Here are comparison pictures, Solo 665 on the left Dolmar/Mikita 6400 on the right. Both saws use exactly the same 47mm windowed single ring piston. Note that the Solo has the decomp positioned on the cylinder top and the Dolmar/Makita on the side.
Interesting that the design of the jugs is totally different. The Makita/Dolmar jug has six open transfer ports while the Solo is a more conventional quad-ported design. What happened? Two engineers at a standoff drew straws as to whose design went on what brand?
The dolmar 6400 7300 7900 was a solo dolmar joint design. Something happen between the 2 companies and solo went on to come out with the 665 675 681. So some changes were made for the better we hope.
After 35 years at Xerox it is more likely that Mikita told Solo we will offer a 60cc saw if you can meet this cost point. So Solo choose a less expensive design. JMO!
This is a from a dealer of both that shared info with me. i posted this in 09. Here is some info that was passed on by a solo,dolmar,redmax dealer when the 681 first arrived. I have found in my own experience that it is difficult to find a local dealer for solo and that the online parts ordering doesn't seem to work for the 681. This was when the 681 was first released. (((((The new Solo 681 is now here, 81cc (80.7) same basic weight as the 7900 but with some major improvements. The 681 has a larger air filter then the 7900, that is why the air filter cover is shaped a little different. 681 also has the good Walbo carburetor not the typical cheap Zama from China like the Dolmar. 681 has the side chain tensioner in the case like a Stihl not in the side cover like Dolmar. 681 also has a large top AV spring. The Dolmar had problems with them breaking. 681 also has a better bracket set up on the AV mount to the top handle. 681 bracket is bolted to the top cover. The Dolmar has a strap that wraps around the handle and it is always turning and putting stress on the top av spring. 681 has a two year YES TWO YEAR commercial warranty. Parts for the Dolmar are a problem, you have to find a dealer. Parts for a Solo can be ordered online from Solo at the same price they cost from the dealer. At first you will think that it is built by Dolmar. That is not the case. This saw is built by Solo. Solo does sell the large Dolmar 9010, they call it the 694 but the 681 and its smaller sisters 675, 665 are Solo built. From what I understand is that Dolmar did not design all of the 7900. Solo designed a lot of the parts for that saw. They were to be sold together in joint venture like the large Dolmar 9010. Dolmar took all of the credit for the 7900 and Solo got the shaft. Solo took it one step farther and made the same basic saw with a larger engine and a few improvements. OK I have had a lot of questions about installing the 681 parts on the 7900 to make it 81cc. Both of the above saws have the same bore size of 52mm. The 681 jug will bolt right on the 7900 case the muffler and carb boot will line right up but the jug is taller to accommodate the longer stroke so you will loose compression with the 7900 crank. I am sure that you can put the 681 crank in the 7900 case. That is a lot of work and with the cost of the crank and labor its not economically feasible.
key points At first you will think that it is built by Dolmar. That is not the case. This saw is built by Solo. Solo does sell the large Dolmar 9010, they call it the 694 but the 681 and its smaller sisters 675, 665 are Solo built. From what I understand is that Dolmar did not design all of the 7900. Solo designed a lot of the parts for that saw. They were to be sold together in joint venture like the large Dolmar 9010. Dolmar took all of the credit for the 7900 and Solo got the shaft. Solo took it one step farther and made the same basic saw with a larger engine and a few improvements.
