48cc in a 70cc block?

[Norman]

here is what I have a dax 70cc almost new with a busted crank, bought it that way the shaft that runs the mag rotor busted off at the key way. I also have a new 48cc needle bearing BGF engine that I don't like not enough ooomhp for me. So I wonder if I can take the 48cc crank and stick it in the 70cc block? I've not measured any of it I do not know the stroke of either so anyone of you out there done or tried this? I'm guessing that the 48cc stroke will be less and the rods end might be different **** I don't know if it will even fit anywhere. this is just some wondering something to try.
Norman

 

[Ghost0]

I am no expert and if I am wrong someone correct me but it is my understanding that the only difference between the 2 is the piston diameter. Same rods and cranks, only the piston size increases the cc's. But I might be all wet.

 

[Norman]

I know I don't know. The biggest difference I can see right off is the dax engine cylinder studs are way larger than the bgf but that is as far as I've fooled with it I may need to go heat up the garage and start playing with it. I might just transplant the bgf crank into the dax and see if it will all fit.
I tried to machine the dax head like I did a bgf 70cc head and can't I'm having trouble with getting the head to true up in the lathe using the sparkplug hole the thing wobbles all over the place the bgf head chucked up with no problems. that figures nothing is easy when I'm playing with fire?!
I do have the dax cylinder ported nicely and kind of itching to try it out. I wanted to really boost the compression on it as well but until I can get that head to turn true on the lathe I'm not going to be able to do much with that head.
Norman

 

[georgeknight]

I was just thinking about if it might be possible to transplant a pocket bike jug & head on a MB. That way I would have a reed valve setup. Just a thought...........George

Just found out that my thinking was far fetched. Seems the pocket bike set up was just another one of my pipe dreams. They have the carb mounted to the engine case instead of the head.. Oh well.......
But it's fun to try to come up with ideas for these bikes anyway!!
They keep us on our toes, don't they????????

 

[Egor]

I have not messed with a 49cc but I have a 55 or somewhere near that, never measured it out. The one thing I have found is that there are two bolt patterns for the head and barrel and two rod lengths. You can figure this out by putting the pistons side by side, or measuring the amount that the gudgeon pin rises above the crankcase. I started a thread about this. If I am not mistaken the 49 has the shorter rod. So you can install the crank in the other block, but you will have to match the correct piston to that crank. It may indeed be that it will go all the way to the largest displacement. Have fun, Dave

PS: I found the thread. http://motorbicycling.com/f4/rebuilding-your-2-smoke-engine-779.html

 

[Norman]

I found out that the 48cc crank is the same as the 70cc in this particular engines both had the bolt on counter weights and weigh the same had the same stampings on both rods .
Guess what I now have a sort of hopped up 70cc. match the porting between the case and cylinder, cleaned up the ports, notched the piston, no base gasket used an anaerobic sealant. now I need to test it and if it runs good I might sell it don't know I need to get rid of some of my bikes hard to ride 4 at once.
norman

 

[eDJ]

I think I was talking with Creative Engineering just after the chat room opened some time back
about the need for having specs for all these engines produced in China, just like you could find
them for anything from an Acura to a Ford Zepher in a Chiltons to a Mitchell's motor manual.
Some here want to seriously build for performance while others are just happy with something
that runs dependably and affordably to have some fun riding back and forth to work or school.

Getting that info into a data base people can access is the question. It would be much more
practicle then, to sit down and calculate a build opposed to just knocking around the shop and
getting lucky. (fine if it's your thing) My thoughts with CE were then directed towards being able to offer a custom "tweek" motor for those who want a speed tuned outfit built by someone who
knows their craft and has the tooling to do it right.

I like the idea George is advancing as it would put reed valves into the mix. Those Russian Motors
had those.

So far in my research I've concluded that a stock kit with recoil start/centrifugal clutch, a
different carb, and a chamnber would be the limit for mine, and putting the real effort into building the bicycle to mount it on. That's just a compromise based on available info.

 

[Egor]

A reed valve can be put anywhere on the crankcase, if installed on a piston port engine the piston is actually in the way. You then have to introduce vent holes in the skirt to let the flow of the charge through. A piston port engine is the least expensive engine to make. The best small engine ever developed for a small motorcycle was the rotary valve Yamaha, though the engine is bulky, as the carb sticks straight out the side. All the fastest Tiddlers were rotary valve engines. The hopped up Russian engines use a reed valve they weld the port anywhere it would fit, they block off the rotary port.

 

[eDJ]

Here is a photo of one of those Russian Motors Carb & Reed induction to the crank
case. I read where the dome over top of the very simple carb could be turned left or
right to partially block the intake holes under it to serve as a choke. (that would make sense
in the cold Russian climate) The air filter is contained in it and it's quite a simple induction.
The carb has a gold color for some reason.

 

[Norman]

the gold color is more than likely an anodizing process look it up on the net "diys anodizing" a guy can do it but the chemicals can be nasty to work with.
I think it would be kind of fun. I know a guy here in town that has a powder coating and chrome shop and I think he also does the anodizing I know the guy through flying we are airport bums he has some really neat planes.
Norman

 

[Moto]

did this ever run? Im pretty sure 50cc engines have a 38mm stroke and 66cc engines have a 40mm stroke.

 

[Greg58]

I search for a thread like this but never saw this one, its five years old so I'm sure Norman found out something. 66cc engines have a 38mm stroke the engines with the 40mm stroke are acualy 69cc. My 66cc engine stroke is 38mm.