New Dolmar 7900 piston

[tdi-rick]

Yeah I know mate...just having a little fun. That was 15 years ago though!

ditto, and what's fifteen years ?

 

[AUSSIE1]

ditto, and what's fifteen years ?

Yeah...well I'd like to have 15 years back! :msp_thumbup:

 

[cheeves]

Yes. Dolmar requests that customers mold a cylinder around the new piston using Play-Doh and have a competitive build off to see which one has the best performance.

E&E will start carrying the cylinders when the winning one gets picked.

You know the Germans aren't just crazy. They're eccentrically brilliant crazy.

 

[ncfarmboy]

New "lightweight" piston for the 7900 for "better motor operation". Weighs 1/2 oz less than the original piston.

They did the same thing with the 510-5100S piston. New piston looks just like the New 7900 no windows. Have not weighed old windowed vs nonwindowed but ribs are thinner. Too thin to cut windows in. Good news on 5100 piston it's about 2mm wider.
Shep

 

[Urbicide]

Lighter? Don't they read AS? A wider skirt would have been No 1 on the list!

I was hoping for a non-limited ignition module. Wonder if improper tuning with the limited ignition was the reason for coming out with a lighter piston? How can I say this, too much of a shock load on the crank, con rod, & bearings as the ignition turns itself off & on at 13,500 RPM? Lower the overall mass of the moving parts & make it easier & safer for the limited ignition to do its intended purpose without undersireable side effects. The piston would be the most logical place to try to reduce the weight without sacrificing strength.
opcorn:

 

[jimdad07]

I was hoping for a non-limited ignition module. Wonder if improper tuning with the limited ignition was the reason for coming out with a lighter piston? How can I say this, too much of a shock load on the crank, con rod, & bearings as the ignition turns itself off & on at 13,500 RPM? Lower the overall mass of the moving parts & make it easier & safer for the limited ignition to do its intended purpose without undersireable side effects. The piston would be the most logical place to try to reduce the weight without sacrificing strength.
opcorn:

I agree on the piston weight. Whenever I port one of my saws I lighten the piston up as much as possible. I like the idea of the new piston being lighter.

 

[steve316]

new 7900 piston

not only did they make the piston lighter the new kit has a tapered pin with would take off a couple of grams.:msp_thumbup:Steve

 

[nstueve]

Reviving old thread...

Ok so I've got a little extra $$$ that I need to get into a P&C combo for my stock 6401... Need something to run a 28in bar on an alaskan mill and i'm hoping that a 7900 kit will be the ticket! Planning on HD filter and P&C kit. I plan to clean any casting from the intake and exhaust to get better flow... just trying to do the best i can with what i got!

3 questions:

1.) I've heard the 54mm big bore 84-85cc kits aren't much better and to go with the stock 52mm P&C???? YES/NO??

2.) Have we come to a consensus on whether the new or old piston is better?

3.) Metal base gasket or yamabond to get more squish?

 

[Jacob J.]

I agree on the piston weight. Whenever I port one of my saws I lighten the piston up as much as possible. I like the idea of the new piston being lighter.

You don't want to go too light though. Apart from affecting the integrity of the piston, you change the scope and "frequency" at which the
motor vibrates. I did this on an 066 once. I took too much material off the piston and the saw ran good, but the change in vibration was
so irritating, I hated running it. I ended up taking that piston out and putting a new Meteor in.

I have found though that you can eliminate some vibration in the bigger saws by making the piston somewhat lighter. I've
never tried a shaved flywheel or lightened crank in a work saw though. Too many potential issues there for me.

 

[nstueve]

any one got a response to the questions I posted 2 msgs ago????

 

[IthacaMan]

Ok so I've got a little extra $$$ that I need to get into a P&C combo for my stock 6401... Need something to run a 28in bar on an alaskan mill and i'm hoping that a 7900 kit will be the ticket! Planning on HD filter and P&C kit. I plan to clean any casting from the intake and exhaust to get better flow... just trying to do the best i can with what i got!

3 questions:

1.) I've heard the 54mm big bore 84-85cc kits aren't much better and to go with the stock 52mm P&C???? YES/NO??

2.) Have we come to a consensus on whether the new or old piston is better?

3.) Metal base gasket or yamabond to get more squish?

I went with the oem 79cc with the metal gasket. Haven't read about anyone using the new piston.

 

[atlarge54]

Ok so I've got a little extra $$$ that I need to get into a P&C combo for my stock 6401... Need something to run a 28in bar on an alaskan mill and i'm hoping that a 7900 kit will be the ticket! Planning on HD filter and P&C kit. I plan to clean any casting from the intake and exhaust to get better flow... just trying to do the best i can with what i got!

3 questions:

1.) I've heard the 54mm big bore 84-85cc kits aren't much better and to go with the stock 52mm P&C???? YES/NO??

2.) Have we come to a consensus on whether the new or old piston is better?

3.) Metal base gasket or yamabond to get more squish?

I just put together a 54mm BB kit and feel the quality has improved over the past couple years. There's a thread on the Baileys forum. Grande Dog mentioned the BB kit might fit milling application better than the OEM 79cc.

 

[parrisw]

I've had no complaints about my BB 6401.

 

[jimdad07]

You don't want to go too light though. Apart from affecting the integrity of the piston, you change the scope and "frequency" at which the
motor vibrates. I did this on an 066 once. I took too much material off the piston and the saw ran good, but the change in vibration was
so irritating, I hated running it. I ended up taking that piston out and putting a new Meteor in.

I have found though that you can eliminate some vibration in the bigger saws by making the piston somewhat lighter. I've
never tried a shaved flywheel or lightened crank in a work saw though. Too many potential issues there for me.

I see what you are saying. Must be I haven't gone too far as I haven't noticed any increased vibration. Here is a pic of a before and after of what I did to one of mine, it doesn't look like I took much off, but I took a good amount off very carefully.

 

[blsnelling]

Ok so I've got a little extra $$$ that I need to get into a P&C combo for my stock 6401... Need something to run a 28in bar on an alaskan mill and i'm hoping that a 7900 kit will be the ticket! Planning on HD filter and P&C kit. I plan to clean any casting from the intake and exhaust to get better flow... just trying to do the best i can with what i got!

3 questions:

1.) I've heard the 54mm big bore 84-85cc kits aren't much better and to go with the stock 52mm P&C???? YES/NO??

2.) Have we come to a consensus on whether the new or old piston is better?

3.) Metal base gasket or yamabond to get more squish?

1. I'd go with the OEM topend.
2. I've never seen one. I doubt it makes much difference.
3. Whatever you measure the squish to be will dictate what you do.

 

[nstueve]

i've heard conflicting thoughts on the BB... it is cheaper and bigger but oem stands to have higher quality from Mahle... I can almost get a HD filter and BB kit for the same cash as the OEM 7900 kit alone. Conflicting thoughts have taken me back and fourth but the gifting of an alaskan mill to me has pushed me to look at more power for milling down logs... I'm wondering what kind of reliability people are seeing from the NWP kits. Good or bad...

 

[parrisw]

i've heard conflicting thoughts on the BB... it is cheaper and bigger but oem stands to have higher quality from Mahle... I can almost get a HD filter and BB kit for the same cash as the OEM 7900 kit alone. Conflicting thoughts have taken me back and fourth but the gifting of an alaskan mill to me has pushed me to look at more power for milling down logs... I'm wondering what kind of reliability people are seeing from the NWP kits. Good or bad...

For milling, I wouldn't even use that saw. The bigger the better. 90cc + all the way.

 

[jimdad07]

i've heard conflicting thoughts on the BB... it is cheaper and bigger but oem stands to have higher quality from Mahle... I can almost get a HD filter and BB kit for the same cash as the OEM 7900 kit alone. Conflicting thoughts have taken me back and fourth but the gifting of an alaskan mill to me has pushed me to look at more power for milling down logs... I'm wondering what kind of reliability people are seeing from the NWP kits. Good or bad...

When I first got into saws it was for firewood and for milling to support my woodworking habit. The first pro grade saw that I ever bought was my 6400 and I did some milling with it wearing a 28". I wouldn't recommend doing that constantly but it held up to it well and cut pretty well up to 20" logs. If you are going to mill with it I think that I would still go with the OEM 79cc p+c for a couple of reasons, one being that the crank and carb are meant to handle up to a 79cc p+c which to me means you can run it richer for milling and the other is that the 7900 is a power house of a saw and seems to do well with milling.