361 Piston question

[billy110]

Hi All!

 

I have taken on a nice project (yes, I'm a DIY user(a very keen one!)) as I have been given a non running Stihl 361.

 

I have partially stripped it down to find the piston and cylinder are both scored and the piston rings have disintigrated. I haven't stripped the bottom end down yet but parts of the piston rings are wedged in amongst the crankshaft and bearings.

 

What is common place here? Obviously the piston and head will be replaced, but is there anything else that would be "routinely" replaced following this kind of breakdown?

 

Also, what is the opinion of using Stihl parts against cheaper alternatives? As stated above, I am a DIYer pottering around in 3 acres and am not relying on my chainsaw as a source of income.

 

Any thoughts or advice gratefully received.

 

Billy

[spudulike]

Hi All!

 

I have taken on a nice project (yes, I'm a DIY user(a very keen one!)) as I have been given a non running Stihl 361.

 

I have partially stripped it down to find the piston and cylinder are both scored and the piston rings have disintigrated. I haven't stripped the bottom end down yet but parts of the piston rings are wedged in amongst the crankshaft and bearings.

 

What is common place here? Obviously the piston and head will be replaced, but is there anything else that would be "routinely" replaced following this kind of breakdown?

 

Also, what is the opinion of using Stihl parts against cheaper alternatives? As stated above, I am a DIYer pottering around in 3 acres and am not relying on my chainsaw as a source of income.

 

Any thoughts or advice gratefully received.

 

Billy

 

My own personal route is to try to salvage the cylinder, I would even use it is slightly scored rather than take my chances with aftermarket. I always try to use a quality piston and favour Meteor if available - they are very close to OEM grade.

 

Failing this, the Chinese kits are good value but are not manufactured as well as OEM, it helps if you know what you are doing and do some simple checks during re-assembly.

 

One word of advice is to find out why the machine failed in the first place, I have seen one or two that have died for no apparent reason but there is usually a reason and this should be found if possible - vac and pressure checks are common and useful tests.

 

Problems I have had on Chinese kit are pistons with direction arrows pointing in the wrong direction, lack of compression, pitting in the chrome/nikasil plating, piston rings breaking, poor ring end to pin fit and poor piston to cylinder fit.

 

Good luck

[agg221]

Several thoughts on this one, based on several saws including an 044 which I rebuilt a year or so ago with a similar problem.

 

Is it possible to tell what instigated the failure? This is key - otherwise you may well find out by ruining a set of replacement parts. It sounds likely that the first thing to fail was the rings, which can happen, but it's good to know why. One thing to check - is the piston a Stihl part or a pattern part? Also, was the piston installed the right way round? If not then it's possible that it failed the first time for a different reason, new piston installed incorrectly and failed immediately so given up as a bad job. The point is, the history before the present failure is important.

 

Secondly, you are in for a long and tedious process removing all the bits of ring from the crank case. Can you turn it right over? If so, it's a good sign that nothing too severe happened. You need to flush out with a little bit of petrol (doesn't have to be 2-stroke) and then wipe kitchen roll round through the crank case, by using the crankshaft to drag it round behind the web. Keep doing this until it runs clean, as all the oily residue will otherwise be trapping little bits of metal which will gradually work free. Once it's clean, you can swill it through and let the residue collect at the bottom of the case and wipe out the remains with a long cotton bud until there are no more shiny little flakes. This took me two evenings.

 

The alternative would be to split the crank case, which is probably quicker but something I personally avoid doing as it requires dismantling bits I don't like dismantling.

 

Once you're confident that the whole thing is clean, you can finally check for play and rough spots. Test the big end bearing for play by pulling the con-rod up and down (not wobbling it, it's supposed to). You need to do this at every position for half a rotation while holding the flywheel to stop it rotating - it shouldn't move up and down. Test for rough spots by very gently rotating the crankshaft using the con-rod and feeling for any sticking, grating or lack of smoothness. Assuming all is OK then you've got away with it.

 

You could replace the bearing end cap seals for good measure, or you could leave them alone.

 

Once you're confident that everything else is OK, and reasonably happy as to why it failed in the first place, you can rebuild it. Personally, I don't have a problem with pattern parts if you can get appropriate ones. Gardenhirespares or Mr Solutions both seem to offer reasonable quality, can't speak for anyone else. Given the use you're putting it to you will probably never wear them out. I have run an 044 hard for milling for a year with no problems on a pattern pot and piston, although it did blow the muffler gasket a week or two back which was a bit annoying.

 

Once it's rebuilt and you've got it started and running, but haven't revved it up hard, it would be a very good idea to get a pressure check done on it. This is not a compression check - it's fairly predictable that this will be OK as it's a new pot and piston. The pressure check establishes that there are no air leaks to weaken the mix, which can cause seizure. Given the failure you know the saw has had it's worth checking that this isn't an underlying problem. Also worth setting the carb back to factory settings, or getting it tuned properly. Factory settings are 'safe' and fine if you're pottering about. Tuning will give you more power without risking damage.

 

Hope this helps.

 

Alec

[billy110]

Many thanks Spud and Alec, I'll let you know how I get on.

 

Billy

[JAMIE1]

There is a stihl 361 piston and cylinder on ebay.