Stihl 661 Rebuild completed

[Derrick Sawyer]

Starting my 661 rebuild project, pretty excited since its a cool saw and never ran a 661, plenty of experience on 066/660s though:

History: Got saw for parts, found a leaking decomp valve, leaking clutch side crank seal, one intake boot o-ring loose and partially blocking intake, and the biggest problem: main bearing on actual crank (big end) was roasted. Looks like a needle bearing was sent up the transfer and made dent in the top of the piston and cylinder, somehow the sides were not scored.

Plans: Got new parts (crank, crank bearings, gasket set, updated intake boot)with a good deal thanks to Johnsobri. I smoothed down the piston and will use it to sand down the raised indents in the cylinder roof and see if I can get this thing running, as I've never seen it running still worried how it might run, but gonna roll the dice on this one.

Pics: See below on the piston and cylinder i'll reuse, case and box of other parts:

IF anyone has any tips on 661 rebuild they learned, please let me know. I plan to test first without any mods or porting, unless the squish is so big I need to shave down the cylinder or delete base gasket. I did try and 066 crank but the counterweights were too big to fit, the 661 crank looks like a larger 460 crank, with the snap ring and longer section of larger diameter on the clutch side, though I thought I could use a 660 oiler worm gear metal sleave to modify the 660 crank to fit but it was a no go.

 

[fin460]

Good luck with your rebuild!

 

[gunnusmc03]

The rare elusive 661 with actual run time

 

[big t double]

do you have any information on how this saw was being used before you got it?

 

[pro94lt]

The rare elusive 661 with actual run time

Lol you beat me to it...

 

[Derrick Sawyer]

I have no idea on its previous life and no black box record but its not beatup too bad so doesn't look like heavy professional use, but other than an issue here or there i've only heard good things about the 661, i do like the fact that the case opens without the dowels of the 066, there are two "integrated sleeves" around two of the bolt holes that stick out about 1/4" to help line up....the cylinder is monstrous in size for sure, as is the clutch side crank bearing. I'm most worried about how the mtronic and coil will work when put back together after sitting for i think 6 months.

 

[Johnsobri]

Looking forward to the rebuild. Glad you got the parts and are happy!

 

[pro94lt]

You don't happen to know the build date do you?

 

[LRains]

Once you rebuild it reset the M-Tronics and break it in, they are a monster when in big wood. Tons of torque, do the muffler mod and let it rip. Good luck with the rebuild.

 

[Derrick Sawyer]

Actual build date was 10/2013 but put in service in 2015.

So far got the crankshaft installed and put in seals and cylinder (no piston in yet) and pressure/vac tested fine. Don't have any special tools, placed crank in freezer and warmed up case bearings with wifes hair dryer, then one side instalsl fine and quickly place case gasket and other side gets close, then finish with the bolts...didn't need longer versions (like the long bolt for flywheel cover that holds the chainbrake in). Used long
sockets just slight bigger than the seals for clutch side but it seemed to hit a stop before I could drive it quite flush, . the flywheel side is embedded so I marked a deepsocket in advance and used gorilla tape wrapped a couple times at that line to try and prevent overdriving it.

Pressure and Vac with an old setup I used when working on cars, with mityvac from brake bleed kit, and for pressure a bulb for blood pressure checking. I have an adaptor for the exhaust side and just gorilla tape the intake hole to hold. I'll check vac/pressure again when I have the saw almost completely assembled again with the intake boot to be sure.

 

[Derrick Sawyer]

Finished up the rebuild and went outside to test. I placed in start position and pulled rope till I got a burp, but I wasn't able to start by continued pulling. I pressed the throttle to put in run position and it still didn't start. I was able to get the saw to start by holding in the throttle when pulling the rope, and it ran and seemed to cut ok but if I released the throttle, saw would just die. It again wouldn't start unless i held the throttle in. Will have to dive in and check everything out with carb and redo vac/pressure check, but this is what i was worried about having mtronic tougher for me figure out what the carb setting is.

 

[Derrick Sawyer]

Did the carb reset today without bar and chain and keeping in start (choke) position for a couple minutes, started real rich but then settled down. Did same but in run position, still have to do some test cuts but at least idles fine now, sound real good too, very loud for sure. Definitely the coolest rebuild I've ever done, can't imagine much cooler of a saw to take apart, really like some of the design features of wiring routing, interesting AV mount and handlebar mount designs, plus cylinder bolts on with regular torx tool-no need for skinnier T-handle, chain guide plate screw also torx instead of flathead screwdriver, air filter and box design is nice too. Only things I found to kind of be a pain is installing the top cover and the cylinder due to its size, harder than on a 660. Since I smoothed out the cylinder roof and the top of the piston with sandpaper, the squish is at .038". It would be perfect if I remove the base gasket (~.018) to make it a final of 0.020", but putting on sealant on this one would take some touch, definitely would want to remove handle for that job. Maybe after a couple weeks of use I'll think about it...I don't have the tools to shave down the cylinder and keep the gasket.