Stihl MS170 crankcase material

rangerfredbob · Jan 14, 2022

Working on a Stihl MS170 for a coworker that he got used from another coworker because it idled high I found the crankcase was cracked on the bottom...

Is it likely aluminum? I'm just hoping it's not magnesium... What's the chances of that aluminum brazing stuff working? I've used it before with decent success, I know the drill on that stuff with the clean wire brush and so forth, was planning to pull the bottom cover and C clamp it to a heavy chunk of steel in as many places as possible and V out the crack with a Dremel then clean the heck out of it before starting...

Would rather not have to replace the whole jug since the only problem is the bottom cap... it's not a fancy model or anything so not worth putting a whole bunch of money into... it runs like a top just has a high idle from the air leak

mrmikey · Jan 14, 2022

It's magnesium, it's lighter than aluminum. If you want to confirm, shave off a piece on a cooling fin or similar, aluminum will come off in a string usually, magnesium will come off in chips. If you want further confirmation, takes the pieces you just shaved off and hit them with a propane torch, aluminum will melt, magnesium will burn very white and hot............Mike

lman · Jan 14, 2022

I used JB Weld to fix a crack on my stihl. That may work for you.

ericm979 · Jan 14, 2022

mrmikey said:
It's magnesium, it's lighter than aluminum. If you want to confirm, shave off a piece on a cooling fin or similar, aluminum will come off in a string usually, magnesium will come off in chips. If you want further confirmation, takes the pieces you just shaved off and hit them with a propane torch, aluminum will melt, magnesium will burn very white and hot............Mike

I've yet to see a saw (or any two stroke) with a magnesium cylinder. Mag cases are common.

On homeowner saws like the 170 the cylinder and the top half of the crankcase is one piece and is aluminum. The bottom half of the crankcase is a simple curved plate that bolts on. That "pan" is aluminum. The chassis that holds the engine and the other parts is plastic.

Replacement parts are cheap, especially the chinese ones. A whole engine from Farmertec is $23 + shipping. Or you can buy the pan and replace the old one if that's what cracked. The pan is pretty thick though. If the saw took a hit hard enough to crack it, it might be damaged elsewhere. Or the crank seals simply worn out and leaking air.

If you're replacing the pan you should replace the crank bearings and seals... and probably the rings and piston pin bearing and maybe the piston and wrist pin too. Might as well get a new engine.

rangerfredbob · Jan 14, 2022

It's just the "pan" as you say, since it started idling high it apparently has less than an hour on it.

I'll try fixing it for now but will keep a new engine in mind, the current owner is kinda weird at times about cheap parts...

I'm betting it's aluminum but will shave some off for good measure... I'm gonna tinker with it here in a few...

rangerfredbob · Jan 15, 2022

Well, that didn't work...

Will have to look into a new lower case half or something...

It's mostly aluminum but there's definitely some magnesium in the mix... too many angles to get an even heat, ended up with hot spots and burned through, if I got it hot enough the filler fell out of the other places it was, was a mess... rather try and fail than not try, only out half an hour of screwing around...

mrmikey · Jan 15, 2022

.....if I got it hot enough the filler fell out of the other places....

Yup, magnesium. if the arc is a purplish/violet color is another indicator..........Mike

Lnk · Jan 15, 2022

rangerfredbob said:
It's just the "pan" as you say, since it started idling high it apparently has less than an hour on it.

I'll try fixing it for now but will keep a new engine in mind, the current owner is kinda weird at times about cheap parts...

View attachment 729225

I'm betting it's aluminum but will shave some off for good measure... I'm gonna tinker with it here in a few...

That is a tough spot to fix. Good try though. Lot of load on that cover when running, reciprocating mass. A new bottom case is definitely in order.

xyz123 · Jan 15, 2022

Just buy another cap and reseal it up.

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