Scored piston on exhaust side - cylinder OK?

[brages]

I mentioned this an my other thread asking for tips in disassembling an MS290... My brother-in-law has a 290 that had been running badly, and now won't run at all.

I took a look at it and it seems to be showing the classic signs of running lean - substantial scoring on the exhaust side of the piston, but the intake side looks just fine, from what I can see of it.

BIL told me that he had recently had a cracked fuel line replaced, so I'm theorizing that the damage occurred due to the bad fuel line. Of course, I'll want to clean out the carb and check the intake boot for cracks when I get into it. I suppose I'll try to cook up some way to pressure test the crank seals, too, or else jsut give them a "visual inspection"...

I looked in the spark plug hole, and I was able to see some vertical marks on the exhaust side of the cylinder. The view was pretty dark however, so I didn't get a great look.

What do you think? Do you think I'll be able to just get by with a $30+ship aftermarket piston and cleaning up the bore? How do these things usually work out for you?

What is the method for cleaning up the bore? Muriatic acid to dissolve the aluminum, followed by a hone (with shoes, or the christmas tree type?) Do you use any emory paper/cloth on any scuffing?

Are the ebay pistons OK? Any other good source for parts?

Thanks!

 

[Mike PA]

Until you see how deep the lines are, you won't know. I recently tore my 038 Mag apart and found some scoring on the cylinder walls, but not enough to warrant new cylinder. I used very fine sandpaper (600) on a sanding wheel to hone it, let hand sanded to add a cross-hatch patern. May not have been neded, but I liked doing it. I was told by a stihl mechanic that the stone hones can break or catch in a port. If they do, you likely damage the cylinder, which is why I went for sand paper. Others may disagree, but it worked for me. Muriatic acid is good for removing transfer, just be careful where you put. Keep it on the cylinder walls.

For the seals, visual inspection will not do it, unless they are REALLY bad.

 

[stihl only]

Get the cart back behind the horse. Might as well get it apart before you worry about it and as far as the seals go, I would be replacing them while you have it apart since you'll have the crank out to get the piston out.

There are some after market top end kits for the 290 and I think they may have the same bore as the 390, so you could step it up IF it needs a P&C.

 

[andrethegiant70]

What do you think? Do you think I'll be able to just get by with a $30+ship aftermarket piston and cleaning up the bore? How do these things usually work out for you?

Prolly, but no guarantees until you disassemble. The streaks are probably mostly alum transfer and will come off. Take it apart asap (be organized, so you know what screws go where), hit the hardware store for supplies, and take a crack at it. Once you start sanding, you'll know pretty quickly. I'm betting you just need a piston.

Welcome.

 

[brages]

There are some after market top end kits for the 290 and I think they may have the same bore as the 390, so you could step it up IF it needs a P&C.

All the aftermarket stuff seems to be for the 290... I think the aftermarket missed the boat on that one!

 

[brages]

I got the piston out last night... cyclinder doesn't look too bad to me. I'm going to hit it with the muriatic acid and sandpaper; wish me luck...

 

[Mike PA]

As long as you don't have big scratches, you should be ok. When I say big, I mean not enough to catch your fingernail, but may catch a knife blade edge.

 

[brages]

There's a bunch of gunk (transferred aluminum) just below the exhaust port, but farther down below it, the cylinder feels smooth. I won't really know until I start cleaning it up, I guess, but I'm hopeful!

Intake side looks perfect.

 

[blsnelling]

You must get ALL of the aluminum out of the cylinder. It must be perfectly clean. Otherwise you'll damage the new piston. Acid alone doesn't finish the job for me. It requires some hand sanding and/or honing to get it right. You can use a brake hone as long as you stay above the lower transfer ports.

 

[brages]

I know this is a dumb question, but where do you get the paper to sand with? Are we talking standard hardware-store sandpaper, or some kind of emory cloth? The only emory cloth I've ever been able to find was in a little strip for plumbers.

I saw muriatic acid the last time I was at ACE... a big gallon jug or something. I hope they have smaller sizes... I don't really care to have a gallon of acid sitting around in my garage forever.