Dolmar 7900 / 7910 piston options

[jacob j.]

Is this a buy the bearing deal or dig for one?
I can dig if other candidates "pop up."

There's aftermarket available but I haven't held one in my hands so I can't speak to quality.

I would probably be tempted to use an 066 piston and get it cut down. I had a machinist turn down an 066 piston for a 288 I put together a few years ago
and that saw turned out strong.

 

[Lightning Performance]

I built up a Makita 6401 a few years back with an OEM 7900 cylinder and piston. I ported it and was very happy with the results. I subsequently gave it to my BIL who got a few years of service out of it. I just got it back to repair because wasn’t running right....

The impulse line was cracked and the piston is scored. Transfer is minimal on the cylinder.

1. What are the current piston options? Multiple old threads mention 046 or 372BB as possible options but none of the comparison pictures are live. OEM 7910 piston is $100.

2. What is the best aftermarket BB kit? I have another 6401 that I might do some swapping with and keep the ported cylinder.

Thanks!

I'm looking to do this for more area in the NPW kit. The longer skirt will help me limit intake timing.

 

[Deets066]

There's aftermarket available but I haven't held one in my hands so I can't speak to quality.

I would probably be tempted to use an 066 piston and get it cut down.

Wouldn't the 288 be a closer fit? And it's single ring

 

[jacob j.]

Wouldn't the 288 be a closer fit? And it's single ring

The 13mm wrist pin would need to be dealt with. Maybe a bushing bearing?

 

[dustinwilt68]

Got mine built tonight, I used a oem 7910 kit in mine. Not worth the risk to me but I get great prices

 

[Deets066]

The 13mm wrist pin would need to be dealt with. Maybe a bushing bearing?

Husky bearing is different size than stihl bearing. That's why you can use a 288 in an 066

 

[NPKenny]

Got mine built tonight, I used a oem 7910 kit in mine. Not worth the risk to me but I get great prices

I love the wrap... I think this chassis is perfectly balanced with the wrap handle.

The full background to this thread is that I built what I thought was the perfect saw a few years ago.... Makita 6401 converted to a ported 7900 with a wrap and 28” bar. I gave it to my brother in law because he coveted it and I was condensing my saw cache to Stihl only...

I’m back into saws and covet my old 7900 back. So I am trying to perform an honest repair on his and build another one for me the way it should be. It’s looking like OEM across the board, but I was hoping for a clever / economical fix.

 

[dustinwilt68]

Totally understand where you are coming from, I love this chasis too, my friend sent me the 6401 as a gift, had tan covers and needed a P&C but I could see the potential and the chasis was well maintained, next I plan on porting it, will be the first saw I do on my new lathe.

 

[jacob j.]

Husky bearing is different size than stihl bearing. That's why you can use a 288 in an 066

I'm well aware of that, having done both swaps myself many times over the years.

In this case though, the ID of the small end of the Dolmar connecting rod is the same size as the small end of a 440/460 connecting rod (15mm), so there's not much room to work with.

You'd be putting a 13mm pin in a 15mm hole.

 

[Deets066]

I'm well aware of that, having done both swaps myself many times over the years.

In this case though, the ID of the small end of the Dolmar connecting rod is the same size as the small end of a 440/460 connecting rod (15mm), so there's not much room to work with.

You'd be putting a 13mm pin in a 15mm hole.

Thought about that after I posted. As I just talked to a guy that put a 272 in a 7900

 

[Poleman]

This is a ported, and reported and again reported NWP Big Bore kit.... It pulled great but was having trouble getting enough fuel to it. I have been playing with modding the Walbro HD’s and ended up with this. It now has good throttle response and feeds it enough fuel.
I have squish set at .018 and it has over 200lbs of compression. It’s hell on starter parts as I do a lot of carb testing with it....you know how that can be....pull, pull, pull....

 

[Backtroller]

Nice Rich

 

[Terry Syd]

"I have been playing with modding the Walbro HD’s and ended up with this."

Nice job Rich, it looks like you've got the HDs pulling like the modded Zama twin jets. Did you split the circuits and drill the auxiliary low speed jet?

 

[Poleman]

Thanks Terry, no I didn’t split the circuits or drill an axillary jet. I went with a straight forward approach with this one. I did do some drilling and spring changes.

 

[Terry Syd]

I like your tenacity. I think I still have to drill the forward air bleed hole on my HD-5 another size bigger. It's a pain on these Walbros to pull the welch plug, drill, replace the plug and then test again. - Then pull the welch plug....

How many times did you have the carb apart before you started getting into the ball park, do you even remember?

You might try splitting the circuits on another carb just to test the difference. You get three benefits from it; first, the low speed circuit will remain richer when you pull the revs down, second, the idle will be smoother and third, you can tune the low speed circuit independently from the high speed circuit.

Its a shame that Walbro didn't use 'best practices' when designing the idle/transition circuit. I like the Walbro venturi and the position of the air bleed holes, if they had done the idle circuit better it would have been a great carb and a lot easier carb to mod. Carb circuits have been around for more than 100 years, so its not like the information is 'secret'. - Both Zama and Mikuni idle circuits use virtually identical idle circuits (the Mikuni has a replaceable jet however).

 

[Poleman]

Yes, I want to try splitting the circuits to see what it does and to learn!!!!!
How many times.........aaaasshhhhhh oooooohhhhh, three that I can recall but it was in a box of carbs for retest that I finally got around too. Had repaired some Zama and a couple single jet Zama copy’s and the line Walbro. I really wasn’t expecting it to work this well but wow.....I really liked it. The main issue I was having was the slooooow throttle response but this one doesn’t have this issue.
I’ve been starting to do all kinda crazy stuff with different carbs.....some work some don’t. It’s just kinda a passion I’ve have and being able to repair older unattainable carbs for good saws that need them. I’m just really slow on some as I’ve got it totally apart checking passages and where they go and what they do....how many passages are connected to it.....Terry I know you know what I’m talking about!! I know you’ve spent hours doing the same. I like thinking about things while I’m running a saw.....nice medicine!! Lol

 

[Terry Syd]

Yeah, what's with the engineers? Do they think they need to come up with a new and obscure carb circuit EVERY time they design a new carb? It is almost like some perverse joke - hold my beer, I got another idea...

You might want to try drilling that auxiliary jet in the HD low speed circuit. The jet discharges between the two air bleed holes. It quickly bumps up the low speed fuel flow as the throttle cracks open. Great for picking up some 'grunt' off the bottom - like for starting a saw in the cut.

 

[NPKenny]

I am the the OP for this thread... I forgot that I hadn't updated this thread after I made the repair.

I cleaned up the OEM 7900 cylinder, then cleaned up the scored piston and installed a 52mm Caber ring. I rebuilt the carb and replaced the impulse line. The saw runs like a champion again. I don't know that I would recommend this for all, but for the intermittent firewood this saw will see, I think it was a fine fix. If I need to go deeper into it in a few years, that will be fine by me as well.

These are great saws.