Clam shell motors....

I have been looking at this forum for the last two months or so. I keep seeing references to the fact that the "homeowner" series of saws by Stihl and Husqvarna have a "clam shell motor" design. I would like to find out exactly what a clam shell motor is and exactly how it is different than the type used in the professional series of saws. I also keep seeing references to the fact that the clam shell motors are a lot harder to work on than the ones used in the professional line of saws. Please explain the reasoning behind this.

First of all a clam shell motor is not a bad design. In today's saws the chainsaw engine is a separate unit from the chainsaw oil and fuel tank assembly which is usually plastic. Clam-shell design is where the cylinder and crankcase is integrated. Thus the crankshaft assembly is held into place with the upper and lower half of the cylinder/crankcase assembly. The older lightweight McCulloch chainsaws in the 10/60/70/80 series where of the clam-shell design which were professional in their day. I like the design as you can service the entire engine with ease in most cases.

BBush said:

I have been looking at this forum for the last two months or so. I keep seeing references to the fact that the "homeowner" series of saws by Stihl and Husqvarna have a "clam shell motor" design. I would like to find out exactly what a clam shell motor is and exactly how it is different than the type used in the professional series of saws. I also keep seeing references to the fact that the clam shell motors are a lot harder to work on than the ones used in the professional line of saws. Please explain the reasoning behind this.

Click to expand...

"Clam shell" engines have the case split parallel to the crankshaft's rotation. They are the same design as snowmobile engines but the top of the crankcase is the same part as the bottom of the cylinder. The cylinder is not as easy to "tweak" because it is not it's own part, it's the top half of the crankcase and is holding the bearings in place. "Pro" engines have the crankcase split perpendicular to the crankshaft's rotation. Cylinders can be shaved down to gain compression because they are their own part separate from the crankcase.

Now the questions will really start....
opcorn:opcorn:opcorn:opcorn:

Quick on the draw....

that thomas...

I didn't know that 2007 was just two months ago.....and only 18 posts?
I must be missing something.

SpaayDawg said:

I didn't know that 2007 was just two months ago.....and only 18 posts?
I must be missing something.

Click to expand...

No, he just (like me) knows how to use the search feature....

What I've noticed most in working on Stihl saws, I can have the piston out of the saw and in my hand in about ten minutes on a 026, or most other vertically split type engined saws. I tore down an 018 a few day ago and it took me quite a bit longer to get the piston out. In fixing the few, (compared to others here) saws I have repaired, it's usually a scored piston or worn rings that brought the saw to me, so I prefer to work on a "pro" type saw. Modified saws are a whole other story...as are Husqvarna type cylinders...

I guess my experience has been the same, mostly with the 1127 series (029/290, 310, 039/390) it seems I have to remove a lot of parts to GET to other parts. I guess that way they're compact. Whereas say my 028, I can have the engine shroud off without taking anything else off. Then it's just muffler bolts and cylinder bolts.

Boogieman142 said:

No, he just (like me) knows how to use the search feature....

Click to expand...

The search feature would bring up alot of build threads with pics I would hope.

BBush said:

I have been looking at this forum for the last two months or so. I keep seeing references to the fact that the "homeowner" series of saws by Stihl and Husqvarna have a "clam shell motor" design. I would like to find out exactly what a clam shell motor is and exactly how it is different than the type used in the professional series of saws.

Click to expand...

a clamshell engine captures the crank by the mated pair of the jug and the bottom cover. the parting line is at the main bearings and is parallel with the crank.

see, for example,
http://www.arboristsite.com/showthread.php?t=129434

a "regular" jug-on-cases construction engine captures the crank by the mated pair of the two side cases containing the main bearings. hence, you need to "split the cases" to get the crank in your hands.

the clamshell is presumably cheaper to manufacture and is used on lower power output engines, while the jug-on-cases design provides better crankshaft support and is used on more powerful engines.

the clamshell type is harder to effectively modify, since you can't change the geometry (squish, etc) due to the fact that the main bearing half-moons have to stay semi-circular.

both engine types operate on exactly the same principles; only the construction differs.

BBush said:

I also keep seeing references to the fact that the clam shell motors are a lot harder to work on than the ones used in the professional line of saws. Please explain the reasoning behind this.

Click to expand...

this is BS in my opinion. i have worked on 2 stroke dirtbikes, saws, trimmers, etc and the dis- and re-assembly of a clamshell setup is no harder than a "regular" construction engine. in some ways the clamshell, to me at least, is EASIER to work on if you are going all the way in.

jim aka the wrooster

thomas72 said:

First of all a clam shell motor is not a bad design. In today's saws the chainsaw engine is a separate unit from the chainsaw oil and fuel tank assembly which is usually plastic. Clam-shell design is where the cylinder and crankcase is integrated. Thus the crankshaft assembly is held into place with the upper and lower half of the cylinder/crankcase assembly. The older lightweight McCulloch chainsaws in the 10/60/70/80 series where of the clam-shell design which were professional in their day. I like the design as you can service the entire engine with ease in most cases.

Click to expand...

Excellent answer. Depending on your skill level and experience, the clam shell design as spoken of here where the bottom of the crank case fits like a "clam shell" to the cylinder is not that much more difficult to work on. Generally, it will take longer to pull the cylinder on a MS290 versus a Stihl 026 type saw. This is due to the fact that one must pull the top cover and all the integrated parts. The cylinder on the pro-type saws can be removed without pulling the carb and many of the other parts. Personally if I am going to rebuild a pro saw (new top end) I like to break it all down and give it a good cleaning and inspection anyway.

There should be plenty of information here on the clam shell type designs. Although many on this site in various threads do not place much value on such saws as the MS290/310/390 and other even smaller models such as the MS210 and the MS250. It becomes somewhat of a preference in my opinion.

never understood why the clam shell concept is frowned apon..maybe from a builder or saw puzzle complication perspective. Some of the toughest two strokes on the planet had that "clam shell" design for decades...Mercury outboards. In particular the 50HP four cylinders and their cousins from the fifties into the ninties..and all those MK15 thru MK58a's ....years and years of reliable operation with a clam shell design.

What do you call a Mac 15 then ? I was looking at 2 that have locked up with rust and that is a whole other ball game lol. It seems you would have to unbolt the rod send it up to the top of the cylinder and must have a retainer plate on mage side then you press it out clutch side.

On the McCulloch 15 you have to remove the crankshaft and then pull the piston/rod assembly through the bottom of the crankcase. The head is not remove able on that particular saw. The higher end build of that style of engine by McCulloch was among the best at the time. I still run a SP125.

weimedog said:

never understood why the clam shell concept is frowned apon..maybe from a builder or saw puzzle complication perspective. Some of the toughest two strokes on the planet had that "clam shell" design for decades...Mercury outboards. In particular the 50HP four cylinders and their cousins from the fifties into the ninties..and all those MK15 thru MK58a's ....years and years of reliable operation with a clam shell design.

Click to expand...

i agree on this,i do not understand the beef w/clamshell design. so what if the cyl. doesn't come off as easy. i have saws that are 25+yrs old,that have never been apart. look at all gas and diesel engines,they are basically a clam design. the crank is sandwiched between the block assy. and the main cap. i know there are guys here that want the jug to come off easy,but to most it is no big deal. i have yet to see any real problem with this design.

must be hard to put the needles in the rod end

It is not bad at all with the help of some grease.

pics of a clam shell husky




pics of a ms280 clam shell




pics of a ms260



scott

Great thread!!

Thanks everyone.
I'm not the OP, but have been wondering what exactly the "clamshell" design was, myself. I had a vague idea, but this clears it up.

Are both designs pretty well the same with all manufacturers in differentiating between the Pro & Homeowner saws?

What about the in between saws like Husky's "Landowner" type?

Also, Echo and Dolmar claim to have Pro type features in some of their small inexpensive saws. What would those be?

To me, note I said "To Me". Just a bit gun shy here...ain't wantin to start no ####, insert nervous laugh here... Just because a saw is or is not a clamshell does not mean that it is or is not a "pro" saw. An example, I have a Ryboi 40cc saw, it is not a clammy, it has a vertically split magnesium case and a bolt on cylinder, but I would in no way consider it to be a pro saw. I take saws apart quite often, older Macs come to mind here, that are clamshell in design, and were most definitely pro saws. I tore down a Poulan 2300 a few days ago, split case, bolt on cylinder...was it a pro saw...no way!