Makita / Dolmar 7900 Piston and Cylinder Install w/ PICS!!

[ECHO-Echo-echo]

So stock50s, do you still have the saw and how is it running now? It should be broke in by now?
I'm looking to do basicly the same thing down the road. I'm scouting around for a deal on an oem 7900 kp/c kit. and the xl air filter.

 

[ncfarmboy]

He lucked out and got an old stock P&C. The piston has windows. The new P&C kits come with a "lighter" non windowed piston. Got two of em to do. One on a new 6401 Makita and one for a 665 Solo to make it a 681. Mine came from Bailey's did not have a gasket of any kind in the box. Be careful tuning the limited coil will fool you.
Shep

 

[ancy]

He lucked out and got an old stock P&C. The piston has windows. The new P&C kits come with a "lighter" non windowed piston. Got two of em to do. One on a new 6401 Makita and one for a 665 Solo to make it a 681. Mine came from Bailey's did not have a gasket of any kind in the box. Be careful tuning the limited coil will fool you.
Shep

You think the old piston is better? Why? There is another thread about it and it is lighter. http://www.arboristsite.com/chainsaw/182257.htm

 

[ncfarmboy]

You think the old piston is better? Why? There is another thread about it and it is lighter. http://www.arboristsite.com/chainsaw/182257.htm

They are lighter I have 2 of them. Don't know personally if they are better just going by what I've read. EC, Ctown etc all run windowed in their WP and race saws must be a reason. TW, EC cut windows in their WP 372, 359 to name a few. Must be a reason. My thinking is that it directs the fuel air charge to the transfers sooner and cools the piston better.
Shep

 

[stock50s]

Update

I haven't been on the forum for a while so I apologize for taking months to respond. The saw is still running strong and well broken in by now. I see that the piston was redesigned without windows. Can anyone confirm the motive behind that? It is interesting they would "improve" the old design without good reason.

 

[nstueve]

I haven't been on the forum for a while so I apologize for taking months to respond. The saw is still running strong and well broken in by now. I see that the piston was redesigned without windows. Can anyone confirm the motive behind that? It is interesting they would "improve" the old design without good reason.

I'm no engineer so this is all conjecture...

The new piston has no windows so the initial flow of gas mix will be slowed. IE less to burn = less emissions
The new piston is also lighter thereby reducing "resistance" and increasing power. letting less fuel through also results in lower fuel consumption.

Basically I would guess it was a move to make same power with less emissions just by changing the piston casting. If I were "building" a saw I would want the old style that can be ground on a little to make them lighter and flow better through the windows.

 

[dntbdmb]

thank you stock50s!

I know this is an old post, but the write up and knowledge of all the other posters on this site has helped me tremendously. I just purchased a Makita 6421, with the mahle 79cc kit. Upon receiving everything, I started the saw, just to make sure it ran and then tore it down. The whole process took about 2 hours, and I have never done one before.

The 6421 is slightly different. The decompression valve was relocated to the top right side, and the h setting on the carb appears to have a double d set screw. The decompression valve has way too much thread to seat all the way in, about half an inch of thread hangs out. I just bottomed it out and snugged it up. I had to drill a new hole in the plastic to relocate it, I did this with a step bit, I was off a little. I am considering buying another valve, so it seats better, but am holding out to do a compression test to see if its even an issue worth worrying about.

I am currently waiting on Amazon to deliver the double d adjusting tool before I run the saw wide open and set it with a tach. I plan on gutting the muffler tomorrow, if time allows.

Again, thank you for a superb write up, made my life much easier!

 

[ancy]

Sounds like your on the right track. I do have a almost new set of 6401 top plastics that would line up with the decomp on the 7900 top end, if your interested PM me.