Notched Cooling Fins?

Fins are notched for various reasons, Notching in the right places will add to cooling by directing the transfer of heat to a specific area of the fin. An example would be the lack of air flow behind the jug. Notching the rear part of the fin "out" of the rest will keep more of the heat forward in the flow. Similar to higher fins up front on the head. I cant see notching adding surface area unless the exposed fin edges have more area than the metal removed for the notch. This would require a very narrow notch. The early dirt bikes had notches on the cylinder with rubber block isolators. I was told this cut down resonance in the fins. Give some thought to the dynamics before cutting the fins onto something that just looks cool. Done right should aid cooling a bit.

China cylinder heads are so marginal I doubt notching would help any more than the loss of material would hurt.
I run a FredHead and a Pedalchopper billet head on my own bikes.
Angle plug head just sux, period.

puch head is best. money wise. water dripping from a bottle is the cheepest oh! we cant waste water here in california. although our rivers flow free to the ocean

It doesn't get very hot in Overgaard, does it? I guess the engine could run hot during a long climb. My China girl did OK when I lived in the Phoenix area.

As thin as these fins are cast, you need as much of the fin intact as possible to keep these things cool, especially when leaning them out or doing anything that boosts performance. Best advice I can give is don't do it...

Another thing I can add is that not all of these cylinders are created equally, if you buy one from let's say BGF on ebay, the fins may only be about .030 to .040" thick, but if you buy from let's say mzmiami or someone else, the fins may be as thick as .060 to .080" thick... It really depends on the seller, where they're getting their parts from, and what they would consider good enough to sell... I've bought cylinders from several sellers before deciding who to buy from, and if they send me a really nice one I'll usually go back and buy 4 or 5 of them since I want to get as many from that particular batch as I can if they're really good or exceptionally good... If I get one with bad looking tiny ports or paper thin fins etc it'll be the last one I buy from that source for a while...

dmb said:

puch head is best. money wise. water dripping from a bottle is the cheepest oh! we cant waste water here in california. although our rivers flow free to the ocean

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I disagree with the puch is best part. Puch engines were mounted parallel to the frame so the air was hitting the top of the head. On our bikes not so much. We need air to be able to flow through the fins.

It wouldn't be too hard to fab up an air scoop to direct the air over the top of a Puch head (similar to those Suzuki GT550's from the 70's) if cooling became a problem, but from what I've seen, the puch head really does a good job keeping things cool without any extra help.

I wish they make a Puch shaped head on the outside and a china girl combustion chamber on the inside! I have never measured but, would have to belie.ve the puch head has more cooling area than any available.

I have never owned a Fred head in order to be accurate.

The problem with the puch in my opinion only, is the squish and the combustion chamber does not match the china girl piston. I believe a certain amount of detonation is responsible for heat when using certain puch heads even when using 98 octane.

Bang ting ting, not ting ting ting

Can anyone tell me what a cool temperature is for one of these 2 stroke china girl engines? How hot should the head measure after 5 miles? What would be a "cool" temp and a "hot" temp?

Typically you want to stay under about 350 degrees F but if you can get one to stay under 300F you're doing really good... a new engine will run hotter so in order to stay under these limits it may mean really short rides at first until the rings free up some. These cylinders are plated so keeping them cool really helps prevent the plating from separating. If you got a way to monitor the temperature while riding like by adding a cht gauge it's always the best way, but you can also take a 5 minute ride then check with a point and shoot infrared thermometer to see what it's doing.
The Puch head or Fred head... or any of the larger aftermarket heads usually do an excellent job keeping the temp down below 300F. I got a Fred head on one bike and it usually runs at around 280F or so after it broke in but it did get hotter while tuning until I got it dialed in.

Thanks Dave, that's good to know! I'm running a Dax F80, 66cc center mount, china girl on my old school steel frame Peugeot 26 inch 21 speed mountain bike. I have a stock slant head, a sick bike parts expansion pipe, a 15 mm RT (Dellorto clone) carb and I'm running Opti-2 at 100x1. I've jetted it with the .65 mm jet and it was 4 stroking quite a bit but still had pretty good power so I left it in. After riding it for about 700 miles with the .65 jet, I decided to see if I could get more out of it by jetting with the .60. The minute I put the .60 in I noticed a huge increase in performance. The 4 stroking is almost completely gone and the motor is running with more power. It's running the best I have ever seen it! My top end speed went up from 28mph to 33mph on level ground but I saw an increase along the entire throttle range. All seems good but I took it out on a 90 degree summer day here in CA. I rode it for about 15 miles and I rode it pretty hard and fast up some pretty steep hills with a 40 tooth rear sprocket. When I drove it home I got out my laser thermometer and pointed it at the head while it was still idling and it was 350 degrees!! I have checked it out before when I was using the .65 and it always registered about 280 at the most after a similar ride. I'm clearly running hotter with the .60 jet.
I mentioned this in another thread and Slogger recommended that I try a cooler spark plug. I was using a NGK B7HS so I tried the NGK B8HS. I ran it for about 1 1/2 miles, up a moderately steep hill at WOT. When I got home the spark plug base measured at 310 F degees. Wow, that's almost 40 degrees cooler!! Okay, it was a cooler day and I didn't run it as long but I ran it at mostly wide open to simulate the first ride but it still seems significantly cooler. I will keep checking my temps and see how it goes. I'm still worried I might be running it too lean, I know it's not good to run it too lean using a 100x1 oil mix! I'm still thinking of getting the Fred Head but this thing is running so good right now, I don't want to mess it up, haha.

The Fred head will definitely help, and if you get it from his site he dropped the price down to $39.99 because of all the clones being made from his design... so might as well get the genuine article if you do get one, but that alone will drop your temp by a good amount and prevent heat soak on the hotter days.
I would also recommend adding a touch more oil during the hottest part of summer and if running at full throttle for any length of time. I was running 100:1 with the Opti2 and it worked just fine for my 1/2 mile run in the morning when it was in the 40's outside, but brought the mix up to 75:1 once it started getting hotter outside or if I was going on a longer ride. The 100:1 mix is safe at cruising and some WOT use, but by adding in maybe about a teaspoon full of castor oil per tank will give your engine the extra protection it'll need for higher rpm use. If you do decide to add in a splash of castor oil to your mix tho, get the high grade stuff meant for small engines as the stuff at the pharmacy is really gummy and bad for the engine.

Oh, I will get the orgininal Fred Head for $40 and I'm certain I will get it sooner or later. It's not like I want to go much faster on this bicycle but if I can run cooler it seems worth it to me. Do you think I would have to re-jet again once it is installed? .60 is the smallest jet that came with the RT carb although, I have the drill bits to make my own if needed. I was actually thinking of trying to make a .62 jet or something close to that and see how it runs, but I think I will add a little more Opti-2, maybe go 90 or 80 to 1 and see if my temp goes down any, it's still like summer here! I hadn't tried castor oil before, I guess it's ok to use it with the opti-2 so I might try that too. Thanks for the tips! Anyway, it seems like I'm still in the safe temp range, I have never measured over 350 F but I figure the closer I can get to 300 F the better.

Would you recommend I use a tea spoon of Castor 927?

wow a fredhead for $40? get the 2 hole head and put a compression release in the 2nd hole, great engine braking [better than a 4 stroke] and a good release of hot gasses as it turns the motor into a airpump.

dmb said:

wow a fredhead for $40? get the 2 hole head and put a compression release in the 2nd hole, great engine braking [better than a 4 stroke] and a good release of hot gasses as it turns the motor into a airpump.

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This is the only one that is $40--> http://www.shop.crmachine.com/product.sc?productId=48&categoryId=15

He sells one with a compression release for $130--> http://www.shop.crmachine.com/product.sc?productId=62&categoryId=15