# 2005 Makita 6401 meets 2014 Dolmar 7900 Piston and Cyl: Very big grin factor :)



## zzr7ky (May 11, 2014)

After reading a lot of encouraging posts at AS, and encouragement here over the years I bit the bullet and sprung for a Dolmar 7900 piston and cylinder.   It's the first time I've cracked open a 2 cycle engine and expected it to run again.  

Going slow and double checking the instructions it took me about 2 hours.  I found a poor looking clutch drum bearing so I had to wait a couple days to tune and try.  I had not tuned one before either.  Several you tube visits and the new Dolkita 7900 is running great.  Plenty of power.  Nice quick work of some 24" willow I had laying here, so I headed out and cleaned up some 18 inch dead standing Elm.  I am very pleased.  Thanks to those who helped me scratch this blossoming CAD rash 

Mike


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## dougand3 (May 11, 2014)

Good deal!


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## TreePointer (May 12, 2014)

Good job!  I did the same upgrade to my 6401 and love it.


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## bassJAM (May 12, 2014)

I'm trying to force myself to wait until this winter to convert mine to a 79ccer.  I've got too many important projects that need my time and attention first.  So please don't post a vid of that beast cutting through wood!


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## Jon1270 (May 15, 2014)

The cylinder on the 6401 I picked up a few days ago is usable, but the piston is toast so I'm debating whether to upsize it or just replace the piston.  If this project has left you with an unneeded OEM piston, I might have a use for it...


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## dougand3 (May 15, 2014)

Jon1270 said:


> the piston is toast so I'm debating whether to upsize it or just replace the piston.


I'd go for the 79cc jug and piston. If you only put piston in, you may say "Man! I coulda had 79cc!" I really want a 79cc Dolkita but surely don't NEED it.


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## zzr7ky (May 15, 2014)

Hi -

Since I have a pair of these and a 17year old son, I'm thinking I better keep a piston and cyl on th shelf .

Also - I think I'd spend the extra cash and get the bigbore kit.  PM me if you like. 

Enjoy!!
Mike


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## jeff_t (May 15, 2014)

When I bought the 79cc kit, my dealer couldn't find any prices for 64cc parts. Part numbers, but no prices. I guess there isn't a big market for them when you can bolt on a horsepower and a half


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## Jon1270 (May 15, 2014)

jeff_t said:


> When I bought the 79cc kit, my dealer couldn't find any prices for 64cc parts. Part numbers, but no prices. I guess there isn't a big market for them when you can bolt on a horsepower and a half



Yep, the 6401 OEM piston is NLA except as part of an entire P&C kit.  In my case keeping in a 6401 with original cylinder and AM piston would get it working very cheaply, but I'm leaning towards getting a 7900 kit.


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## TreePointer (May 15, 2014)

+1

I vote for the 79cc kit.  If you go that route, you will possibly add a few tools (long torx bit, t-handles, compression/vac tester, etc.) and experience that you can use to diagnose/repair/mod future saws yourself.


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## Jon1270 (May 15, 2014)

TreePointer said:


> +1
> 
> I vote for the 79cc kit.  If you go that route, you will possibly add a few tools (long torx bit, t-handles, compression/vac tester, etc.) and experience that you can use to diagnose/repair/mod future saws yourself.



I already have the toolkit, and have repaired/rebuilt several saws over the last year or so.  Just deciding what to do with this one.


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## jeff_t (May 15, 2014)

I just needed a piston, too, though it looked nothing like yours. I thought about trying to locate one, but just briefly. 

The saw really is a pleasure to run. I have run a 440, a 460, and a 372, and they all cut similarly by my SOP meter, but I just like the way the Dolmar feels. Maybe just 'cause it's mine.


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## D8Chumley (May 16, 2014)

I'd like to have a 79 cc saw, but what would I do with it? I guess I'd have to cut bigger wood, but that sounds like a lot of work  Looking forward to a video of it running


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## Jon1270 (May 16, 2014)

D8Chumley said:


> I'd like to have a 79 cc saw, but what would I do with it? I guess I'd have to cut bigger wood, but that sounds like a lot of work  Looking forward to a video of it running



I have no practical need for a saw in that class, either.  Such a saw works just fine on smaller trees, and it's fun to buzz through wood in a fraction of the time it takes otherwise, but I only burn 2 - 2.5 cords a year; it's not as if I'm actually cutting often enough that I need to save time.  I could do fine with just a 50-60cc saw, but the big ones are fun.  Bought used, they hold their value well so it doesn't really cost that much to have one for a while and then re-sell it.


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## D8Chumley (May 16, 2014)

That's a good idea. I have 5 saws at the moment, if she found that out I'd be selling one or two haha


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## Jon1270 (May 16, 2014)

D8Chumley said:


> That's a good idea. I have 5 saws at the moment, if she found that out I'd be selling one or two haha



For me the dynamic is different because I only buy saws if I think I can sell them at a decent profit.  That usually means they need work, which is fine because I really enjoy working on them.  It also means my saw hobby produces a little income, rather than being pure indulgence, so my wife doesn't mind a bit.


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## D8Chumley (May 16, 2014)

Right, I could sell one or two if I got in a bind. I have the 025 now, the 170 might just take a back seat or get delegated to the jobsite wood cutting machine. I wasn't looking for this last saw but I couldn't turn it down either


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## zzr7ky (May 16, 2014)

I could also get by fine with the Dolmar 5100.  But then I got by better, with a little more power with the 6401...  Then I saw You Tube video of Dolkita 7900 vs Stihl 460... and now I have a project for it.  A 60" diameter Ash is on the roster of jobs .  No rest for the wicked


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## TreePointer (May 16, 2014)

Jon1270 said:


> I already have the toolkit, and have repaired/rebuilt several saws over the last year or so.  Just deciding what to do with this one.



I just realized that I confused a couple different discussions.  The idea of rebuilding the engine and getting tools and experience was for the "RIP 036" thread.  Sorry about that.


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## Jon1270 (May 17, 2014)

TreePointer said:


> I just realized that I confused a couple different discussions.  The idea of rebuilding the engine and getting tools and experience was for the "RIP 036" thread.  Sorry about that.



No problem at all.


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## jeff_t (May 17, 2014)

zzr7ky said:


> ... and now I have a project for it.  A 60" diameter Ash is on the roster of jobs .  No rest for the wicked



I whacked a solid 5' silver maple trunk a couple of years ago. Ran out of gas about a third of the way thru the second round  It was fun, though. Had to kick piles of chips out of the way so I could keep cutting.


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## Jon1270 (May 18, 2014)

I went to a get together yesterday, and a guy had a ported 7900 that repeatedly went through a 12" square poplar cant in just over 3 seconds.  The trouble with that sort of fun is that it's over so quickly.


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## Firewood Bandit (May 20, 2014)

Yesterday I weighed the 7900.  It is 1# less than the 372XPG and has substantially more grunt.


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## mstoelton (May 20, 2014)

If one of my 6421's goes down, I'm going with an aftermarket 84cc P/C kit.


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## AK13 (May 20, 2014)

Is this a difficult project to do? Does it require any specialized tools? Does someone who is mechanically inclined but has never taken apart a small engine have a chance to do this swap successfully?


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## dougand3 (May 20, 2014)

AK13 said:


> Is this a difficult project to do? Does it require any specialized tools? Does someone who is mechanically inclined but has never taken apart a small engine have a chance to do this swap successfully?


No, Maybe, Yes. I did Husq 55 for first time and it was simple and straight forward. I needed a long 5/32" Allen to go thru cyl fins to get to 4 mounting bolts.


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## Jon1270 (May 20, 2014)

AK13 said:


> Is this a difficult project to do? Does it require any specialized tools? Does someone who is mechanically inclined but has never taken apart a small engine have a chance to do this swap successfully?



It's pretty straightforward, roughly on par with an automotive brake job.  On the Makita, as on newer Stihl saws, the only even slightly unusual tool you absolutely need is a long T27 torx driver because that's the sort of fastener used to hold the cylinder in place (as well as most of the rest of the saw).  A short T27 bit in an extension will not work because extension won't fit into some of the holes it has to reach through.  If you are methodical and don't take shortcuts like trying to re-use the base gasket, you can get away with that torx driver, needle-nose pliers and not much else.  However it's really a good idea to vacuum and/or pressure test (preferably both) the engine after reassembling it, because an air leak resulting from some misstep can ruin your nice new top end.


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## zzr7ky (May 20, 2014)

Hi -
Is this a difficult project to do? :  I didn't find it difficult.  It was my first time cracking open a 2 cycle and expecting to to run again.

Does it require any specialized tools?   It was handy having a real T27 screwdriver, a $12 set of plastic piston ring compressing bands (off eBay), and a good awl to flip the piston wist pin retainer out.

Does someone who is mechanically inclined but has never taken apart a small engine have a chance to do this swap successfully?   Yes.  I did it.  There are guys who can do them in less than a half hour.  It took me 2 hours.

CLEAN it well first.  I spray with engine degreaser, let sit a bit, then rinse with hot water.  Repeat until clean.  I found good instructions on AS.  PM me if you like.


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## TreePointer (May 20, 2014)

Judge for yourself by reading this excellent step-by-step info posted on AS:

http://www.arboristsite.com/communi...00-piston-and-cylinder-install-w-pics.122884/

There's a 79cc OEM kit and a 84cc big bore kit.  The NWP 84cc kit page at BaileysOnline has a link to the REPAIR MANUAL in the Specifications tab:

NWP 84cc kit

Repair manual  -->  [1.62MB PDF] http://www.baileysonline.com/PDF/6400to79001_repairmanual.pdf


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## zzr7ky (May 21, 2014)

The instructions Treepointer supplied are the one's I used.  Worked like a charm. 

While cleaning the saw before the swap I found a few partsI wanted to replace.  On mine also replaced the plate that is between the saw and the bar, and the clutch drum.

Resistance is futile....


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