Plastic Crankcases

[jokers]

I know when I first heard of this particular development I cringed. It just didn`t seem like a good idea, then I heard all about the Stihl 029/MS290 air leak problems attributed to the plastic case. Suspicions confirmed. The point is, there are other saws out there with plastic crankcases, probably even other Stihls, and I`m not hearing alot of bad news about them. The main difference between the Husky 350 and 351 is that the 350 has a plastic case and is cheaper. I believe that the 340 and 345 also have plastic. I haven`t been hearing any horror stories about these saws, so I`m wondering what is the real shortcoming of a plastic crankcase if all the other components are the same? I know the perception is that they suck, but can anyone provide any sensible thought on this matter? Russ

 

[Fish]

It is not actually a plastic crankcase, but the block, or powerhead,
or whatever you wish to call it is bolted together to a plastic
frame. The bottom of this block holds the crank on and sealant
seals the mating surfaces. The mating surfaces are at the
crankshaft line so the seals are in this sandwich as well.
The same fasteners that hold the plastic saw body to the engine
also hold the engine together. Just a design I think is poor,
and a bear to work on.
Easy and cheap to crank out of the factory, a lot of parts
interchangeability is the long term goal I am afraid. Instead of
keeping and using a saw for a decade, they want everyone to
warm up to the idea of just buying a new one every couple of years.
Not just Stihl,but all of them
Fish

 

[Treeman14]

I think the term for it is planned obsolescence. We are living in a disposable society. Nothing is built to last anymore, including cars, toys, appliances, and marriages. We're expected to just throw it away and get a new one whenever something wears out or we just get tired of it.

 

[Walt Galer]

Howdy,

My reading on this would generally go along with Fish. I have softened a bit though after really looking in on it on the mechanics bench. True the mechanic really cusses having to unglue one of these, and then get it back together without leaks. Personally, I wouldn't want to attempt it without some very specific trainning. (For example, how many of you know how to start a screw in a used hole in composits? You are allowed only one go, and then it is ruined if you miss!)

The idea has been around at least since Partner tried it, generally successfully, as long as the displacement was kept under 50cc. Where the 029 and 031 look questionable, is this displacement thing. When saws get that large, they tend to be used for jobs like cutting firewood, which may entail enough continous operation that heat begins to accumulate in the case. Plastic (composits actually) do not dissapate heat worth a darn, so the main bearings tend to sit there and cook their lubrication. If you couple this with good engine speed, you are in trouble. On top of this the bearings used are not the top quality professional models of the bearings, and are somewhat minimum size.

My limited experience with these is that as long as they are never run continous for more than a tank of fuel, and spend most of their life hanging up on the shop wall, they should be expected to last a good 10 years. After that, the material will tend to embrittle enough that the case should be expected to crack at the slightest excuse, if it hasn't already. Such failures occur sooner if the saw has been left in the sunlight.

If the saw is used to cut 8 or 10 cords of firewood per year, I would expect these saws to last mabe 3 or 4 years with reasonable care. The embrittlement problem seems to be to accelerate with this heavier use, and I have seen a few cracked cases with only a couple-three years use. (I'm not sure how much abuse, but you can bet there was plenty). I also kind of question how much of the leakage problem is paired with the user trying to pry the saw through the cut, stretching the composit case?

So I guess, it is a matter of "you almost get what you pay for", at least a bit. If you figure the cost of that 8 or 10 cords of wood after including the cost of the saw, the deal may not be so attractive after all.

I have a friend who is experiementing with welding up cracks in these new "plastic cases". His results look encouraging, but you have to wonder. If it cracked once, what is there to say it is not generally embrittled and will only crack again?

Regards,
Walt Galer

 

[Thumper]

Bearing Heat

Based on conversations with many dealers I agree with the fact that the composite body saws cannot hold up to extended use under full thottle and load. Stihl and Husky dealers both told me to go with a magnesium or aluminum bodied saw if I wanted a saw for the long haul. Not only because of ease of rebuild but but they disapate heat particularlar from around the crank bearings much better than their plasic counterparts. I have a plastic saw that is 4 years old now and hasn't seen any real action until this past year. The saw still works ok for what it is but I don't expect it to last for another 4 years at the rate it was worked. I guess if you want a pro type saw or if you wish your saw to last many years, the choice isn't plastic. If you can get by with a low cost plastic box store saw for the short term as I did, then spend less and get what you pay for. For me it now time to belly up to the bar, spend the cash, and get a real (non-plastic)saw. I all depends on what you expect from you saw.

Thumper

 

[homelite360]

hell even craftsman chainsaws still have a cast allumium crank case. so this proves my theory that stihls are junk saws.

 

[HUSKYMAN]

Andrew what do you think the new Homelites have in their crankcases

 

[WRW]

I think abuse was the key to Walt's reply. I've had a plastic crankcase saw for ten years and have cut the 8-10 cords of firewood/year mentioned with no problems as yet. If the saw died tomorrow I wouldn't replace it, but I don't figure it owes me anything.

 

[Walt Galer]

Howdy WRW,

What model is that 10 year old plastic saw? My real question is: Is the rear handle bottom one piece with the crankcase section that clamps the main bearing and seal? (Like Husky and Stihl).

I have a theory that lifting on the rear handle to pry a cut or reefing on it to pull out of a pinch is what causes these leakage failures, and possibly also a good part of the case failures.

Regards,
Walt Galer

 

[sonny]

My theory is a dull chain causes many internal damage.

 

[WRW]

Walt,
It's an 025, which meets your criteria of under 50cc.
As for heat on the crank and seals, bogging till the clutch slips would be the quickest way to build up that kind of heat. I can't imagine it doing the composite any good either. Wrenching in addition to that abuse sounds like a death sentence.

 

[homelite360]

whell i won't even touch the new homelites if i don't have to. it just pisses me off how they went downhill so much in the past few years. that is one reason why i do not like to even consider buying a homelite that is newer than a 330 cause i knwo they are not worth the material that they are made out of. but i do feel a great deal of dissapointment in them lately. and it looks like stihl is goign in the same direction. but with the trend that they are goign at the next thing is to use a space age plastic to make the cylinder and piston out of plastic. sure the saw will weigh about 5 or 6 pounds but what will be the life of them. and they will still cost you a ton for a saw that will wear otu in a few years. it is enough to piss you off.

 

[HUSKYMAN]

I agree it is pathetic, Andrew. Saw a McCulloch 33cc at the farm store yesterday, what a piece of junk! They lure people in to think they are buying a good product with a name and they get a piece of crap

 

[WRW]

I wouldn't waste my bile over the quality of saws in department stores. Some purchasers will get a surprising amount of service from them. Others would fare no better from a professional grade saw. Either way, they get what they pay for.
I know of one case of a saw being discarded for not running where the problem was a clogged air filter. How can you feel bad for that owner? I guess an air injection system would have doubled the length of ownership time, he might have gotten a year out of it.
There are probably plenty of other war stories out there.

 

[jokers]

Hi All, many thanks for your input. I`ve got a feeling that we are going to be seeing many more name brand saws with plastic cases in the years to come. I`ve also got a feeling that this isn`t necessarily a bad thing either. Of course there is a learning curve for the manufacturers in what constitutes solid engineering using composites, and technicians, such as Fish, will have to learn a few new tricks, but we need to recognize that composites may be the technological advance now that aluminum and magnesium were in the early 1900`s. Maybe that juncture where the handle meets the case needs a closer look, too many users reef on the handles to ignore this area. I don`t really subscribe to the belief that composite cases are leading to an increased incidence of bearing failure due to poor heat disipation. I think Walt nailed it when he said that the bearings he has seen in these saws appear to be of a lower quality and smaller size. I know you`re all thinking I`m nuts about the heat thing, but consider this, lack of heat conduction is also know as insulation. What is far and away the hottest part of the saw powerhead? The combustion chamber/cylinder right? Heat doesn`t care what way it travels in regard to up and down, only from hotter to cooler. So what`s to say that a significant amount of heat doesn`t transfer to the crank bearing area, through conduction, from the cylinder in a typical metal cased saw. I`ll gaurantee that it does and I know that the manufacturers perform modeling tests on the hot spots. A composite case can minimize this heat transfer. What is really important to cooling and lubricating the bearings is the proper amount of quality fuel and oil and air flowing through the crankcase. I agree also that smoking the clutch and running dull chains are far underrated causes of engine damage. I think the manufacturers have to do a little improving on these cases before they will be accepted on a wide scale, but I also think that we as consumers have to be open to new technology if they can make saws cheaper and lighter and still give good service. This could easily be another one of those Stihl vs Husky type posts, or one ring saws are junk, but the truth is that there are no universal truths about any of those topics. We need to keep cautiously, open minds. Russ

 

[tundraotto]

now those are some VERY good points there Russ...i could not agree with you more.

 

[WRW]

Russ,
Good post, good thread. Especially the CAUTIOUS open mindedness part.