Makita 6401 to 7901 Plus porting the 7900 and the Solo 681

[Mastermind]

Yeah, we are covering a lot of ground in this thread, but these saws are all similar, so I thought we could lump em together and maybe see what makes em all tick.

First of all we have a Makita 6401 here that I've had for about 4 years. It's always been a damn good running saw, so I never took the plunge into turning it into a 7901. I ended up getting a rough looking 7900 that had a hole in the fuel tank......the top end was very nice, so.........why not?





To make one of these into a 7900 all you have to do is swap the top end, the intake boot, carb, muffler, etc is all the same. Just change the jug and slug, and you go from 64cc to 79cc.

 

[Mastermind]

So, we have two Dolmar 7900 cylinders, and one Solo 681 cylinder here. Can y'all tell which is which?

 

[Mastermind]

Let's take a look at the 681 jug.....

 

[Mastermind]

The 681.



Now a 7900.



See the differences in the lower transfers?

 

[Jacob J.]

Those Makitas sure are ugly with that HD filter cover.

 

[Mastermind]

Those Makitas sure are ugly with that HD filter cover.

Bite your damn tongue. This saw is for sale.

 

[mdavlee]

The 681.



Now a 7900.



See the differences in the lower transfers?

So that was worth the extra $200 for the Top end. [emoji6]

 

[Jimbo209]

They look about half the dolmar

 

[Mastermind]

So that was worth the extra $200 for the Top end. [emoji6]

Not to me either.........it is interesting to see any differences. You and I both know how bad ass these 681s are stock.

They look about half the dolmar

The Dolmar has had some relieving done on the right hand side. Making this comparison was an after thought.

 

[mdavlee]

Yeah I never really looked at the 681 top end close. It must be something to It to get that 5% over a 7900.

 

[Mastermind]

OK, now we're gonna look at how we cut the squish.

I've got a bunch of different plates made up with different bolt patterns for different saws. All you need is a 4" x 4"x 3/8" or thicker steel or aluminum plate, or some all thread.



Now, this is sorta important.........you need a pivot point under the jug.



Some washers and nuts, and it's ready to go in the four jaw.

 

[Mastermind]

Yeah.....you need a independently adjustable four jaw.

 

[Mastermind]

A little trick to make it easier.......

Once you find the top of the plating with the indicator, make a mark that you can come back to on the ways.



Same thing when you find the point at the bottom of the bore you want to indicate at.......

 

[Mastermind]

Then you have to get all the runout out, at the top, and at the bottom of the bore. Back and forth, till it is perfect.

 

[Mastermind]

Once all the indicating is done, cutting it takes maybe 5 minutes max. I also cut the cylinder extensions while the jug is mounted.





Then slick it up with a diamond burr.

 

[Mastermind]

Now for a few pics of the roughed in port work. This is a 7900 jug in these next pics BTW.



I had just unshrouded one side of the lowers in these pics........I went back and cut out the others later. Sorta changed my mind after looking closer at the 681 jug.

 

[Mastermind]

Now a few shots of the finished jug......

 

[Mastermind]

I'm working on the 681 now........

Here's the stock port timing numbers with a base gasket.

Ex: 102°
Tr: 130°
In: 79°

 

[Mastermind]

We forgot to take pics of the muffler work........but here's the 7901 outlet.

 

[Mastermind]

That's all I got for now........more on the Solo tomorrow.