Stihl 076 rebuild......

[Lakeside53]

That has seen better days... I'm going to assume the cylinder wall is NOT pitted and the pics are just exagerating the dirt in the mix, but...

I see rings clogged with carbon, a piston that is scoured by dirt rather than run lean, and what is all that junk in the mix oil?

I'd be tearing it down, cleaning it out and measuring the piston. The ring grooves are likely backed up with carbon, so it's only a matter of time before you get a stuck ring.


Your compression test is likely distorted by the oil in the bore.

 

[Lakeside53]

OK, I think in that case it is a bin job. I checked the guide rails and in some spots the gap is 2.1mm and it should be 1.65mm so that I believe is too much slop. Oh well, might be able to use the worn bar to make some hand-plane blades.

I am now converting the saw to 3/8" pitch and .063" gauge and ordering an new 36" solid nose bar.

Get a roller tip bar... Duromatic is from the past... and unless you are digging dirt with it, the roller is far better.

 

[martrix]

Those pics I took straight after I bought the saw and before I cleaned it.

The air filter was completely clogged with oily dirt and shavings. All squeaky clean now and had it running after a few other fixes.

I just took some new pics now, and it much cleaner. There is no pitting on the cylinder wall, that was crud. Doesn't look to be too much carbon in the rings.

What do you think now?

The brown discoloration on the piston skirt is just a thin film of oil on it. Its very clean

The gritty look on the piston around the rings is just pixelation from my cheapy digital camera. It is clean and smooth.

Yeah, good tip about the bar. I think I will go for a sprocket nose.

 

[SmokinDodge]

I'd be tearing it down, cleaning it out and measuring the piston.

Lake what is typical measurement of the piston vs. the bore? Obviously you can't have a 2" bore fitted with a 2" piston. I know on some things (cars) the clearance is so many thousands per inch of bore, just wondering if it's the same on saws? Or is there a top secret wear chart?

 

[Lakeside53]

I see 3-5 thou less than bore on a typical new 50mm piston. It is hard to measure accurately though...

 

[SmokinDodge]

I see 3-5 thou less than bore on a typical new 50mm piston. It is hard to measure accurately though...

Thanks for the info. How far worn is too far? Eight - ten thou?

 

[Lakeside53]

Yep,, or when it sounds bad (slaps) and spits back bad...

 

[BobL]

Thanks for the discussion guys - I'm learning heaps.
Cheers

 

[cbfarmall]

Ummm, I may be stupid but that looks like a broken piston skirt through the intake port. I don't see that it was mentioned anywhere in the post.

Chris B.

 

[martrix]

Ummm, I may be stupid but that looks like a broken piston skirt through the intake port. I don't see that it was mentioned anywhere in the post.

Chris B.

You know what Chris, that thought has crossed my mind more than once while looking at the piston. It is hard to tell for sure, but I was thinking it was maybe just a rough edge on the skirt.

.........crap, I'm gonna pull the piston tomorrow and see whats going on.

 

[GASoline71]

If you "shake" the saw a little do you hear a piece of metal clankin' around in the crankcase?

Gary

 

[martrix]

If you "shake" the saw a little do you hear a piece of metal clankin' around in the crankcase?

Gary

Hey Gary, no I cant hear anything clanking around in the crankcase apart from the oil pick-up.

If a part of the skirt has broken off, would that affect the performance of the saw?

I have had it running (no chain) and it seems to be running strong.

 

[GASoline71]

If it is a big enough hunk it could be catastophic if it wedges itself into the crank and rod. Not to worry you... but if there is a hunk of piston skirt in the crankcase, it would be best to remove it.

Gary

 

[martrix]

yeah, understand. Best I pull it out tomorrow and investigate. If there is a broken piece, it might be sitting in there just waiting to pop out.

I will post my results tomorrow some time......gotta go to bed, its 1:15 AM here in Melbourne.

Just found this page on Piston Failure Diagnosis.

 

[Lakeside53]

If it's broken, it will definitely affect the performance of the saw - the intake timing will be really messed up. Also, it's usually indicative of very bad piston skirt wear...

tear it down...

 

[martrix]

OK, got the jug off and from my first thoughts are that things arent too flash.

Turns out I may very well have an 051 and not an 076. The badge on the airfilter says 076AV Super, yet the piston measures 52mm? Maybe someone changed the piston and cylinder over?

I have lots of pics, but first off, have a quick look at this Video of the crankshaft needle bearing. I knew nothing about chainsaws or 2 stroke motors a few weeks back, so I'm still learning.

How much play should there be with the crankshaft and the needle cage?

 

[martrix]

Here is a pic of the serial number on the crankcase above the oil filler cap.
Anyone know what this identifies?

Piston Skirt. Definitely broken off. Couldn't find any metal shavings or the piece floating around in the crank case. Must have been removed in a previous repair.

Pic of the cylinder gasket with a sizable piece missing. I guess that wouldn't have helped with the performance of the saw?

Pic of the gouging left by the broken piston skirt.

The piston measures 52mm in diameter, so I am starting to think that this is actually an 051 AV and not an 076 AV like the badge says on the air filter. Maybe they got re-badged for the Australian market?

 

[Tzed250]

hmmm...time for a total rebuild...imho

 

[martrix]

Here are some more pics to help drive those remaining nails into my coffin.


These pics of the cylinder show the chrome has worn off between the intake/out-take.

These pics probably help confirm that it is actually an 051 P&C. From what I have read here, the decompression valve is to the right of the spark plug on a 076, and on the 051 it is almost directly above the spark plug like mine.

 

[SmokinDodge]

Damn man when it rains it pours. Your not alone though, my 064 was in about the same condition of everything you have posted. Looking at the carnage you really wonder how it even ran................................