Stihl 066 RUD during milling

[tom_]

Hi there, I'm an amateur woodworker and have recently gotten into milling and drying my own lumber using a Stihl 066 (and lo pro milling chain and bar). Halfway through a nice bit of oak, the saw cut out and the pull cord wouldn't move to re-start. Turns out we'd had a RUD (Rapid Unintentional Disassembly - to borrow the term) of the clutch - one of the arms had sheared and the clutch must have fallen apart whilst the saw was running.

Took it to a Stihl dealer/repair shop for a look and they quoted for a new cylinder and piston (looks like some scoring from looking through the exhaust port once I've taken the muffler off), clutch, drum, and a couple of other bits to make this all work (needle bearing, washer etc which all seemed to go missing when the clutch fell apart). I'd rather do the work myself so would appreciate your thoughts on the below.

I'm new to this and trying to learn from here / Youtube. Would value your advice and input on:

What else should be replaced when changing the cylinder and piston? - I have a new cylinder/piston kit from Meteor which seems to be a good choice from looking at other posts on here. I'm comfortable installing this and have new gaskets (cylinder and muffler side). Whilst it's off, are there any other parts which you'd recommend replacing? The saw is c. 20 years old so might be time to replace a few bits?

Do I need to worry about replacing anything else on the drive side? I'm conscious that the oil pump is just below the clutch so if there were parts of the clutch flying around in this region should I be testing the pump or anything else?

Here's a couple of photos that might help:

 

[J D]

Looks like that shaft has gotten helluva hot... I'd start by checking & measuring that.
Then, clean it thoroughly, vac/pressure test it (before dismantling & after reassembly), pull old cylinder & inspect (they usually clean up surprisingly well), check rod bearing for vertical play & main bearings are smooth, replace seals if they leaked or just do it anyway for piece of mind. Dry fit P/C & measure squish... adjust or delete gasket accordingly. Inspect & replace fuel line, fuel filter, air filter. Clean & kit carb as necessary.
Oil pump will likely be fine if there's no obvious damage, just ensure everything spins freely before you fire it up & check it's oiling before before you start cutting

 

[tom_]

Thanks JD, this is really helpful.

Just to check a couple of bits:

Any guidance on how to check and measure the shaft would be great. When you mention check the main bearings are smooth, I've checked for any play in the shaft and there isn't any. To check for smoothness, I presume I'd pull off the starter side and just give it a spin?

Which seals should I be checking? I'm presuming these are different to the gaskets for the cylinder?

I've had a search for what kit carb means but I'm afraid I'm still at a bit of a loss. Is this buy a kit to refurb the carb?

I've pulled the old cylinder and this is the scoring that's present (and on the piston):

One of the reasons for going with a new cylinder is a) I don't have any honing tools and b) there's more play with the new piston in the old cylinder and virtually zero play with the new cylinder.

I'll check the other bits you've mentioned.

Thanks again.

 

[cookies]

looks like only a little aluminum transfer, a little elbow grease and a correct new piston, new clutch and vac/pressure test and your back in business. If your going to mill your going to need to learn to adjust the carb to prevent damage under such heavy use.

 

[J D]

Any guidance on how to check and measure the shaft would be great.

Inspect it for gauling or unevenness... a decent set of vernier can be used to ensure the diameter is consistent along the shaft. The amount of variation that is acceptable is a bit subjective though as the clutch bearing only "works" when the saw is at idle

When you mention check the main bearings are smooth, I've checked for any play in the shaft and there isn't any. To check for smoothness, I presume I'd pull off the starter side and just give it a spin?

Yes, spin from side feeling for any catching or grittiness. Working the rod up & down by hand is another way to do it

Which seals should I be checking? I'm presuming these are different to the gaskets for the cylinder?

There are seals on each end of the crank shaft. These should hold both pressure & vacuum. A leak down test will check the saw is air tight & is critical to ensure there are no leaks that would result in a lean meltdown after a rebuild

I've had a search for what kit carb means but I'm afraid I'm still at a bit of a loss. Is this buy a kit to refurb the carb?

Yes, it refers to a kit of parts to refurbish the carb... I usually start by cleaning the carb & will buy a kit if any of the gaskets, diaphragms, etc need replacing

I've pulled the old cylinder and this is the scoring that's present (and on the piston):

That looks like aluminium transfer from the piston & should clean off just fine

One of the reasons for going with a new cylinder is a) I don't have any honing tools and b) there's more play with the new piston in the old cylinder and virtually zero play with the new cylinder.

These cylinders aren't honed, they are lined with incredibly tough Nicasil. They can generally be scrubbed clean of aluminium smearing with little or no detriment to the cylinder. So far as I'm aware, the 066 doesn't have the slight variations in cylinder sizes some other saws have so the inconsistency in size may be significant

 

[muddy42]

That cylinder doesn't look too bad at all. It should clean up fine with some gentle hand sanding. Plenty of videos online - you want to eliminate those vertical marks by sanding in a circular motion. If at all possible you should keep the original cylinder, it will be better made than any replacement. You want to get to a stage where the cylinder feels smooth to your fingers.

You don't show a picture of the piston, but given how little damage there is to the cylinder it could be salvageable with new rings? However normally pistons receive more scoring and skirt wear so its ok to replace it before the cylinder. Make sure a new piston and cylinder fits well with no knocking.

Often saw dealers are quick to diagnose scoring to sell new saws! Of itself that clutch failure shouldn't really cause cylinder and piston issues? Yes it looks like the clutch side got hot at sometime, but I have seem plenty of saws survive with way more heat damage to the plastic around the clutch.

You can see the crank seals with the clutch and flywheel removed, they are circular areas around the crank. But unless you have serious damage, the only way to check them properly is to pressure and vac test with a mityvac once the cylinder is reinstalled. You then block of all the holes to the cylinder except one and then apply vacuum and pressure to the cylinder (normally through the spark plug hole, but you can use the impulse line if you have one). Some leaks will tolerate pressure, but not vacuum, you need to check both. Rotate the crank slowly during the test. You can use soapy water to locate a pressure leak.

 

[tom_]

This is amazing info, thank you very very much @J D @cookies @muddy42!!

I'll look into all of this. Great to hear about the cylinder likely being okay to re-use once I've removed the aluminium transfer. I'll see if I can do a little more digging on the slight size variation re the new piston not quite fitting.

The piston did look a fair bit worse, I'll see if I can grab a photo which might be useful reference for anyone else who comes across this thread.

 

[ericm979]

The shop manual should have a spec for cylinder to piston clearance, and ones for acceptable cylinder and piston diameters. You need a bore gauge to measure the cylinder, either the snap type which you then measure with the same outside mic that you use for the piston, or the more expensive kind with a gauge on it.

 

[Bob Hedgecutter]

Couple of things that have not been mentioned for my money- it looks like the tip is busted off the activating arm of the oil pump worm drive. And there looks to be some damage to the brake band where the band is scarfed and rub wear from an unbalanced clutch drum on the band where is sits near the bar studs.
Myself, for a clutch side of the crank to be that blue- I would be highly suspect of the PTO side oil seal and be checking all the bearings on the crank really carefully before tidying up the top end or new top ending it.
Milling is hard on powerheads, constant deep heavy bucking can be hard on powerheads- anything involving constant heavy cutting is going to amplify any short comings really quickly.
To be a good milling rig, the powerhead has to be in tip top condition, tuning has to suit the workload, chain has to be near perfect sharp and angled more correctly on the cutter- and the operator has to be on the ball. Milling is a lot more demanding than falling or limbing and a lot less forgiving.

 

[HumBurner]

Milling is hard on powerheads, constant deep heavy bucking can be hard on powerheads- anything involving constant heavy cutting is going to amplify any short comings really quickly.
To be a good milling rig, the powerhead has to be in tip top condition, tuning has to suit the workload, chain has to be near perfect sharp and angled more correctly on the cutter- and the operator has to be on the ball. Milling is a lot more demanding than falling or limbing and a lot less forgiving.

Truth

 

[tom_]

Thanks for the additional thoughts!

I've taken the piston off and the rod has no vertical play but does have a fair bit of lateral (horizontal) play, I'm guessing this is normal?

Also awaiting a carb kit to arrive and looking at pressure/vac testers (though would prefer to avoid the cost of a Mityvac if possible!).

I'll check the oil pump is working, I *think* the wire that drives the worm gear is okay but we'll see when I start it up.

Will also definitely be checking out how to set the carb up for milling - thanks for the tip.

I'll let you know how I get on with the repairs, fingers crossed!

For future reference, here's a couple of images of the piston (I've removed the rings):

Exhaust side


Inlet side

 

[J D]

I've taken the piston off and the rod has no vertical play but does have a fair bit of lateral (horizontal) play, I'm guessing this is normal?

Yup

 

[muddy42]

That piston needs replacing. It looks scored on the exhaust side, that looks like a chip on the top ? plus there is skirt wear at the bottom which will cause it to rock. You can save the cylinder with some polishing, but buy a decent aftermarket piston from Meteor or golf etc. You can measure the diameter of the old one to double check they are the same.

If you want my advice just get the mityvac pressure and vac tester. I also tried to avoid the cost and bought a cheap tester from ebay - it leaked itself which was pretty frustrating! You need a reliable tool and the ability to test both pressure and vacuum.

 

[cookies]

she got a weeee bit hot, swole up and started rubbing the cylinder like a stripper that has seen a wad O cash. I recommend a OEM piston over any aftermarket considering your use, it will live longer making more power. Good luck...and may the odds be ever in your favor!

 

[ericm979]

The rod should be able to slide side to side a little but if you can rock the small end of the rod back and forth more than a bit the big end bearing is worn. That's called "side shake". Of course if there is any detectable up and down play the big end is shot.

I agree that piston's no longer useful for anything other than a paper weight. What are the markings on the exhaust side crown? Was that done during disassembly?

The failure could be from overheating or lack of oil or both. If you're 100% positive you put in enough good oil then it overheated. Ash or really burnt oil on the underside of the piston crown could also indicate overheating.

 

[jweier111]

That cylinder looks very similar to the one I pulled off my 046 project a couple of weeks ago. That definitely looks like it can be saved. Take a little extra time and check the intake, exhaust, and transfers for any sharp edges and knock them down. I would highly recommend a Meteor piston and ring set from Wolf Creek saw shop. As others have mentioned, go over the lower end with a fine tooth comb before you order parts. As long as you have it opened up, replace your crank seals (Bellhopper on youtube has a great video on a simplified process for replacing seals on the 046, and I'm sure it's probably applicable to the 066 as well). When you do get back to rebuilding the clutch side: Either go OEM on the clutch, or at a minimum get OEM springs. Otherwise, you risk ending up right back where you started with another broken clutch. And watch out for that f-ing "E" clip. Make damn sure it's seated. I reused the old one in reassembling the 046 and it didn't even run for 2 minutes before that clip and the washer went flying. Luckily we shut it down before anything more than the drum bearing got buggered. I checked it twice for fitment prior to cold start. But once we finally found it (after sweeping the whole garage) it looks to have bent during installation. Thankfully I had already ordered a replacement that arrived later that afternoon. As others mentioned it's worth replacing the worm gear for the oil pump while you are in there. I thought my wire looked fine too (it was oiling...) until I set it next to a new one . I have no idea what was driving the oil pump. Magic? Will to live? IDK.

 

[ericm979]

Always fit the E clip so the sharp edge side is to the outside. If its the other way its easier for it to be pushed out of the groove.

 

[tom_]

Hey,

Quick update and question if anyone would be so kind. I've bitten the bullet and got a Mityvac and have pressure/vac tested the crankcase. Results are:

Pressure - 8 to 7 psi over 6 mins. There were no bubbles I could see from soapy water on the seals on either end of the shaft.
Vacuum - 15 to 12 inHg over 6 mins (hopefully I'm remembering that unit correctly)

What do you think about those results?

===

To answer a couple of questions further up:

The marks on the piston top are from me scratching off some of the carbon.

And I'm sure that I used a 50:1 mix of gasil. The gas I used contains no ethanol. If anyone reads this far - would you recommend a lower ratio for high and long intensity applications like milling?

 

[ericm979]

I'd recommend a higher ratio of oil, which is lower numerically. For example 32:1. And really good synthetic oil. And tune the carb rich on the high speed adjustment. More fuel means cooler running.

Also keep the cooling fan air intake clear of chips.

 

[Maintenance supervisor]

This thread is the best argument for the clone MS660 chainsaws.