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Dolmar 7910 rebuild. Need some help

t4driller

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This is gonna get a little long winded so bear with me. I bought a 7910 in pieces in a box that had a scorched top end. The cylinder was cleaned up when I got it and everything looked good in it. No transfer left behind. I took some scotch Brite and ran threw the n cleaned with wd 40 until rag came out clean. Bought all new oem bearings , seals, piston , rings, gaskets, and couple other little parts. Got some of the parts off my local dealer and some off nate. Once I got the case split I sent all the parts threw the washer to be cleaned then reinstalled everything. Once I got the piston in and jug on i spun it over a couple times and it seemed to have just a very slight drag to it. I figured it was just new rings in a cleaned up jug. I assembled everything with stihl 2 stroke oil. Went outside to start it and it fired on the third pull. It just popped for a second or 2 on choke. Then wouldn't start on high idle. Each time I would pull it , it got a little harder to pull over. So I stopped and pulled the muffler and saw that the piston had scuffing/ groves almost on it. uploadfromtaptalk1457706341240.jpguploadfromtaptalk1457706354784.jpgfirst assumption was wrist pin bearing or con rod bearing letting the piston rock back and forth a little. Checked them out and they are nice and tight with no up and down movement. So started taking some measurements of piston and cylinder. Now I know this isn't the best, most accurate way to measure. But it's what I have at the moment. It took me almost an hour of going back and forth measuring to come up with consistent numbers. uploadfromtaptalk1457707143200.jpguploadfromtaptalk1457707157256.jpguploadfromtaptalk1457707203022.jpgso this is where I'm at. [emoji17] [emoji17] I gave nate a message lastnight to see what he thought. He said make a thread to get some opinions / input. Thanks

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t4driller

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I agree with Mike lee. The dolmars always have sharper port corners than most saws. Put a little bigger champhers on all ports. Hit it with the scotch Brite again and retry! :)
All the ports have nice bevels on them. I don't think I have a picture of the cylinder before I put it on. I have a couple after the fact. I didn't clean the cylinder or do anything to it since this happened. uploadfromtaptalk1457709851117.jpguploadfromtaptalk1457709877000.jpg how much does a piston expand once running? Also doesn't it expand quicker then the cylinder? Not saying that's what happened here cause it didn't run long enough to get warmed up.

On the edge of the piston skirt is where it looks like it all started from. There is a good bit of wear there for the brief moment it ran and the couple times I pulled it over afterward. Why it's there??? Just want to figure out why. Thanks. Keep the ideas coming

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t4driller

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I can try and get some better pics of cylinder tonight when I get home. Also to have a better look at the ports.

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SteveSS

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I'm just spit ballin' here, but I think you said that you got this saw in pieces in a box? Did you make sure that there wasn't anything gunky in the crankcase before you put it together?
 

huskihl

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51.95 mm on the piston skirt and 51.96 mm at the bottom of the jug. That's under .002" clearance. Might be too tight.

I'd scotchbrite and wd the bore again, and run around every port edge with 180 emery cloth. Try some 220 or 320 grit on the piston to smooth it up. Put the piston (without the ring) in the bore and feel for a burr or tightness as you go in. It should fall in if you let go.
 

t4driller

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The scratches on the exhaust side look like they run from BDC on the piston where it would be on the exhaust port to me.
There is fine lines that are very hard to see in the pic that run to the bottom of the jug. Then there is some slight transfer that starts about 3/4 of an inch below the exhaust port that run a little above the exhaust port. Where the sides of the piston skirt meets the jug is where the very fine lines are alot more concentrated. I was waiting to clean the cylinder back up incase I needed some more pics.


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t4driller

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I'm just spit ballin' here, but I think you said that you got this saw in pieces in a box? Did you make sure that there wasn't anything gunky in the crankcase before you put it together?
Yes got it in a box o pieces. I split the case and installed new bearings and seals. While everything was out and apart I cleaned them up real good.

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t4driller

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51.95 mm on the piston skirt and 51.96 mm at the bottom of the jug. That's under .002" clearance. Might be too tight.

I'd scotchbrite and wd the bore again, and run around every port edge with 180 emery cloth. Try some 220 or 320 grit on the piston to smooth it up. Put the piston (without the ring) in the bore and feel for a burr or tightness as you go in. It should fall in if you let go.
I've had a couple people tell me that it's not the most accurate way to measure, and I agree. So the measurements might be off by a smidge[emoji4] [emoji4] . It took me awhile to get consistent measurements. Even had a friend come over and do it and came up with almost identical numbers.

I plan on running scotch Brite threw it then see what it looks like from there. For the piston, I'm gonna clean it up and check it out some more. But I don't think I'm gonna reuse it. Was looking at aftermarket ones because oem pistons aren't the cheapest. I haven't been able to find a meteor and was told they don't make one for it. So... what to do??[emoji15] [emoji15]

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riverrat2

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51.95 mm on the piston skirt and 51.96 mm at the bottom of the jug. That's under .002" clearance. Might be too tight.

I'd scotchbrite and wd the bore again, and run around every port edge with 180 emery cloth. Try some 220 or 320 grit on the piston to smooth it up. Put the piston (without the ring) in the bore and feel for a burr or tightness as you go in. It should fall in if you let go.

Good plan

I will pay shipping + $10.00

Hey Randy Put the Ice Pack down and look @ this @Mastermind
 

t4driller

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Good plan

I will pay shipping + $10.00

Hey Randy Put the Ice Pack down and look @ this @Mastermind
Gonna keep the piston, sorry[emoji4] [emoji4] . But looking at aftermarket pistons, which would you use? Haven't seen any meteor. And there's some that are windowed. My plan was to get it back together stock, make some cuts then start to do mods in steps to see how it does. Also was hoping the unlimited coil comes out soon.

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mdavlee

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Not many choices for aftermarket. I've used a few different ones that don't originally go in the 7900.
 

Four Paws

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Or like Mike said, swap in apiston from a different model.
 

riverrat2

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51.95 mm on the piston skirt and 51.96 mm at the bottom of the jug. That's under .002" clearance. Might be too tight.

I'd scotchbrite and wd the bore again, and run around every port edge with 180 emery cloth. Try some 220 or 320 grit on the piston to smooth it up. Put the piston (without the ring) in the bore and feel for a burr or tightness as you go in. It should fall in if you let go.

May be some merit here in the post by huskihl,

that is less than ,001 on each side if your measurements are correct. if you can find the high spot and sand it down on the piston, scotch bright the bore, touch up the port bevels it may work

Gonna keep the piston, sorry[emoji4] [emoji4] . But looking at aftermarket pistons, which would you use? Haven't seen any meteor. And there's some that are windowed. My plan was to get it back together stock, make some cuts then start to do mods in steps to see how it does. Also was hoping the unlimited coil comes out soon.

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understand,, I was just poking fun
 
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huskihl

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May be some merit here in the post by huskihl,

that is less than ,001 on each side if your measurements are correct. if you can find the high spot and sand it down on the piston, scotch bright the bore, touch up the port bevels it may work
I've never really been into measuring piston vs bore clearance. But I've never had a failure either. I've heard it mentioned that (I believe) .0025 - .003" clearance is optimal. But I really have no idea as to the accuracy of that statement. I know several meteor pistons per model are available that are maybe + and - .001
 
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