It's the little things......

[Mastermind]

Dolmar 7900/10 piston weights.

Old style windowed piston.

 

[Mastermind]

New style piston.

 

[Mastermind]

 

[dustinwilt68]

Wow Randy, no wonder everyone recommends the 7910 piston, I have a showdown later today with a 7900 that is getting a new oem 7910 P&C.

 

[Mastermind]

Wow Randy, no wonder everyone recommends the 7910 piston, I have a showdown later today with a 7900 that is getting a new oem 7910 P&C.

I wish they would have left the ring locating pin in the same place......but for some reason they put it over next to the transfer port.

 

[Simondo]

So with the lighter and "None Windowed" piston (like iv seen inside the 5105 ) any change in the cylinder ?
We have this situation with only being able to get 1 design of Meteor piston (none windowed) for 242 xp where they were windowed pistons from factory.
Be interested how/if Dolmar have been able to put a slab side in if they haven't made cylinder changes. Whada you think ?

 

[TJ the Chainsaw Mechanic]

Outta be a little snappier.

 

[Mastermind]

So with the lighter and "None Windowed" piston (like iv seen inside the 5105 ) any change in the cylinder ?
We have this situation with only being able to get 1 design of Meteor piston (none windowed) for 242 xp where they were windowed pistons from factory.
Be interested how/if Dolmar have been able to put a slab side in if they haven't made cylinder changes. Whada you think ?

The jugs are the same design.......with the exception of the exhaust port (because of the muffler change) and the port widths. The way it feeds from the bottom is the same. The non-windowed piston seems to be to provide more primary compression, and better direction to the transfer charge.

 

[dustinwilt68]

I would have to think a 7910 piston swap would make a saw feel like it has a better throttle reaponse.

 

[Mastermind]

I would have to think a 7910 piston swap would make a saw feel like it has a better throttle reaponse.

I'm doing a 7900 with the new piston right now.

 

[srcarr52]

Darn, that sucks about the ring pin location. Other than that it's a great piston design, light and strong.

 

[Mastermind]

Darn, that sucks about the ring pin location. Other than that it's a great piston design, light and strong.

I gave up widening the uppers a few years ago anyway.

 

[dustinwilt68]

Let us know please what you think when it is done. Could be a nice simple mod for the regular guy.

 

[mdavlee]

That's my complaint on the piston with the ring ends. I've had good luck with the windowed pistons though.

 

[srcarr52]

I gave up widening the uppers a few years ago anyway.

I don't really widen them much towards the intake any more either.

 

[Mastermind]

I don't really widen them much towards the intake any more either.

And in your reply I can tell that you are likely doing the same thing I am on certain saws. Deepening the pocket and widening toward the exhaust when needed.

 

[srcarr52]

And in your reply I can tell that you are likely doing the same thing I am on certain saws. Deepening the pocket and widening toward the exhaust when needed.

Yup, a lot of work under the bridge.

I'd still like to see a better ring pin location... they scare me when they are that close to the transfer port.

 

[Mastermind]

Yup, a lot of work under the bridge.

I'd still like to see a better ring pin location... they scare me when they are that close to the transfer port.

Agreed.

 

[wcorey]

So with the lighter and "None Windowed" piston (like iv seen inside the 5105 ) any change in the cylinder ?
We have this situation with only being able to get 1 design of Meteor piston (none windowed) for 242 xp where they were windowed pistons from factory.
Be interested how/if Dolmar have been able to put a slab side in if they haven't made cylinder changes. Whada you think ?

The 7900/7910 transfers are bottom fed so not as dependent on the piston windows. Not sure why they used the earlier design.

The 242 however is side fed so the windows play a much bigger role. If you're stuck with using the non windowed piston it might help to drag/grind the 242 lowers down as much as the base/skirt allows.

 

[Nitroman]

Drill those windows and make the piston lighter. Pull the pin, drill the hole undersized, ream undersized, warm piston to 700-800* and press pin back in. Peen top and bottom so will never come out.