Finally Going To Do One, Just To See

Been reading and been pestered by one customer to build a saw with a stroked crank. The Solo branded versions of the Dolmar 64/73/7900 saws used a 1mm stroked crank, but the saws are long gone. However Solo claims to still have the crankshaft available as a spare part. So, I ordered one, we will see if they actually ship the part.

I have a NOS Dolmar 7300 and a complete current OEM 7900 C/P to use. So the process will be to remove the 7300 crank and replace it with the Solo stroked crank with no other changes. Run it for a while to see how it goes then do a basic WP on the saw and see how it goes then.

 

Cool. Cant wait to see how it turns out.

 

In theory, how should a stroked crank affect the way a saw runs? If I understand correctly it lengthens the piston stroke and increases displacement, but I guess that also affects port timing and all sorts of other stuff I don't really grok.

 

I'm no expert by any stretch, but is there enough clearance at the bottom of the stock jug for that?
Oh, and.......

 

Correct Jon1270, it should give about 2cc more displacement and because of the 1mm longer stroke increase torque. There have been all sorts of stuff published about doing this thing and its' advantages and problems. Supposedly the best method is to use the C/P off a current 7900. However you have to place a 1.5mm spacer under the jug to get back to correct timing.

You are also correct papadave hence the need for the spacer!

I also have a 'box stock' 7900 that I can run this saw against to see/compare the differences. All of this is however dependent on Solo actually being able to supply the crankshaft!

 

So how does increasing displacement by making the stroke longer compare to doing it with a bigger bore?

 

Bigger bore gives more horsies, and longer stroke increases torque.
Don't know how or why.......cue KRL.
Will the increased length make up for the lost compression by moving the jug down with a spacer?
Trying to get this straight in my feeble mind.

 

I know years ago the race crowd was using the 681 for a 5 cube since you could buy them off the shelf instead of hoping to find a thin ring 281. There is a difference in power between the 7900 and 681. The cylinders are different also. The solo jug is way more expensive than the dolmar.

 

Actually the spacer is raising the jug to get back to proper stock timing, so there should be no loss in compression. Since the crank has 0.03937 more stroke, the timing will be off if you don't use it and most likely the squish would be dangerously close. Longer stroke equals more torque and if you think about it torque does all the work, chain speed is all well and good however the saw is pulling the chain through the wood and that takes torque. The more of the bar you have buried the more torque it takes.

If you look at the older designs of saws (Jonsered 820 comes to mind) they did not turn anywhere near the RPMs the newer saws do, but they were impossible to stop in the wood because of the torque they made! There are three ways to design an engine. The relationship of bore to stroke can be square B=S, under square B>S or over square B<S. Under square is an RPM machine, over square a torque machine and square a balance.

 

Mdavlee, correct, Solo jug P# 22-00-522 current price $236.00. And IIRC you have to putz around with the muffler. Might call Solo on Monday to see if the P/C kit for a 681 is still available!

 

Increasing the bore can do two things increasing the bore in a square engine will move the engine toward under square and increase RPM, increasing the bore in an over square engine will move the engine toward being square, RPMs will decrease but the max torque and max RPM curves should come closer together.

If you look at older MX bikes like a 400 Cross (over square B=81mm and S=86mm) these engines were taller than say a Suzuki 400cc machine because the Suzuki was WAY under square. Saw manufacturers take the size of the finished saw into consideration when designing the engine. It becomes a balance of power and the compactness of the finished package.

It would be crazy to put some long stroke thumper type engine in a chain saw!

 

....because, I assume, the RPMs would be so low and the torque so great that it would yank the saw out of the operators hands, smash in the front of the crankcase and/or break the dogs instead of cutting anything?

 

Not to mention the fact that the saw would be extremely unweildly, and all the other stuff you mention. Although they did do it, look at a Strunk!

 

The are already out there IMO. Look at the torque monster saws with their longer strokes and smaller bores in same cc range as others. Love torque in a saw.

Owned a stroked 3120 and if I wanted to pay $400 more I could have had a 2mm stroked crank in a 372 I had.

 

Stroked 3120 on muffler with a dubbed up pretty dang dull square chain. Just goofing off making some cuts with 3/8 12T or 13T rim sprocket.

 

Yowza. But then the biggest saw I've ever run was about 85cc so I have no idea how much of that to attribute to the stroked crank and how much is just normal for a 120cc saw.

What does "dubbed up" mean in this context?

 

Also, does the variety of rim sprockets and chain pitches available make the question of whether a saw tends to be high-revving or torquey kinda not matter much, since you can just gear it up or down?

 

A retired saw racer above, watch the seconds on the video and see how fast Kevin cuts three cookies off the stick, if you can count and keep track of the saw, I cannot!

Torque really is king and watching Kevin you will notice that the saw just rams its' way through the wood! The 3120 has a 60mm bore and a 42mm stroke in stock form, so adding the stroked crank to the saw bumped up its' torque but Kevin does not say how much stroke was added to the crank!

 

Normally if the engine has enough torque to pull the taller gearing, no problem however some saws can't pull the tall gears because of low torque. some chain is easier for the saw to pull but race chain is usually square cut. Overall gearing changes can only take best advantage of what the saw has, they will not increase power or torque! I think Kevin was referring to the chain not being up to what he would actually run in racing conditions. You will also note his reference to 'just goofing off'!

 

Gearing does not change the saw at all, it just makes the saw more suitable to your current purpose. The same reasoning is why your 4X4 has 4L,4H and 2H.