of them can be lightly felt with a finger tip so I estimate them to be on the order of 0.001" to 0.002" deep. Marks are visible above the exhaust port, but cannot be felt. The cylinder measures round and straight +/- 0.00025" above the exhaust port. It looks like some dirt or carbon or something got between the cylinder and the piston skirt while the saw was running.

With the rings removed, I can slip a 0.003" feeler gauge between the piston and the cylinder at the exhaust port and a 0.0025" feeler gauge on the sides and a 0.002" on at the intake port. If I rotate the piston 180°, so the intake side of the piston is on the exhaust side of the cylinder, then the measurements follow the piston -- the 0.003" fits on the cylinder's intake side instead, etc.

The rings were visibly worn even before my brain-dead son fiddled with them and snapped one.

So I think I have found the cause of the low compression. Now, what to do about it. How much out-of-round and how much clearance is acceptable with these tiny pistons?

 

[7sleeper]

How about just trying out a new piston ring and going on from there.

7

 

[H 2 H]

This MS 250 .325 18" b/c

This saw has Meteor piston kit in it

I've been only using Meteor piston kit the last dozen rebuilds I've had no problems with them

 

[torch]

Yes, all reviews of Meteor are good. However, Meteor apparently does not make one for this saw (42mm), only the later version (42.5mm).

 

[Stihlofadeal64]

you can search for a used OEM piston and cylinder. That may be the ticket for you. If I bought a used piston/cylinder I'd buy the Caber rings (same ones used on the Meteor). If you replace both piston and cylinder you can upgrade to the 42.5mm set up with no problem on your 025.

 

[torch]

A number of retailers are advertising 42mm Caber rings for the 025 -- but claiming the same rings will fit a number of other saws that Caber indicate use 1.5mm thick rings (eg: 024). I contacted several of them and confirmed the rings they are selling are indeed 1.5mm thick so they won't fit the 025.

However, Northwood Saw did confirm that they have 42mm x 1.2mm rings, which should fit the 025. Caber's website shows they do make rings in that size -- listed for the Stihl FS450 brush cutter. The greek guy shows those rings as fitting both the FS450 and the 025 too.

So I ordered the rings (and some other odds and ends like the coil, new filter, hoses, seals) from Northwood. I'll let you all know if they fit properly in the end.

I'd still like to know what the maximum piston clearance should be. I'm thinking of chucking the piston up in the lathe and knurling the skirt, to avoid piston slap in the future. Does anyone have a spec on the piston clearance?

 

[torch]

Update:

I ordered the parts from Northwood last Thursday. They arrived yesterday, which is pretty good time considering the border crossing.

The rings are the wrong size. Oh they're 42mm diameter all right. They fit in the bore with an 0.008" end gap, which is probably about perfect (the other rings -- at least, the one my son didn't break -- are worn to a 0.028" end gap). However, the new rings are 1.5mm thick instead of 1.2mm thick. I have sent an e-mail off to Northwood outlining the problem. I will update this thread when they respond.

View attachment 294653

 

[torch]

Northwood replied within an hour:

"Sorry about that our mistake, we will send the correct set out tomorrow."

Sometimes $#!+ happens. Nice to see a company that will make it right without a hassle.

 

[torch]

Updade:

The correct rings arrived today. Compression is good, saw runs like a champ.

However...

I'm not sure if I'm missing a part. At 0:28 of this video: Repair of Stihl MS250 Chainsaw PARTIAL ENGINE REBUILD - Part 4 of 4 - YouTube he shows a washer that I don't have. Now, he's rebuilding an MS250 and I've noticed at least one other change from my 025 (fuel tank vent pipe is different) so I'm not sure if

a) the 025 never had that part in the first place,
b) the Stihl dealer forgot to install it when he changed the clutch years ago, or
c) I lost the damn thing when I tore the saw apart.

I'm inclined to doubt c) as I carefully sorted all the parts into magnetic trays as I stripped things down. Everything else is there. There doesn't seem to be any play between the clutch and the circlip. The oiler works well. The chain brake works properly. Can anyone verify if that part was actually part of the 025? Alternatively, can anyone point me to a parts diagram for the 025?

 

[Stihlofadeal64]

The correct rings arrived today. Compression is good, saw runs like a champ.

However...

I'm not sure if I'm missing a part. At 0:28 of this video: Repair of Stihl MS250 Chainsaw PARTIAL ENGINE REBUILD - Part 4 of 4 - YouTube he shows a washer that I don't have. Now, he's rebuilding an MS250 and I've noticed at least one other change from my 025 (fuel tank vent pipe is different) so I'm not sure if

a) the 025 never had that part in the first place,
b) the Stihl dealer forgot to install it when he changed the clutch years ago, or
c) I lost the damn thing when I tore the saw apart.

I'm inclined to doubt c) as I carefully sorted all the parts into magnetic trays as I stripped things down. Everything else is there. There doesn't seem to be any play between the clutch and the circlip. The oiler works well. The chain brake works properly. Can anyone verify if that part was actually part of the 025? Alternatively, can anyone point me to a parts diagram for the 025?

The clutch washer you are speaking of is not included in the Stihl 025 (unless someone upgraded the clutch to the newer version). The washer was included with the newer MS250 clutch to help protect the case and oiler in the event the clutch came apart.

The tank vent assembly on the two saws are entirely different as well. The 025 has a clear translucent line with two small grub screws. One of the screws should be inserted on the tank side of the nipple and the other on the top -- just enough line at the bottom for it to cover the tank vent nipple.

(The MS250 had a black tank vent that extends all the way over the top of the carb and is anchored in the carb adjust boot. Personally I don't like them, but that's another story). Sounds like you're in business with your saw. Congratulations! Happy Sawing!

 

[torch]

Sounds like you are intimately familiar with the differences between the two. You are absolutely correct about the tank vent. The dealer upgraded the sprocket from a nylon or delrin type material to a steel one. I don't think that necessetated changing the clutch itself, but I don't recall the details now. But your info seems to mesh with what I'm seeing so my mind is now at ease. Thanks.