STIHL 034 AV rebuild... How far do I go?

[MiloFrance]

Hey folks it's been an awfully long time since I was here. Mostly because my work changed and I was no longer waving chainsaws around on an almost daily basis. I now have a full-time job at a very beautiful Chateau in the south of France, but anyway on with the question:
Way back in the you suck thread of 2021 I got hold of a couple of saws at a good price. As the title says I am now looking at the 034.
It's issue was that although it would run fine and idle it lacked power, so looking a bit further the piston and cylinder were quite badly scored.
I've now got the service kit for it but was under the impression that on this model you couldn't change just the piston and cylinder you have to split the crankcase to get even that done.
It now looks to me like I could just change the piston and cylinder and give it a good clean... See photos.
The question is in your much more experienced opinions than mine, is the low power purely going to be due to the scoring? I get the feeling that the seals and bearings are okay although on the big end there is a very very small amount of wiggle. So is it actually worth splitting the crankcase on a saw this old that in all probability will just need the piston and cylinder changing or sell I just do the minimum to get it clean and probably running well again? I've never done a crankshaft and bearings on anything hence my hesitancy and quite a lot of reluctance!

 

[Gord404]

something took that piston out and will do it again- bad bearings and seals- boot???

 

[MacAttack]

Dumb question...is that a plastic crankcase?

I just went through a rebuild on a 390...OEM Stihl parts are expensive. Chinese made parts are cheap. I guess it all depends on how much you want to invest in it.

 

[ZeroJunk]

I would have nuked it with a pressure washer before I took the cylinder off.

 

[ZeroJunk]

Dumb question...is that a plastic crankcase?

Magnesium most likely.

 

[Mad Professor]

I would have nuked it with a pressure washer before I took the cylinder off.

Yup, needed a good bath and a pres/vac test.

Dumb question...is that a plastic crankcase?

Magnesium, it's not a clamshell like the 390.

OP, I would totally clean and inspect it before buying parts.

Hopefully crank/bearings are OK. Get a Stihl gasket/seal kit and determine if the OEM cylinder can be reused.

Get a piston kit if so, not a Chinese one. You can use OEM 036/360 parts if You need to replace cylinder.

Replace and/or inspect all the rubber parts/hoses/filters/AV buffers, and put a carb kit in it. Check the clutch springs/carrier. Maybe put the clutch upgrade kit in it that comes with a new rim and bearing. Dress the bar and get a chain.

 

[MacAttack]

If that's all magnesium housing and crankcase that actually sounds like better construction than the clamshell and plastic housing on my 390. I feel like that would definitely be worth fixing.

 

[MiloFrance]

Thank you for all the replies folks...
Having seen through the exhaust port that there was scoring on the piston I knew I was in four a new piston and cylinder, so I bought a kit from a reputable company here in Europe. I've used them before to change piston and cylinder on a 260c.
The kit comes with plenty of other spares so apart from the big end bearing I think I have what I need. Looking at the pictures rather than bad fuel could it be a worn bearing giving rise to piston slap (I think that's the correct term)?
And yes I'm very much regretting not getting a jet wash on it knowing it was coming apart anyway. The inside of the cover on the clutch side was a quarter inch thick of congealed chain lube.

 

[MiloFrance]

If that's all magnesium housing and crankcase that actually sounds like better construction than the clamshell and plastic housing on my 390. I feel like that would definitely be worth fixing.

Way back when I got this along with an 039, I realized the 039 was almost spotless. However a friend in need got the 039 in exchange for something else as I knew that the 034 was more pro quality with magnesium crankcase halves. All the favours he's done me in the past he deserves something good and will probably use it twice a year. I've just given myself a bit of extra work to have a chainsaw for life now I'm not regularly chopping things up.
Incidentally it's a 1987!

 

[Mad Professor]

Good chance that OEM cylinder would clean up.

Piston is well worn on intake side, but ex side still has machining marks. The place where it scored usually indicates it got too hot. Either from an air leak or the gunk caked on the cylinder.

 

[Bob Hedgecutter]

How far do you go?
Well that depends entirely on your wants and needs for the saw- if you need a dependable 60ish CC saw that will supply your firewood needs for the next decade or so- go full hog on it.
If you simply want to experiment and see if you can make it run but never really use it- put a Chinese top end on it and keep your fingers crossed.

Given the state of it and the evidence on the piston- its been hot- to the point it near locked up with the piston smearing just to the righthand side of the exhaust port.
That could be a combination of all the filth, dull chain and being pushed hard- or it might well have other issues.

If it were mine (as mentioned above) most all the built up filth would have come off before any tear down of parts and a pressure vacuum test done. I have (and you can buy a cheap aftermarket version of) Stihl case splitting tools- it is not that daunting of a task- if you can do pistons- you can do bearings.
Gasket sets for Stihls (OEM) come with main oil seals and are usually cheaper than buying genuine seals alone. Genuine bearings from Stihl are not cheap- but worth it to have peace of mind in a saw you expect to last longer than one season.
So, me, if I wanted it for myself and need to depend on it for home heating over the Winter- case split and up total rebuild. But, its not mine- its yours and so you have to decide is the investment worth the cost.

 

[Gord404]

that might be a score or a heat mark- iwould give it a try cleaning up the cylinder if its not a deep score