Solo 665 to 681 Conversion

Spoke to Dexter briefly last night. If time permits, we are going to test the 665 conversion to 681 with the stock windowed Solo piston and with the slab sided Dolmar 7900 piston. So we may be able to have a report on how the Dolmar works in a stock 681 compared to the Solo windowed piston.

Waiting for the 681 plaques, then the kits, gaskets, plaques and the 7900 piston are off to Dexter and Dewey on Monday perhaps Tuesday. Dexter's OCD REQUIRED that if the saw was going to end up a 681 it needed the appropiate plaque stating that it is a 681! Good for him, I would have blown that detail off, LOL!

 

Interesting feature on the original 7900 piston is that it has asymmetric windows on the piston which was likely to stagger flow on the transfers and improve exhaust scavenging/ lessen charge/air losses out the exhaust. Some engineering/thought went into its design so hard to understand why the change to the slab side. Thought I read a while back that the 7900 crank design was changed? Perhaps this had something to do with the piston. Lightened??

 

I always wondered about the staggered transfers...
Never even occured to me the engineers would try it via piston window size...

Learn something new every day...

 

Good stuff, Adam. I have a windowed Dolmar 7900 piston round here, I'll compare it to the Solo. Better yet I'll get a picture of the Solo piston and put it up here. I also read that the case volume is so 'on the edge' in the 7900 that putting a BBK on them provided less than satisfactory results.

 

Solo piston out of the kit on the left, Dolmar slab side on the right.

Oh and the plaques came this afternoon.

 

They are a step down in performance IMO... But will pull a big bar decent..

 

MM, great idea, unfortunately, I have the crank but no saw to try it out on. I'd do it, but don't have a dolnar saw to try it on, thats why I'm looking for a very clean 6400.

 

Dex, packed, ready to go! UPS will pick up Monday!
Cheers
KRL

 

Tried the BBK on a stroked 7900 and it worked ok. Wasn't worth the $$$/effort

 

OK, Adam, I guess it was a nice idea but no more!

Thanks
TJ.H

 

What I would be more interested in is a 675 to experiment with. I have some ideas that may or may not work. I'll find out eventually though.

 

What are you thinking Mike?

 

50mm bore gives a ton more piston options in solid side and windowed. Pick one to match the intake timing with no windows and then make a spacer. 372 piston is going to be close. It's just a good bit taller so it will take a spacer and maybe a little machine work. Thinking it might be like building a stroked 372 but not having to pay for the stroker crank. I haven't had time to put it together and see where the timing will end up.

 

Mike, I have a 665/675/681 NOS crank here, 675 P/C kits are actually harder to find than the 681! Why, who knows! If I could find one I would try it. What if you used a Dolmar 7300 as a donar? Don't know about timing differences or porting between the 7300 and 675 but it is a thought.

 

I have the 7300 top end. I just couldn't bring myself to start swapping parts onto that 665.

 

It would be an extra 1mm on top of the other 3mm to get the piston to have squish.