Why is everyone hung up on machining?

[CanadianCarGuy]

Just curious to finding out WHY is everyone stuck on machining everso thin based cylinder to bump up compression? I see some of the smarter guys are machining the combustion chambers, but why not just fill in and reshape the combustion chamber? Seems like everyone is trying to reinvent the wheel when you are already rolling. Guys machining the heck out of something and then tryin to figure out how to get port timing back to stock numbers. These cheaper disposable saws only take 190ish psi compression for a few months of steady use anyways (that is only from personal experience and for approx. 30ish other fallers). I know I have tried it all, yeah you can make power through the roof, but the silly cylinders fall apart after a month or two on a work saw. That last 390 muffler I had brazed (Thought my welder was too hot and making them crack premature) only last 25 work days exactly and completely fell apart. I think it is a common misconception out there that a good work saw can only be made with complex and tedious machining, but I feel that to not be completely right. Sure there are guys out there who can make a boatload of torque off idle and you can push on the saw hard as you want and the chain still spins, but isn't chain speed and rpm in a cut just as important? I am curious all if those guys who run a ported saw day in and day out what do you look for in a saw and how long are you getting out of your port saws, what sprockets and what chains do you run? If you are gonna say a 7 tooth is all you need, you never ran a real saw before, cause that thing will be pinging on the limiter every cut if it is over 80ccs at least that is my experience, unless your are running those silly spikes that take out the clutch side bottom end bearing every few months. Before I get stomped on by some super talented guy that should be a head engineer at NASA, just think about the point I am getting at. 30 years ago welding a big glob on an 076AV piston was the norm for bumping up compression to make more power. Now we have saws that set their own ignition timing. I am sure we put our heads together and come up with something that can compete with fancy machine work without the expense and hassle. I mean how hard can it be to bump up compression without altering port timing or affecting the saw's rpm for max torque? Also what is the point of unlimited coils if you are not pinging off your limiter, I always wondered that?

 

[mdavlee]

How long is a 390 with a compression bump lasting for you guys? I'm getting ready to port another one and I'm curious how long it will last. I'm going to be milling with it some with a 30" mill as well as bucking bigger logs that will be milled by the 088.

 

[CanadianCarGuy]

We are seeing between 3 and 6 months most of the time before something goes. Most of the time is the cylinder coming apart. Then either the piston or clutch side bottom end bearing. I like 180ish psi compression on my works saws, seems like a better balance to longevity and you can run more ignition timing as well (I do very little machining for myself). Seems like anything over 220ish psi compression don't go past 6 months. This is all with basic pop ups set 20 to 22 thou squish, I have no experience with the squish bands being machined, and I don't have much machining experience myself, but I got friends who prefer machining over timing, so I get it shipped out and done for em. I moved to 54mm 385 cylinders from the 55mm 390 cylinders and see another month or so in longevity. I started keeping track of how long things last on paper after loosing 30ish 390 cylinders premature over a period of about a year and a half. I started making guys run their saws for 3 months before bringing em to me, that weeds out of the poor castings a lot of the time (and it helps when you can see the wear marks from the piston skirt on the cylinder wall for porting) I see bout a week over three months to the day almost on my personal falling 390s before the cylinder pukes out. (No machining other than back of flywheel and clutch, base gasket removed, no changes in port timing with the exception of raising rear upper transfers, 60% cylinder bore exhaust port width, I keep one transfer port closed, one open, widen intake slightly match it exactly to the carb boot and then just polish, polish, polish, until you are using jeweller's rouge, so simple a monkey can do it) If you do a good job, the saw will be pinging on the limiter dawged in with a 7 tooth sprocket and hitting it occasionally with the 8 tooth. That with good chain, if you don't have that yet why are you running a ported saw?


This whole conversation come up at the bar last night with some buddies of mine, seems like the biggest hassle of machined work saws is you can't just bolt another cylinder or put another piston on in camp, you gotta get another made and flown in or you have to wait until you get the saw back to the guy who made it.

 

[mdavlee]

Ok good info Daniel. Are the cylinder bases cracking on those too?

Machining the squish let's you pop another piston in and go back to work. There's no cutting on the piston tat way. I need to get me another compression gauge and test my 046 and 088. The 390 I ported a while back but squish is still too high. I want to get it back down to .020-023". I was planning on machining the squish and base bit now I'm wondering. It's going to spend some time on a mill with long cuts so I don't want it to die premature.

 

[Jacob J.]

What I've been doing on several saws is having the squish band machined and leaving the base alone and using a different piston. This works well for me in 046 and MS-460 saws, using an 064 piston. I'm getting ready to do a couple Jonsered 920s this way with 066 pistons and a MS-440 using a 266 piston. I built a couple pretty good running big bore 372s that way, also using 064 pistons for members here.

 

[mdavlee]

What I've been doing on several saws is having the squish band machined and leaving the base alone and using a different piston. This works well for me in 046 and MS-460 saws, using an 064 piston. I'm getting ready to do a couple Jonsered 920s this way with 066 pistons and a MS-440 using a 266 piston. I built a couple pretty good running big bore 372s that way, also using 064 pistons for members here.

I never did put a 064 piston in a 46. I thought about and should have tried it on the one I have now.

 

[CanadianCarGuy]

I used to be a real fan of super tight squish bands, but keeping above 22 thou or so helps out a great deal with the cylinder surviving ring failure and bearing failures. The tighter the squish the greater chance of the piston shattering causing catastrophic failure. I just had a 390 on Friday lose power (My partner ran it for a day with his 7 tooth sprocket, me thinks he got a little rpm happy??) I had to put an 8 tooth on cause it quit pulling the 9 tooth and had to keep turning the idle screw up during the day. I got the day out of it and didn't really think anything of it, packed it out took it home to change the muffler (no pipes allowed) and found what you see in the picture. I honestly wasn't expecting that!!

The 288 will take a 660 piston with widening of where the con rod goes through at the wrist pin bearing. Makes a heck of saw. I've done a 920 with a 066 piston before as well, it works well too! I prefer keeping the stock pistons on the 72s cause you are limited with port width with the narrower skirt on the Stihl piston. I think some other builders use an older Jonsered piston with pop up already in it on the 50mm 372 cylinders.

 

[mdavlee]

Did it lose the big end bearing?

 

[CanadianCarGuy]

I got some more pics, here is a 372 pop up with lower con rod bearing failure. Just sanded the marks off the squish band area of the pop up with a dremel mini sanding drum. For the cylinder squish band marks I just use the larger dremel cut off wheel attachment on a CC specialties (foredom copy) grinder and take my time until the squish band is smooth again. On the 385 cylinder that the piston in the pics from the previous post come from I even left a chunk of ring stuck in the squish band, I don't think its going to come out it is in there good

Here is where I get most of the cylinder failures, either under the exhaust port or through the intake. Stock or ported they all seem to fail the same, just a matter of time. The cylinder is a 390 from three or four years ago. I am a lot cleaner with my porting now and not so aggressive any more.

 

[CanadianCarGuy]

Mdavlee:

No not as far as I can tell, just ring failure. I will find out soon enough I guess. I can usually tell from what kind of indentation is in the piston what failed which bearing, wrist pin clip, or whatever else. I recently had a screw from the choke butterfly come out and go through the intake and jam up between the crank and casing while trying to start my saw one morning. Also had a crankcase bolt come loose and jam up underneath the flywheel recently, talk about lucky both times. Coulda been way worse.

 

[CR888]

where are the builders.........

 

[rwoods]

Probably in the chainsaw forum. Ron

 

[mdavlee]

Mdavlee:

No not as far as I can tell, just ring failure. I will find out soon enough I guess. I can usually tell from what kind of indentation is in the piston what failed which bearing, wrist pin clip, or whatever else. I recently had a screw from the choke butterfly come out and go through the intake and jam up between the crank and casing while trying to start my saw one morning. Also had a crankcase bolt come loose and jam up underneath the flywheel recently, talk about lucky both times. Coulda been way worse.

Ok just wondered. So that's a good bit of run time on it. I replaced a crank in a 390 that was a month old a few years back. Took out the p/c when it did it. I had an extra cylinder so I put the money in it to make a runner again. Just curious what oil ratio you guys are running up there.

 

[one.man.band]

someone correct me if i'm wrong. don't do this for a living.

looking at the second set of photos, this is what i see:

3217 - the wrist pin shows sign bluing high heat marks. the flange holes might do better if they had a chamfer on them to relieve stress.

3221 - plug color on insulator, resembles high speed glazing. possible misfire.

3223 - intake port floor slanted, with unmatched side radii. uneven wear marks from piston skirt on the cylinder wall spigot blow the intake floor.

don't know if any of these caused your issues, just putting it out there for discussion is all.

regards
-omb

 

[CanadianCarGuy]

Yes you are right the 372 had con rod bearing failure and was ran until it wouldn't run anymore

Yes you are right, I had to finish a cut up tree with the cylinder broken with choke on leaned right out. This cylinder had been sucking air slightly for a couple days before failure. You can tell when they are about to go, as they start running really good for some reason, and the idle starts coming up ever so slightly

And yes right again, I fudged up and raised the intake too much trying to use a dull rasp. This was not a new cylinder when I ported it for myself, I do not use new cylinders, this one that was severely scored and I just sanded out the scoring that is what you running up the cylinder wall. One look at that port and you can see I was struggling trying to get the rasp to bite. When you need a saw for the next day and everything is closed or you are in camp, you are limited in what you can do sometimes...

 

[mdavlee]

I just have to wonder if more oil would make them last longer if you could stand it. Losing the big end seems to be too much rpms or lack of oil and getting real hot.

 

[CanadianCarGuy]

Don't forget we are hard on our saws, they take the odd tumble down a bluff or take a hit, or get squashed sometimes. I find too, if you are running a loose or broken muffler for any amount of time, it is a death sentence for the cylinder falling apart. Same goes for taking big hits on the muffler, the bracket that holds the muffler to crankcase is for vibration more than structure, it does little for supporting the muffler when it gets struck by something, cast cylinders aren't exactly designed for impacts.

I really like the idea of the additive metals is talking about. I think awhile ago when 385s first come Walker's was putting additive in for break in newer saws to harden the chrome inside the cylinder, but the guy working behind the counter had no idea what I was talking about when I asked last time I was in there.

 

[rmh3481]

Boron is what is used in products like 'slick 50'. Maybe theyre mixing it into the fuel.
Here is an article;
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=729116

 

[WHSH]

This is cool info and photos. I do ported work saws at my shop cut base and squish, nothing race oriented, I appreciate trying to simplify the whole process, by limiting the maching. I don't have endless wisdom and still on the learing curve with two strokes, so take that into account with my input. I have been reading a bunch on the value of the proper squish band size and shape on the Motocross 2 stroke forums. This is what i have found the it may shed light on the burned, piston photos. Squish band size serves to keep the pistons from smashing the cylinder, influences flame front burn charactoristics and piston crown cooling. A tight squish burns effienctly but does not allow piston crown enough fresh charge for cooling. Too Large a squish band the the turbulance of the flame front is week and the burn time is and effiecenty is reduced. On 65cc and up saws that I mod I have between .025 and .030. This way if I tune the saw well I know that I am getting good piston crown cooling. A cooler crown, reduceds the risk of detonation (second rogue flame front) and I think that is the I see in the photos. I am not expert, and still taking the information in, but that is my best guess.

 

[Deets066]

I have an 046 with an 064 piston .040 pop up and base cut to get .020 squish. This brings compression to about 206 lbs. when I had the the saw apart I put new bearings and seals in and mild exhaust and intake porting and just took the casting edges off the transfers. I haven't had any issues whatsoever, I also run 100 octane with full synthetic oil and Lucas oil upper cylinder lube. Have been running this for about four years now. I don't run it everyday, but it sees quite a bit of use