That is good to know. Cause I am really wondering if I truly need a balanced crank. I mean I am wanting to run a aftermarket ignition box on this to back the timing down. Ive seen lots of people swear that alone really smooths out the engine. But the hot rodder in me says for $100 to get a balanced crank go for it as the bottom end will most likely out last the cylinder and pistons.
66cc, picking a starter engine
- Thread starter Rusty_S85
- Start date
FurryOnTheInside
In Memoriam
What's a balanced crank?
Is it this?
Even if it goes wrong and I do take too much away, there's still a satisfaction that comes from building my engine with my own hands, and learning how to do it better next time. I think you would enjoy this kind of stuff too!
A balanced crank is basically a rotating assembly that is balanced, crank to rod to piston to provide a smooth balanced rotation. It is very hard to get a perfect balance with a single cylinder its easier with a countering piston. But it does remove vibration and harmonics which is vibrations does in 4 stroke engines at least cause issues.
I am currently looking at the balanced bottom end that CRM sells it goes for $275. I don't have a problem with doing some tinkering to make things better. I just want to ensure I have a good base that will give me no troubles and if there are problems the bottom end can be salvaged and reused.
I am currently looking at the balanced bottom end that CRM sells it goes for $275. I don't have a problem with doing some tinkering to make things better. I just want to ensure I have a good base that will give me no troubles and if there are problems the bottom end can be salvaged and reused.
FurryOnTheInside
In Memoriam
Yeah but to put it another way a balanced crank has had two little holes drilled in it.
Idk if these you're looking at are balanced in some other way.The member Jaguar has written lots about the crank "balance" or getting the vibration as minimal and gentle as possible, and what I have drawn from his writings is that it's rpm dependent. To "balance" the crank you have to be able to predict not just the weight of the other parts but the engine speed.
I didn't want to build, and then find out how many rpm, then drill balance holes. That's why I just went for the trial and error approach, drilled out the lowest estimated depth that I had read and I'll be able to drill a little deeper after some testing.
The bearings may be upgraded in those bottom ends, and the crank may be trued.. idk. I just wanted to demystify the balance thing, afaik it's just two holes.
Well if its just two holes then theres no reason to spend $100 more.
Don't get me wrong I am not afraid to break a stock GT5 engine in and then make changes for improvement. Its just that I want to ensure I am starting off with a good base. Plus I want to ensure that I can get the engine set up to give me the max speed I am looking for so I wont have to be playing around with drive sprockets so much.
I'm really looking at the MPG-38 engine without the $100 balance upgrade. I can use that $100 extra to put towards the Mikumi 18mm carb I want to run, or put it towards the CDI.
Don't get me wrong I am not afraid to break a stock GT5 engine in and then make changes for improvement. Its just that I want to ensure I am starting off with a good base. Plus I want to ensure that I can get the engine set up to give me the max speed I am looking for so I wont have to be playing around with drive sprockets so much.
I'm really looking at the MPG-38 engine without the $100 balance upgrade. I can use that $100 extra to put towards the Mikumi 18mm carb I want to run, or put it towards the CDI.
gary55
Well-Known Member
Rusty look into this a bit more. Hopefully some of the others here will chime in on this. Their is more to this than drilling a couple of holes in the crank. A good deal of this method was entertained before manufactures were paying more attention to balance. Now there are already holes drilled in the crank for balance that gets them close and all modification from there is fine balancing for those that are hot rodding or just like to have things done as well as possible .
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CrazyDan
Well-Known Member
From what I understand to balance your engine the best you weigh all moving parts conrod up then make all the other moving parts from crank to flywheel weigh the same by drilling holes to remove material.
FurryOnTheInside
In Memoriam
Jaguar has made his calculator app available, to figure out for you how much mass/material to remove. It's like a spreadsheet thing and it's free. But you need to own a computer to use a spreadsheet.. idk maybe someone has a phone that'll do it. I'm not technological, so I chose the bit-at-a-time method.
"Balanced" it would seem is short for "the optimum amount of mass removed from the conrod side of the crank cheeks"?
I'm sure you can only get the optimum balance for a particular RPM: Jaguar said to do it for the top RPM that you will run the engine at. So if you get an engine already balanced, surely the maker needs to tell you what RPM it's balanced for?
And if it's "precision" balanced, what precisely does that mean?
It's true the reputation of the Grubee Skyhawk GT5 is good including what people say about the balance/vibration level. Seems that they read the forum posts about the vibration problem and it's cures and decided to do something about it. So it might already be "balanced" optimally for your top RPM?
If you do want to remove more mass then obviously you would need to take into account the starting point for that particular engine. You definitely should read everything you can find though and get as much expert advice as possible, I quite agree with that!
What kind of engine do you want anyway? What top RPM?
"Balanced" it would seem is short for "the optimum amount of mass removed from the conrod side of the crank cheeks"?
I'm sure you can only get the optimum balance for a particular RPM: Jaguar said to do it for the top RPM that you will run the engine at. So if you get an engine already balanced, surely the maker needs to tell you what RPM it's balanced for?
And if it's "precision" balanced, what precisely does that mean?
It's true the reputation of the Grubee Skyhawk GT5 is good including what people say about the balance/vibration level. Seems that they read the forum posts about the vibration problem and it's cures and decided to do something about it. So it might already be "balanced" optimally for your top RPM?
If you do want to remove more mass then obviously you would need to take into account the starting point for that particular engine. You definitely should read everything you can find though and get as much expert advice as possible, I quite agree with that!
What kind of engine do you want anyway? What top RPM?
Steve Best
Well-Known Member
Not as simple as all that Dan, and yet there is a simple answer to all this.
Buy a kit from a reputable seller. I chose Grubee.
Check it over as your skill allows.
Assemble it as the manufacturer recommends. (even the hardware is good enough now)
Take great care that all is mechanically right on your bike and kit.
(good brakes are a must)
Use a good quality oil and good fuel.
Ride it as intended and enjoy!
If you wish to modify anything, do it after break-in, one item at a time.
One item at a time so you can see if it has any effect.
And keep checking the nuts and bolts, spokes and fittings that all stay torqued.
From my experience you can get long service from one of these kits if you follow this advice.
Crank balance? It is fine for these engines used as intended.
I buzzed my Grubee up to 10,000 rpm and it was still fine.
Bolt that engine snug to the frame and all will be well.
Well I finally received a reply to my email. Apparently that motor I was looking at the MPG-38 would not be capable of 50 mph top end as its not designed for use with tuned expansion chamber exhaust and all that. I was recommended the GT-5 SuperHammer engine which is priced right at $539.00.
I don't think I can justify spending $539 on an engine. I know I shouldn't cheap out but at the same time I just think I should keep my price more around $300 - $350 max especially after finding out I am pushing $1200 on my build already and I still have a small list of parts to buy.
I did see for $105 he sells a trued and balanced Super Rat Crank. I asked just now if that requires a special modified case or if it will fit any 66cc case. If it will fit any 66cc case I am seriously contemplating just buying a GT5 Grubee 66cc engine and strip it down and swap in the Super Rat Crank for $105 that would put me right at $250ish for the engine. Which they sell a balanced GT5 bottom end for $230 but still have to by piston, cylinder, cylinder head, intake, exhaust, piston rings, piston bearing. Where as with the GT5 I could in theory reuse the cylinder and do some port work on it myself to see if I can make a difference with the exhaust I am wanting to run.
In any case all I want is for this bike to pull hard without pedaling but at the same time be capable of cruising 30 - 40 mph without screaming its head off. Why I really had my eyes set on the MPG-38 engine as it is said to build low end torque in the 1500 - 7000 rpm range.
I might just cheap out and go with the GT5 Skyhawk engine and just do some hotrod work on it. My big thing is I don't want to be going down the road with the engine screaming its head off out the exhaust and only going 25 mph. I know in my mind 2 - 3 hp which is about what the GT5`s are rated at should in theory be strong enough to pull a good amount of weight from a stand still. I mean with gearing if a 30 hp tractor can pull close to 8,000 lbs to 30 mph easily then 2 - 3 hp should pull 300 - 350 lbs (estimated bike and rider weight) off the line just fine with the right gearing. I just have to buy a 36T sprocket so I can try it. Plus its not hilly here so I might need to look into running the 36T sprocket off the bat anyways for break in. But now after I move there are some huge hills that I would have to climb. So might be better to go with a higher dollar engine such as SuperHammer
Also just came across this.
http://www.zone8cycling.com/Bike-En...ngle-Fire-Slant-Head-Bike-Motor-Kit-p625.html
Price is right, but I don't need all that extra garbage such as the black chain guard (I ordered a chrome one), or the fuel tank (I have one built in the frame).
I don't think I can justify spending $539 on an engine. I know I shouldn't cheap out but at the same time I just think I should keep my price more around $300 - $350 max especially after finding out I am pushing $1200 on my build already and I still have a small list of parts to buy.
I did see for $105 he sells a trued and balanced Super Rat Crank. I asked just now if that requires a special modified case or if it will fit any 66cc case. If it will fit any 66cc case I am seriously contemplating just buying a GT5 Grubee 66cc engine and strip it down and swap in the Super Rat Crank for $105 that would put me right at $250ish for the engine. Which they sell a balanced GT5 bottom end for $230 but still have to by piston, cylinder, cylinder head, intake, exhaust, piston rings, piston bearing. Where as with the GT5 I could in theory reuse the cylinder and do some port work on it myself to see if I can make a difference with the exhaust I am wanting to run.
In any case all I want is for this bike to pull hard without pedaling but at the same time be capable of cruising 30 - 40 mph without screaming its head off. Why I really had my eyes set on the MPG-38 engine as it is said to build low end torque in the 1500 - 7000 rpm range.
I might just cheap out and go with the GT5 Skyhawk engine and just do some hotrod work on it. My big thing is I don't want to be going down the road with the engine screaming its head off out the exhaust and only going 25 mph. I know in my mind 2 - 3 hp which is about what the GT5`s are rated at should in theory be strong enough to pull a good amount of weight from a stand still. I mean with gearing if a 30 hp tractor can pull close to 8,000 lbs to 30 mph easily then 2 - 3 hp should pull 300 - 350 lbs (estimated bike and rider weight) off the line just fine with the right gearing. I just have to buy a 36T sprocket so I can try it. Plus its not hilly here so I might need to look into running the 36T sprocket off the bat anyways for break in. But now after I move there are some huge hills that I would have to climb. So might be better to go with a higher dollar engine such as SuperHammer
Also just came across this.
http://www.zone8cycling.com/Bike-En...ngle-Fire-Slant-Head-Bike-Motor-Kit-p625.html
Price is right, but I don't need all that extra garbage such as the black chain guard (I ordered a chrome one), or the fuel tank (I have one built in the frame).
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