Custom 70cc engine build

Is it worth it or stupid?

  • YES

    Votes: 21 63.6%

  • Hell no

    Votes: 2 6.1%

  • Kinda

    Votes: 4 12.1%

  • YES

    Votes: 3 9.1%

  • Nah I'll pass

    Votes: 3 9.1%
  • Total voters
    33
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AnubisBC said:
Hey Z, I've been following this thread for a while. Very impressed with your stick-to-it attitude despite the many challenges! I'm really excited to see where this project goes.

If you plan on using a rotary tool for any length of time, you might want to look into getting a Foredom flex shaft with a foot pedal (stick with the Foredom brand). They're not cheap, but they're a worthy investment. I use this one in jewelry making constantly (https://www.amazon.com/Motor-Plastic-Pedal-Speed-Control/dp/B00CFSBO0A/) and it blows my dremels out of the water in every respect. They have a huge amount of torque compared to the smaller options. You'd be surprised at how accurately you can cut metal using a tungsten bit with higher torque, lower speeds and a foot control.

Sorry, didn't mean to thread jack, I'm not a rep for the company or anything, you just seem like the kind of fellow who would be able to do some awesome things with the right tools.
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Thanks! Yeah @dougstar suggest foredom as well, I do plan on getting one in the future ?
 
Seandonato73 said:
Doug will like that. he said something about them a few pages back lol. I have one if the "beefier" dremel tools and yeah they lack finesse. Been scoping them out on Ebay try to get a decent used one. They arnt cheap!
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Too true! They're not cheap, but once you get one, you'll know that they're definitely worth it! hahah!
 
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Z.hb71 said:
Found my candidate for a rotary tool, https://www.homedepot.com/p/WEN-101-Piece-Rotary-Tool-Kit-with-Variable-Speed-2307/203736986 had good reviews, looks like a good deal and sounds 30k rpm apparently. I'll be giving this a go ? tho I'll order it thru Amazon prime so I can have it in 2-3 days vs home Depot shipping is 5-8 days or something like that ? and I'll get some diamond grinding bits cuz those are for steel so it should go through aluminum just fine ? or if I can find me some tungsten carbide ones I'll get those instead ?
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Bummer homedepot link is ( acsses denied? ) just a small tip when working with flexshaft you must have the least amount bend in it as possible? with a tight bend in the shaft adds a load of friction inside the shaft and it will melt:oops:(n) thats if your rotary tool has a shaft?
 
dougstar said:
Bummer homedepot link is ( acsses denied? ) just a small tip when working with flexshaft you must have the least amount bend in it as possible? with a tight bend in the shaft adds a load of friction inside the shaft and it will melt:oops:(n) thats if your rotary tool has a shaft?
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Mine doesn't have a flex shaft, probably should have got one since the version with the shaft was just $5 more, but it is what it is. But thanks for the tip!
 
Z.hb71 said:
The crank goes in fine, you just have to use alot of lube. The crank is the exact same as Alx (the people who made the crankcase)
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It was only a watch out kinda thing. We use a lot of cranks in our engines. Welp... went to go put one together once and would not go into bearings. Ignition side shaft was .0028 thousand inch bigger.
 
Are you positive your piston skirt is interfering with the cases? I suggest installing the cylinder then rotate the crank slowly. If you feel a abrupt bind then yes you have an issue. Sometimes you will encounter a "false" piston interference when rotating the assembly without the cylinder installed. Are you sure your crankcases are compatable with the piston bore and crankshaft stroke combination you are attempting to use ?
 
Polar_Bus said:
Are you positive your piston skirt is interfering with the cases? I suggest installing the cylinder then rotate the crank slowly. If you feel a abrupt bind then yes you have an issue. Sometimes you will encounter a "false" piston interference when rotating the assembly without the cylinder installed. Are you sure your crankcases are compatable with the piston bore and crankshaft stroke combination you are attempting to use ?
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Yes its not a compression illusion, just stops abbruply close to bdc. This crankcase was meant for this cyl and Piston, the only difference is, my crank. The crank meant for this crankcase was a 39.3 or a 39.4 I believe, I'm using a 39.6, so it will allow the piston to go lower @ bdc and higher @ tdc, I didn't buy the 39.3-39.4 crank only because they are impossible to come by nowadays. So I believe that is my issue
 
OK, so you're basically adapting a "stroker" crank. Your going to have 2 problems. 1) is what you are experiencing which is the base of the piston skirt interfering with the crankcase. The 2nd issue you're going to have is at TDC your cylinder was designed for a 39.3mm stroke. Where your now running a 39.6mm stroke your potentially going to have a piston squish clearance interference issue. Your basically adding about .012" of total stroke. Divide this by 2 and your adding .006" to BDC and TDC. What might be a far less risky fix would be to remove the needed material off the base of piston skirt rather than attempt to relieve the cases with a high speed cutter. If you attempt to use a high speed cutter on your cases I about guarantee the tool will "grab" the aluminum and immediately gouge a huge divot where you did NOT want material removed. Have a close look at exactly where, and how much material will be required to gain the needed clearance. Again, I can't be of 100% help but at least I can throw out some less risky options. As for your issue with squish, that will be more easy to correct and that is to simply add an additional .006" base gasket and verify your squish clearance with a piece of lead solder. Here's a good 2 stroke vid with a cutaway view about 2 stroke engine operation.

<iframe width="560" height="315" src="
" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
 
Polar_Bus said:
OK, so you're basically adapting a "stroker" crank. Your going to have 2 problems. 1) is what you are experiencing which is the base of the piston skirt interfering with the crankcase. The 2nd issue you're going to have is at TDC your cylinder was designed for a 39.3mm stroke. Where your now running a 39.6mm stroke your potentially going to have a piston squish clearance interference issue. Your basically adding about .012" of total stroke. Divide this by 2 and your adding .006" to BDC and TDC. What might be a far less risky fix would be to remove the needed material off the base of piston skirt rather than attempt to relieve the cases with a high speed cutter. If you attempt to use a high speed cutter on your cases I about guarantee the tool will "grab" the aluminum and immediately gouge a huge divot where you did NOT want material removed. Have a close look at exactly where, and how much material will be required to gain the needed clearance. Again, I can't be of 100% help but at least I can throw out some less risky options. As for your issue with squish, that will be more easy to correct and that is to simply add an additional .006" base gasket and verify your squish clearance with a piece of lead solder. Here's a good 2 stroke vid with a cutaway view about 2 stroke engine operation.

<iframe width="560" height="315" src="
" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
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So wait, I get the squish thing, I can just add a thicker gasket or another thin gasket (I have a whole gasket pack with me), but I don't understand what you are saying with removing meterial of the crankcase, I will be as careful as possible and put the tool on a low setting to it doesn't slip and gouge the heck out of my case, or I could use a less agressive tool, but removing meterial from the piston would be more risky than carefully removing meterial from the case becuase of port timing right? What would you do? I'm not fully understanding sorry
 
Take it out of the case. Just be very careful. It's not rocket science. I mean I'm capable of doing it and I'm not that bright. I cant even upload a video to YouTube for crying out loud. It would be good to take a few measurements and market the case out as well. Take your time. Like polar more or less said. Once metal gone it's gone.
 
Seandonato73 said:
Take it out of the case. Just be very careful. It's not rocket science. I mean I'm capable of doing it and I'm not that bright. I cant even upload a video to YouTube for crying out loud. It would be good to take a few measurements and market the case out as well. Take your time. Like polar more or less said. Once metal gone it's gone.
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Will do ?
 
My suggestion of removing some material from the piston skirt was a suggestion based upon the fact you might only have to remove a small amount at the 4 corners of the skirt. You won't be altering the port timing in this particular case. I have no idea to what extent you need to remove metal, all's I was trying to do is get you to think in both directions. Remember this quote with respect to "grinding" engine port work : "It's WAY easier to loose 2 hp , than gain 1 hp with a grinder".
 
@Polar_Bus Ok point well taken. Z Take some measurements and let's see if we can come up with something that will work the best for what you need. I think you said it's a Reed case so some skirt removal may be ok. But we want to make sure taking material off the exhaust side wont affect the timing. The worst imo grinding the case will do is effect the case volume. From what I can tell in your pictures it doesn't look (to me) like it would be a lot to take off. Opinions?
 
Seandonato73 said:
@Polar_Bus Ok point well taken. Z Take some measurements and let's see if we can come up with something that will work the best for what you need. I think you said it's a Reed case so some skirt removal may be ok. But we want to make sure taking material off the exhaust side wont affect the timing. The worst imo grinding the case will do is effect the case volume. From what I can tell in your pictures it doesn't look (to me) like it would be a lot to take off. Opinions?
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Ok so what exactly do you want measured? Also I'll have to remove some from the intake side as it touches. What effects would messing with case volume do?
 
How much farther down does the piston need to go? how much material of the side of the case would have to be removed. Changing the crankcase volume can affect how the engine runs. Think of it like this. The piston going up sucks in your air fuel charge. Then when it goes down it pressurize the charge in the crankcase forcing it into the cylinder. Increasing or decreasing the case volume can affect that pressure and how the air fuel enters the combustion chamber. Now small (remember it's a 70 cc engine) changes that I think you will need to do shouldn't really hurt it. Imo. But let's also think about this for a moment. You have a longer stroke rod so room needs to be made for the piston to move. This is also a common thing to be done on a g320 to turn it I from a 32 cc to a 34 cc. Longer stroke con rod. Make sense?
 
Seandonato73 said:
How much farther down does the piston need to go? how much material of the side of the case would have to be removed. Changing the crankcase volume can affect how the engine runs. Think of it like this. The piston going up sucks in your air fuel charge. Then when it goes down it pressurize the charge in the crankcase forcing it into the cylinder. Increasing or decreasing the case volume can affect that pressure and how the air fuel enters the combustion chamber. Now small (remember it's a 70 cc engine) changes that I think you will need to do shouldn't really hurt it. Imo. But let's also think about this for a moment. You have a longer stroke rod so room needs to be made for the piston to move. This is also a common thing to be done on a g320 to turn it I from a 32 cc to a 34 cc. Longer stroke con rod. Make sense?
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Ok so the crank doesn't hit anywhere in the crankcase, this is the only issue. The piston needs to go down about 8mm more. 15633315782897587276032385797137.webpboth intake and exhaust side. The 2 marked spots are were it needs to be modded 15633317259431936439230098949540.webp
The backside I was being a bit generous but it's the general idea.
 
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