What is a clamshell design?

[traktorz]

I've seen in some threads here mentioning a saw has a clamshell design, I think among budget chainsaws. What exactly does it mean?

And if not a clamshell design, what other design types exists?

What are the pros & cons on each types? I guess economical reasons for clamshell being more popular among budget saws ?

 

[XSKIER]

I guess the most direct comparison would be the "pro" MS 261 C-M to the homown MS 271. They share many external parts and follow the same basic form. The 271 will have a plastic hull that the "clamshell" motor is bolted into. The 261 motor is built into its magnesium hull, saving room and weight.

A clamshell engine is only split in one place, horizontally about the crankshaft. A pro engine is split horizontally above the crankshaft, and then vertically about the "hull".

 

[sachsmo]

I guess a 125 Super Pro was NOT a 'pro saw'?

 

[XSKIER]

Only if it has one of your good and proper muffler mods.

 

[traktorz]

...
A clamshell engine is only split in one place, horizontally about the crankshaft. A pro engine is split horizontally above the crankshaft, and then vertically about the "hull".

Thanks for the explanations.
So horizontally split like a clamshell or flip-phone, however the phone is folded via a hinge. No hinge on the saw though.

 

[Wagnerwerks]

I guess a 125 Super Pro was NOT a 'pro saw'?

Please... You know mac saws are far inferior in general...

 

[sachsmo]

Only if it has one of your good and proper muffler mods.

FAIL!

also points deducted for a 'low blow'!

 

[Chris-PA]

Here is a picture of a clamshell in a chassis. You can see the horizontal split line at the crankshaft centerline. There is only a small metal cap at the bottom. The engine is a self-contained unit that can be built (and tested?) separately and bolted in the chassis at a final assembly stage, rather than having to be assembled as part of the build up.

Some Husqvarna models don't use a metal lower cap, rather have the plastic housing from the bottom of the engine, while newer ones mold the metal lower cap into the plastic housing as an insert.

As has been pointed out, clamshell construction isn't much different than the way many older horizontal engines were made.

 

[sachsmo]

Good one Chris,

A picture says a thousand words.

 

[Franny K]

Probably ought to just stop at the clamshell one the split goes half way through main bearings and the other one the cylinder has no half a bearing pocket.

There is a grey area if all of the above posts are correct. The Stihl 150 mini saw does have a plastic hull if that is a fair term while having a vertically split case and bolt on cylinder. Some Husqvarna models have magnesium bottom of the engine module that incorporates the bar mount so plastic or perhaps more correctly composite in this matter might be left out.

Removal and inspection, replacement, modification, and similar of the cylinder is possible in probably most cases where the case splits vertically and the cylinder bolts on. It seems to me the bolt on cylinder offers more manufacturing options to make passages.

 

[sachsmo]

Here's a typical 'split case design:




See how the 'jug' is completely separate from the crankcase.

Much more robust and easier to work on.

Another plus is it is far easier to adjust squish on the 'split case design.

 

[Chris-PA]

Much more robust and easier to work on.

I gotta disagree with the robust part, at least for the engine itself. There are zero differences in strength or the way the porting can be designed in a stock engine between the two designs. Some kinds of service are easier with a clamshell (bearings) and some are harder (pulling the cylinder). The difference is primarily in modification.

 

[sachsmo]

OK I'll give you that.

However some/most modern clamshells incorporate a block o' plastic for the bottom half of the crankcase.

Not so robust in my book!

Give me a magnesium 'split case' any day.

Ever seen one o' them plastic fantastics get their studs pulled out o' the case?

 

[sachsmo]

I gotta disagree with the robust part, at least for the engine itself. There are zero differences in strength or the way the porting can be designed in a stock engine between the two designs. Some kinds of service are easier with a clamshell (bearings) and some are harder (pulling the cylinder). The difference is primarily in modification.

You may want to re-think that notion?

 

[Chris-PA]

You may want to re-think that notion?

I'll stick with what I said, which was in regard of to the strength of the engine itself. You can make the argument that a plastic chassis is weaker than a magnesium alloy chassis, but that is a different claim. There is actually no requirement that a clamshell engine be placed in a plastic chassis, although this is the typical implementation.

 

[Wood Doctor]

One thing for sure, if you intend to rebuild a clamshell engine, be prepared to take the whole saw apart--lock, stock and barrel. Your workbench will be covered with parts all over the place. I recently enhanced a Stihl 310 and made a 390 out of it. That was indeed a learning experience and twice the work of upgrading an 034 to an 036.

I've also been told that the crankshaft seals and bearings don't last as long on a clamshell engine, but I have no data to back that up. I do know that both were shot on the 310.