parrisw
Tree Freak
Ya I bet he was, that really sucks. I'll do that with one of the pistons, when the wife isn't looking! Or actually I'll just stick it on the wood stove.
Husqvarna 390 pistons?
- Thread starter parrisw
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Ya I bet he was, that really sucks. I'll do that with one of the pistons, when the wife isn't looking! Or actually I'll just stick it on the wood stove.
parrisw
Tree Freak
I got one 385 together and running yesterday. Checked comp tonight, came in at 180psi, that's with just flatten the squish with sand paper on the mandrel and machining the cylinder down to get .020" squish. Pretty happy with that. Not going to be porting these, just not worth it when just flipping the saws, unless someone specifically asks for it and will pay.
Putting a 390 together right now, did the same thing, will see how comp comes in at, suspecting it will be lower.
Putting a 390 together right now, did the same thing, will see how comp comes in at, suspecting it will be lower.
parrisw
Tree Freak
390 done, and pulls 165psi. Only a few minutes of run time.
parrisw
Tree Freak
Well something happened to the 390, first few minutes of run time and the saw just bogged out. I took it apart and the piston is scored pretty good, the cylinder appears to be saveable, little bit of transfer. I can't see what caused it though. The pins never backed out, scored on the ex side, lots of oil in the saw, so that's not it. Doesn't act like an air leak, to me it almost looks like something went through the saw? Or possible the alu cast is cheap and just came apart. What say you?
duramax99
ArboristSite Operative
Judging by the looks of the top ring id say something went through it thats alot harder than aluminum.
parrisw
Tree Freak
That's what I first thought? I don't see anything in the saw though, and I had it down to the case half's. Everything cleaned really well.
duramax99
ArboristSite Operative
Didnt have any plating chip or anything like that?
parrisw
Tree Freak
Doesn't appear to be?
The muffler you had on this saw- new or used? If used, how bad was it for carbon build-up?
One issue with these aftermarket pistons- low silicon content which means they're softer than most OEM.
One issue with these aftermarket pistons- low silicon content which means they're softer than most OEM.
parrisw
Tree Freak
Used, it was pretty clean.
Ok, and the bevel/chamfer around the exhaust port is good?
parrisw
Tree Freak
Yes. It was completely stock.
Hmm...air leak, carbon score, physical defect of some kind, water in gas- I'm running out of ideas here. All I have left is what you mentioned, just the piston being too soft or of inadequate material.
Those black streaks on either side of the damage indicate a carbon score to me. Maybe check inside the muffler for any telltale signs of loose carbon?
Those black streaks on either side of the damage indicate a carbon score to me. Maybe check inside the muffler for any telltale signs of loose carbon?
parrisw
Tree Freak
I'll take a look.
You guys have seen way more of these failures than I have, but I have never seen a jug scored offset from the exhaust port.
Do you have a way to check the jug for roundness or high spots? Just a thought.
Offset scoring is not unusual, although I can't explain it.
Mastermind
Work Saw Specialist
I'd double check that jug for any traces of transfer from the last time. If any is left behind that can happen fast.
parrisw
Tree Freak
Well, too hard to tell now. But I'm pretty sure I got it all good! Who knows though. May use another jug.
Mastermind
Work Saw Specialist
Well I've ruined a piston or twelve ya know. :hmm3grin2orange:
parrisw
Tree Freak
LOL, I might have ruined a couple two or three.
