What is full crank and does it matter?

[cruzenmike]

Last year I had tried a Ryobi 2-cycle "full crank" power head and I was not satisfied with how it worked. I am not sure if I had a bad unit or if the experience I had was due to a significant change in the engine design over my previous power head. Long story short, the new power head just didn't seem to have the power that my older unit had. The engine would not rev up as high when connected to an attachment or when loaded up. This was most noticeable when using the blower attachment; it just wouldn't move as much air as it did with the older engine.

Exactly what is "full crank" and should I even consider this to be important in a new power head?

I am looking to replace my current power head and I can either get a new old stock power head that is the same as mine, but no warranty, or the newest Ryobi/Toro/Cub Cadet unit that under warranty and still compatible with my attachments.

 

[Romore]

The 'full crank' has bearings at each end and is definitely the stouter of the two. The cantilever or 'half crank' has the pin sticking out in mid air, it is much cheaper construction and can be spotted easily by the rewind between the engine and the shaft. I won't buy or work on them.

 

[bertsmobile1]

think of a built up crankshaft on a motorcycle

There is the drive shaft which presses into one flywheel which then has the big end journal followed by the other flywheel and then the other shaft , usually with the alternator or cam gears on it.
That is a full crank, usually running in a pair of bearings near where the shafts join into the flywheels.

Now imagine the output shaft is twice as long with a bearing at each end.
This shaft presses into the flywheel and the flywheel has a journal with the big end on it, retained by a bolt through the journal and a thrust washer so there is no second flywheel and no second shaft.
That is a 1/2 crank, or over hung crank.

Principle advantage is fewer moving parts & easier assembly so it is up to 2/3 cheaper to make.
Downside is all of the forces created by the piston's movements & the rotation of the crank are supported from one end so it is unbalanced.
SO where you can cheap out on the crank shaft, you have to make the crankcase a lot stronger.

I could start listing the pro's & con's of either design, but much like an oil thread it would go forever & no 2 people would be in full agreement.

My all time favourite push mower engine, the Victa Power Torque has an overhung crank and there are hundreds of thousands of the original 1982 engines all around the world still going strong, except in the USA where they were banned.

When it comes to small hand helds it is purely for cost reasons.
Cheaper brands, Ryobi, Homelight & many store brands principally use 1/2 cranks

Quality long life brands like Stih, Echo, Shindawa, Kas all use full cranks

Generally a 1/2 crank has the pull start at the shaft end, because there is nothing to pull on at the other end.
Having said that I have pulled apart a couple of cheap home brand ones that are using the Victa system which has the starter at the other end as it would be well out of patient by now.

 

[primerbulb120]

Last year I had tried a Ryobi 2-cycle "full crank" power head and I was not satisfied with how it worked. I am not sure if I had a bad unit or if the experience I had was due to a significant change in the engine design over my previous power head. Long story short, the new power head just didn't seem to have the power that my older unit had. The engine would not rev up as high when connected to an attachment or when loaded up. This was most noticeable when using the blower attachment; it just wouldn't move as much air as it did with the older engine.

Exactly what is "full crank" and should I even consider this to be important in a new power head?

I am looking to replace my current power head and I can either get a new old stock power head that is the same as mine, but no warranty, or the newest Ryobi/Toro/Cub Cadet unit that under warranty and still compatible with my attachments.

As far as what "full crank" means, Bert's explanation above is the most detailed explanation you could ask for.

About those Ryobi Full Crank power heads: They are all wimpy, it's not just yours. The engine isn't all that powerful to begin with, and it's also tuned too lean from the factory to achieve full power. Also, the clutch bearings like to fail if you use them a lot with an edger or blower attachment.

The new Toro power heads are a red version of your Ryobi. Same exact engine, same lack of power. If you don't like the Ryobi, you won't like the Toro.

The Cub Cadet/Craftsman/Troy-Bilt 27cc power head is actually quite powerful when tuned correctly. It's too lean right out of the box and won't achieve full power without adjustment.

Another unit to consider is the Husqvarna 128LD or 128CD. Doesn't matter which one you get, it's the same power head but the CD comes with a curved shaft attachment and the LD with a straight shaft attachment. It's the closest thing to a commercial grade unit that will still work with all your attachments. The engine is large, has good power, and doesn't vibrate as much as the cheaper power heads do. It will still need tuning right out of the box to achieve full power.

Stihl and Echo power heads will not work with your attachments.

 

[cruzenmike]

Thanks to everyone for the explanations. Now I completely understand the difference. It also makes sense why the pull cord is on the back side of these units. I guess I was not only disappointed but surprised that the new Ryobi Full Crank model didn't perform as well as my half crank SS26. My trimmer still runs like a champ but due to an issue with the plastics on the trigger assembly, the throttle cable is slipping and I am losing revs. I have tried a few times to fix/adjust it, but at this point I fear that the parts to make it right again are too much to justify and I could just get a new power head. I have gotten almost 10 trouble free years out of it, so I have thought of moving on to something newer/better.

On the flip side I have considered replacing my whole system with a Stihl or Honda and their respective attachments. I have only heard of positive experiences with Stihl and from my own personal experience with Honda equipment has been exceptional. I will certainly have to look at the Husqvarna as a viable alternative to my existing power head if I am not going to abandon ship on the Expand It attachments altogether.

 

[primerbulb120]

Thanks to everyone for the explanations. Now I completely understand the difference. It also makes sense why the pull cord is on the back side of these units. I guess I was not only disappointed but surprised that the new Ryobi Full Crank model didn't perform as well as my half crank SS26. My trimmer still runs like a champ but due to an issue with the plastics on the trigger assembly, the throttle cable is slipping and I am losing revs. I have tried a few times to fix/adjust it, but at this point I fear that the parts to make it right again are too much to justify and I could just get a new power head. I have gotten almost 10 trouble free years out of it, so I have thought of moving on to something newer/better.

On the flip side I have considered replacing my whole system with a Stihl or Honda and their respective attachments. I have only heard of positive experiences with Stihl and from my own personal experience with Honda equipment has been exceptional. I will certainly have to look at the Husqvarna as a viable alternative to my existing power head if I am not going to abandon ship on the Expand It attachments altogether.

Which one is the one with the throttle cable slipping? If it's the SS26, the issue is a warped throttle trigger housing. You can buy both sides of the housing brand new for $6.85 on Amazon. It also comes with a clip to help hold the cable in when the new part warps.

Part number is 308869002.

As far as I know, you can not buy a new power head for $6.85.

 

[bertsmobile1]

Ryobi are consumption throw away tools.
Work well if you get a good one ( most are ) but not economic to repair, even if you can get repair parts.
As such they use 1/2 cranks to cut down costs.
One of the advantages of 1/2 cranks is the lower rotating mass so it will be a lot more responsive to throttle.
The down side is less inertia so they need to rev faster to make up for it.
In 4 stroke terms rather like the slow reving side valve engine than goes chuff chuff chuff, regardless of how much work it is doing compared to the OHV engine that dances around its rev range.

Tools designed to run off ryobi power head will not work well if connected to a Honda engine or any other 4 stroke.
The best replacement would be a Husqvarna 2 stroke power head as they tend to run higher revs than Stihl do cc for cc

 

[cruzenmike]

Which one is the one with the throttle cable slipping? If it's the SS26, the issue is a warped throttle trigger housing. You can buy both sides of the housing brand new for $6.85 on Amazon. It also comes with a clip to help hold the cable in when the new part warps.

Part number is 308869002.

As far as I know, you can not buy a new power head for $6.85.

Thank you for providing this information. I had only given slight consideration to fixing it, but I guess I can take it apart and see what all I need. If it ends up working out I may find myself getting a dedicated trimmer and keeping the Ryobi power head for the other attachments. My one regret on my Ryobi is that when I broke by straight shaft timmer attachment I replaced it with a curved shaft. Now that I am trimming over 500 linear feet each time I mow it is starting to take a toll on my back.

 

[cruzenmike]

Ryobi are consumption throw away tools.
Work well if you get a good one ( most are ) but not economic to repair, even if you can get repair parts.
As such they use 1/2 cranks to cut down costs.
One of the advantages of 1/2 cranks is the lower rotating mass so it will be a lot more responsive to throttle.
The down side is less inertia so they need to rev faster to make up for it.
In 4 stroke terms rather like the slow reving side valve engine than goes chuff chuff chuff, regardless of how much work it is doing compared to the OHV engine that dances around its rev range.

Tools designed to run off ryobi power head will not work well if connected to a Honda engine or any other 4 stroke.
The best replacement would be a Husqvarna 2 stroke power head as they tend to run higher revs than Stihl do cc for cc

In my experience this Ryobi has worked well, and it is certainly rev-happy. And maybe that was another difference that I noticed between the old and new is that the throttle response of the SS26 was much quicker than the full crank replacement. They just had to fiddle with something good....

 

[bertsmobile1]

Getting off topic a little but most new gear runs slower than what it replaced.
You can thank the EPA for that as emissions are a fixed amount measured at operation speeds.
The better ones specify it be under load & the poorer written ones allow the reading to happen without load.
Also emission restrictions mean the engines have to run leaner and lean = lots of speed but no power.
I always believed that new riders / drivers should be made to learn on old manual vehicles where you have to do everything , including control the timing.
It is amazing just how much more power comes on line from dropping the spark back a few degrees.
Now days it is all done by computers so the driver never learns basic engine management.

Then again very soon it will all be electric so maybe I should go reside with the rest of the extinct species at the local museum.