Full Circle Cranks photos

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trappermike

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Please read my previous replies on Full circle cranks and 372 crank mods first for more info please. I finally founda couple photos of one of the full circle titanium stroker cranks I built for competition. Unfortunately the pics are kinda dark,the light setting on the camera must have been incorrect. I'm sure some of you guys know how to lighten the photos,it may help,if you can,please post them here.
Anyway the cranks had a 2mm longer stroke,knife-edged con-rod,polished counterwieghts,and the rod was modified for better oiling to the upper and lower bearings.
The crank is shown beside a stock one.IMG_0003.JPG IMG_0004.JPG
 
To undertake this project I really needed a good book that explains metals and alloys,and most important have all the charts that compare all metals in strength,hardness,and weight,and I found a perfect one- Metallurgy & Plastics for Engineers ( ISBN # 0 08 021684 6 )
Fortunately the book is concerned mostly with metals not plastics.
This is a great book for anyone if you can find a copy.
 
The old McCulloch method of building full circle cranks for racing used steel but the method used to balance them was rather archaic,instead of punching 2 large hole beside the rod big-end pin,they added heavy "Mallaroy" metal to the opposite side of the crank,the resulting cranks were almost 3 times the weight of the stocker...
 
Higher crankcase compression is very beneficial in a high speed 2-stroke because the fuel-air charge in the crankcase must be "transfered" up into the cylinder from the crankcase in an incredibly short time. More pressure in the crankcase will deliver more charge in the short time the transfer ports are open.
For example in a saw motor that has a transfer port duration of 120 degrees, at 12,000 rpm the transfer ports are open for only .00166 of a second, that's only 1.66 thousandths of a second to transfer all the fuel charge up into the cylinder!
 
I was lucky to get 3 new con-rod assemblies from the factory to work with which included the rod,big end bearing and pin. Of course in chainsaws you cannot buy a replacement rod assembly,only a new crank. I had to sweet talk someone I met from the factory to send me them. Otherwise I would have simply pressed apart a new crank and taken the parts from it to use.
 
Many,including motorcycles and snowmobiles since 1974. But I'm retired now,I'm not trying to promote or flog any work or parts. I don't mind providing some knowledge and experience to help people interested and wanting to learn,but I do resent people who try to steal my ideas and designs and then sell them as their own inventions as has happened in the past...
This winter when I was trying to clear out my remaining racing parts I was able to help a really nice guy in Australia,amazingly I helped him build his entire racing saw on Skype (My help free of charge),he had a good webcam,and it was like I was standing right there in his shop helping him doing measurements and showing him what to do. It was a lot of fun and he did a great job building his first competition saw with a pipe, and it runs fantastic.
I'm not selling any parts or work anymore. however.
I may post more informative articles in the future if some people are genuinely interested. How about a 372 piston swap with a simple machining mod that really makes power I invented in 1998 ? Or carb fuel circuit mods,or how Ram Effect produces more top end power in a 2-stroke,or better spark plugs for modded work or competition saws.
 
I don't know if the book is available to buy anymore,but I'll bet you can get a copy to read from your library system,just copy any data you want to keep. If you wanna modify things or make special parts it's really valuable to have that info.
 
I don't know if the book is available to buy anymore,but I'll bet you can get a copy to read from your library system,just copy any data you want to keep. If you wanna modify things or make special parts it's really valuable to have that info.
Mike, were you in on the carb boring thread? I haven't had time do attempt it yet, but it didn't sound real difficult. And a good way to power up.
 
Yes,if you understand that engines need more air and fuel to make more power,and that the stock small carb bore is quite a restriction to airflow into the motor,and costs no money to bore and get a good boost in power. Enjoy the free power. I've been boring saw carbs since 1985,motorcycle and snowmobile carbs (Mikunis) since 1975.
 
Yes,if you understand that engines need more air and fuel to make more power,and that the stock small carb bore is quite a restriction to airflow into the motor,and costs no money to bore and get a good boost in power. Enjoy the free power. I've been boring saw carbs since 1985,motorcycle and snowmobile carbs (Mikunis) since 1975.
Stupid question, but why don't the saw manufacturers do the upgrades in the first place?
 
A legitimate question,mostly to save cost and weight,the ports are small to save piston and cyl. wear,a bigger carb would cost more and cost a few grams more of weight etc. Frankly I would like to see one of them make a true "Professional" model with a little more aggressive ports, bigger carb,and ideally a modern reed valve assembly,and a removable cylinder head which would cost them a few grams more. They will stick with the stock "porkchop" type crank because it's cheaper to make but most importantly it's a lot lighter and saw makers try to trim every gram off the saw for sales reasons.
 
And emissions.

The stuffer cracks in factory saws are coming. Look at the 572, they shrunk the bore and increased the stroke then added the suffer to get the top end back. I think it's brilliant.
 
I'd like to see some photos of these new cranks. Most saw motors traditionally have a pretty short stroke compared to their bore "Oversquare" to reduce piston speed and increase the rpm's they can safely turn. However a short stroke in a 2-stroke also means shorter ports,they are not as tall and give up some area,and some 2-stroke designers went back to longer strokes for taller ports. The 064 is an actual example, it's bore and stroke was not as oversquare as previous saws. Suzuki did that too with their newer motocross engines.
But it sounds like an good idea,and a stuffer crank should increase power throughout the rpm range.
Ah yes emissions...
 
I don't have any pictures. I did notice too playing with Jennings formulas that longer stroke makes getting desired port time areas much easier.
 
Here is a perplexing problem with saw cranks, the counterweights I'm guessing are about 1/3 of a circle so lower crankcase compression ratio. But take the piston and cylinder off your saw and turn the crank to BDC so it's at the bottom of the downstroke- when the transfer ports are open... Now look at the crank,it's counterweights are at the top of the crankcase and a large part of your fuel/air charge(about 2/3 of the crankcase volume) is trapped in the bottom of the crankcase with the counterweights above blocking your fuel charge from getting up past them to the transfer ports,considering the short amount of time the transfers are open I dought if much of that charge will actually make it into the cylinder. So in engines like these I thought about somehow creating a couple "tunnels" from the lower part of the crankcase to the transfer ports to utilize the trapped charge obviously for more power and to remedy the problem.. But the crankcase castings are too thin,but I have seen one 2-stroke(the only one ever) engine that actually had these tunnels I wished to make,it is the old 275 and 380(125cc) Canadien saws from the 60's,they had the porkchop type crank too but actually a well built tunnel in each side of the crankcase going from the bottom right up into the transfer ports,I think they obviously saw the problem and tried to remedy it. However the extra volume created by the tunnels lowered the crankcase compression even more of the saw, it was already suffering low compression from the crank design so unfortunately I've never seen tunnel ports again. Now when you have full circle crank or maybe one of these new ones less or none of the fuel/air charge is trapped at the bottom of the crankcase,where it probably won't contribute much to power.
 
I believe the 572 also feeds the transfer ports from the front of the crank case in the middle instead of the sides so it also wouldn't have that problem either. If course that means the transfer tunnels hug around the hot exhaust port, but on a 2 stroke I don't think that's all that bad. Yes the air fuel mix is warmer, but it's basically already metered into the engine at this point, so being warmer would aid in combustion speed. It would make it more prone to detonation, but the seems to rarely be an issue with chainsaws.
 
You sound right,stock saws are too low performance to create any real "cylinder pressure" (not compression) so rarely suffer detonation or piston melting problems that high output 2-strokes have to be careful of. I would like to see some photos of the 572,or someone please lighten those dark photos of my crank and re-post them here so they are more visible,I have 2-stroke oil in my veins and I'm not very good with computers,or I would have done it myself.
 

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