bmp01

Nice pictures - pictures always good for adding info. Extra saw history helpful too. As you say age isnt a big concern, pictures suggest the insides are in decent condition. The light scuffing above top ring I've seen once before - it was on a saw i did porting on, assumed I'd left grinding grit in there. Probably fine brick / concrete grit in your case, suspect its just a visual tell-tale. Worth noting: sometimes the top few mm of the cylinder increases in diameter a smidge (it's a manufacturing thing, honing to top of a closed cylinder is difficult) - its a good place for crap to get trapped. Compression numbers, thats low at 115psi but depends on your tester. Compare your numbers to what you get on another 'good' 2 stroke with same tester... Two stroke mix down plug hole or flooding the engine will give higher numbers through partial ring sealing - like your picture through ex port which looks shiney / oily. Personally, I'd keep going with carb cleaning / fuel investigations before anything more drastic, because that picture says the piston is happy. But keep in mind that compression number is low. Fingers crossed for you.

How old is it ? How much use has it had? Have you checked cylinder health with compression test ? Can hear any piston slap noise when running ? You could pop the exhaust off to see the exhaust side of piston - might give you a clue as to general piston health. But really you want to see the inlet side. I'm wondering if you've got a worn piston which is no longer closing the inlet port, that might lead to your soggy filter. A bit more saw history would be useful. ...

T27 Torx bolt head, might get a hex driver to work but it aint right.

Only buy in 5L qtys. Was surprised to find Screwfix sell 5L Oregon bar oil at 15 quid a go - about as cheap as online prices.....

I have limited knowledge regarding cutting angles but if you do some digging I'm sure you'll find info on here.

My advice would be to do a proper health check on saw before you start milling - paying attention to fuel system and check for air leaks around crankcase seals, intake boots, gaskets etc. As you're probably aware milling is harsh, it'll find any mechanical niggles in short order. Might be best to do some cross cutting first too, make sure it behaves itself. I'd set the saw up to run a little rich which will keep piston temps a tad cooler. Depends what value you put on the saw. And can it be rebuilt if it seizes a piston? I did all the above with a seconhand 390XP and it still shat itself in a 12" milling cut. Later I learnt it had an aftermarket piston and repaired cylinder in it..... thats the lottery of secondhand saws.

Think I'd put the breather at the top of the oil tank ....

Well that's different ! Clearly it's a filter. The location is good for the oil pick up so I'd say that's what it is. How does it work though? Given the height of filter and the fact you can't leave the top part exposed to air (coz then the pump will then only stuck air) - I imagine the filter goes into sealed a plastic tube, top of the tube connects to pump inlet, bottom of tube should connect to oil tank.... Have a look in the hole the filter came out of and see if there is a connection to the tank down low. If there is a "cross drilling" I bet its blocked. You might also see something useful through the filler plug hole ....

Never seen such a thing (sump plug), are you sure that's what it is ? As you've fitted a new pump you'll know which drilling or hose feeds the pump, clean that line out and check the oil filter for 'blockedness'.

There's a lesson for the design team - all failures straight down the centre of the rib I bet. I suppose the rib was put in to relieve the stress associated with expansion 'n contraction but they've concentrated all the load on a very tight inside radius, so it work hardens and cracks. Clever and daft in equal measure.

How about a turned (lathe) insert epoxied in ? Could have a rim on it .... so a "top hat" insert, put in from exhaust side. Benefits are, it won't come out, top hat rim is thinner than a nut, can have the location nose on it, can take a proper thread most likely a size down on the self tapper cack. Concerns: is there enough meat to centralise the hole and can you then file / drill a reasonable hole (doesnt need to be perfect if the insert is glued in).

Here we go again.... Think OP is going to find Spuds answer slightly more useful. For entertainment value we'll give you 9/10 for effort Stubby. I'm in agreement btw.

Ok, as simple as.

How did you go about doing that ? Seems like a plan if you've got a half used can kicking around.

3 spare plugs for one saw sounds a bit excessive. 3 spare plugs for your collection of saws doesnt sound enough 😆

In terms of overhang from crank bearings its a few mm in a handful of mm - big percentage for the engineer's sums. With you on the rest of it.

First part: No, brake band is a wibbly wobbly thing. And gets hot if rubbed too much....oo-er-misses. Second part: Lets exagerate things a bit and imagine a top handle saw with the bar and chain half a meter away from the engine; top handle above the engine. Apart from being horribly cumbersome, I think you can see it would be almost imposible to react the chain pulling force in a cut, single handed. So again, its an advantage to have chain as close as possible to engine centreline. As you say, the manufacturers seem to agree.

That. It doesnt matter the route the forces take, clutch or cltuch bearing route.

Exactly. It did seem to work surprisingly well without brg. Lucky the brake band held the clutch approximately in the right place I suppose. The transition from from idle to chain runnning though .... horrible ! I should say I didn't cut with it like that, took it apart because it didnt feel quite right. I know the MS261 story. But even the substandard cranks were hardened steel , I still think it's a valid statement to say bearing works hard. Ok, sensible. But I have an issue with the clutch not running as true as one would hope. They're not that accurately made, carrier is held on with a thread, obviously play between the components for the weights to move. My guess is the bearing clearance is tight enough to do a job of keeping the clutch assembly concentric while clutch is driving the chain. So its carrying a force - I think. How much, dunno.

Emmm... dunno. The 390xp i bought s/h started and ran ok'ish, slightly strange behaviour as clutch engaged. Turned out the clutch bearing had been pinched out of the saw 😲 On t'other hand the early ms261 fiasco says the clutch bearing/crank has to be up to spec to survive - it clearly works for a living, that says its not just a spacer IMHO.

Yep. Space for chain end float same for both too. I was thinking about the forces on the crank. Forces ought to be smaller with chain closer to crank bearings. Then I got stuck, considering the load path from crank to chain. I suspect theres a better case for the outboard clutch, chain forces supported through clutch bearing which is closer to engine centreline. But when the clutch is engaged does the bearing take all the chain force .... or does the clutch ?

Which one puts the chain closest to the centreline of the saw ? Does that mean the crank dimensions can be smaller (ie lighter) ?

I know. But its skirt trimming on inlet side as you said and crown trimming on exhaust / transfer sides (not crown trimming on inlet side, which is what needs editing...). STICKY..... STICKY..... STICKY..... 😀

This ought to be a Sticky at the start of the thread. Quick edit of '...crown inlet side of the piston...' maybe ?

Aw crap did it happen again.....