How to balance a Crankshaft???

[crmachineman]

Hi everybody! I've been very concerned about vibration as well. I have investigated the crankshaft and found a few things that were wrong. First, the worst thing I found was that the crankshaft assembly had angular misalignment. All three engines that I have done had pretty much the same defect. As you probably know, the crankshaft is an assembly that is pressed together; well, for understandable reasons, when the assembly is pressed together, it is not perfect. If you were to put one of these crankshafts in a lathe and spin it, you would be shocked!, I mean really shocked! The misalignment is very pronounced. You don't need a dial indicator to see it; it is totally visible! The crankshaft ball bearings were never designed to operate in conditions like that. The angular misalignment also throws the flywheel weights off, so that just adds to the vibration. When I build an engine, I straighten the crankshaft, then take a skim cut to true up the flywheels. It is a quick way to get reliability from the engine. The bearings just cannot cope with the shaft torquing from side to side due to that angular misalignment. All the crankshafts I've seen are bent the same way, and I'm sure your crank is bent too! It's terrible! Another thing I would like to share with you all is that the flywheel weights are lightened from the factory on the connecting rod side. On the GT-5A (and I presume the GT-5SR as well) motors, you can see the drill holes, but on the older models, the flywheel weights are held in place by 3 screws. If you were to remove one of those weights, you will see that they milled some slots on the inside of those weights for balancing purposes, and when they are attached to the plates, you can't see them. I tried further lightening the flywheel weights on the connecting rod side, and it made the vibration worse. I might start selling reworked crankshafts for people who want to redo their engines, but don't have a machine shop. I predict I'll get to that in about a month.
Thanks,
-Fred

 

[biknut]

Hi everybody! I've been very concerned about vibration as well. I have investigated the crankshaft and found a few things that were wrong. First, the worst thing I found was that the crankshaft assembly had angular misalignment. All three engines that I have done had pretty much the same defect. As you probably know, the crankshaft is an assembly that is pressed together; well, for understandable reasons, when the assembly is pressed together, it is not perfect. If you were to put one of these crankshafts in a lathe and spin it, you would be shocked!, I mean really shocked! The misalignment is very pronounced. You don't need a dial indicator to see it; it is totally visible! The crankshaft ball bearings were never designed to operate in conditions like that. The angular misalignment also throws the flywheel weights off, so that just adds to the vibration. When I build an engine, I straighten the crankshaft, then take a skim cut to true up the flywheels. It is a quick way to get reliability from the engine. The bearings just cannot cope with the shaft torquing from side to side due to that angular misalignment. All the crankshafts I've seen are bent the same way, and I'm sure your crank is bent too! It's terrible! Another thing I would like to share with you all is that the flywheel weights are lightened from the factory on the connecting rod side. On the GT-5A (and I presume the GT-5SR as well) motors, you can see the drill holes, but on the older models, the flywheel weights are held in place by 3 screws. If you were to remove one of those weights, you will see that they milled some slots on the inside of those weights for balancing purposes, and when they are attached to the plates, you can't see them. I tried further lightening the flywheel weights on the connecting rod side, and it made the vibration worse. I might start selling reworked crankshafts for people who want to redo their engines, but don't have a machine shop. I predict I'll get to that in about a month.
Thanks,
-Fred

Yeah these cranks are the worst. Every one of them I've checked was off quite a bit. It's hard to find one that's even close to true.

Either your bearings go bad, or the bearing wobble around on their shafts, and wear out the block holes they're pressed into.

What I'm finding though, is it's surprisingly easy to adjust the crank with a hammer. It's easy to adjust, but that doesn't mean easy to make true.

So far I've only been looking at one shaft at a time. When I adjust one, it makes the other side go off. In a little while I'm going back to harbor slave and get another dial indicator so I can see both shafts at once. I hope that will help.

 

[crmachineman]

Hi Biknut.
Okay, that's alright; if it works, it works! I do them on a lathe. And I've found the flywheels need to be trued up, but I think straightening the crank is the most important thing, and you are doing it. Even though I'm running a straightened and trued up crankshaft from a GT-5A engine, it still starts to vibrate more than my liking at high RPMs. I would like to also be able to dynamically balance them, but I don't have the technology for that, yet.
-Fred

 

[biknut]

Hi Biknut.
Okay, that's alright; if it works, it works! I do them on a lathe. And I've found the flywheels need to be trued up, but I think straightening the crank is the most important thing, and you are doing it. Even though I'm running a straightened and trued up crankshaft from a GT-5A engine, it still starts to vibrate more than my liking at high RPMs. I would like to also be able to dynamically balance them, but I don't have the technology for that, yet.-Fred

It's probably not worth the trouble. You can't really balance a single cylinder. All you can do is make it smother at some speed. If you make it smooth at red line, it will vibrate at slower speeds. If you make it smooth at slower speeds, it will vibrate at red line. You just have to decide where you can best live with the vibration.

I think a lot of stock motors are balanced for about 15 mph.

 

[ivan H]

I dont have a real vibration issue. It aint perfect, but the triangle mounting of shift kit (which could b done with any non shift kit motor) & eliminated chain tensioner really helped, also, my CDI. Proffessor Dave, when he built his CDI from the schematic I had posted commented on how much it smoothed it out both down low & up high, & let it rev higher. I'll finish in next post as I'm outta phone screen & typing blind

 

[ivan H]

Also, I think many people tend 2 mishandle the easily distorted crank . When I replaced bearings I bottomed both crank & the 1 countershaft bearing thats a tight fit in the cases, then thru gasket in torqued them up & inside mic'd between the mains, outside mic'd the crank 2 end float, then with other countershaft bearing on shaft & loctite bearing retainer in the case 2 seat it, I pulled the crank spindles thru the mains 2 bring the case halves together

 

[ivan H]

I then aligned the countershaft, torqued the cases up & left 4 the bearing retainer 2 cure. This alone gets rid of a lotta rattle & vibration & by pulling the crank thru the bearings u avoid distorting it. With everything combined its relatively smooth all over, a **** of a lot better than it was when first fitted new. Cheers

 

[biknut]

I made this short video today showing how to check crank runout

http://www.youtube.com/watch?v=CZv_sVNC_q0

 

[ivan H]

Hi Bikenut, good vid, even tho I couldnt c as well as I'd like on a phone, but yeah, we know these cranks aint perfect. I think theres alotta misconception about balancing 2. Theres no perfect balance factor, tho thats a lot different 2 "true" also. We just gotta make do with wot we got. By using good techniques & approaching vibration in a "whole deal" sortta way they can run smoothly enough, sortta. Cheers

 

[ivan H]

i guess wot I'm saying is that vibration doesnt come from just the crank. These things dont produce wild rpm for the crank 2 cause huge vibration & vibration is caused by a good many aspects of the whole package. Cheers

 

[biknut]

Even though the shafts are straight, the flywheels are flapping around like elephant ears. It's easy to see why after you get the shafts straight, you then need to lathe the flywheels. I'm not able to do that, I wish I could though.

 

[ivan H]

Hi Biknut. Yeah they're manufactured to pretty sloppy tolerances. If someone offered an aftermarket alternative I'd deffinitely b interested, sepecially if they used a solid crankpin, taper fit 2 the flywheels rather than pressed. I think a 5 piece would b preferable 2 a 3 piece so any metal removed 2 obtain balance factor wouldnt lower crankcase compression ratio, as it aint good 2 start with. We can only hope,,, we might get lucky one day. U have good patience Biknut, I would'a lost it way before 4days. Cheers

 

[biknut]

Hi Biknut. Yeah they're manufactured to pretty sloppy tolerances. If someone offered an aftermarket alternative I'd deffinitely b interested, sepecially if they used a solid crankpin, taper fit 2 the flywheels rather than pressed. I think a 5 piece would b preferable 2 a 3 piece so any metal removed 2 obtain balance factor wouldnt lower crankcase compression ratio, as it aint good 2 start with. We can only hope,,, we might get lucky one day. U have good patience Biknut, I would'a lost it way before 4days. Cheers

LOL I lost it everyday. Now I'm working on the other crank, and I've already lost it twice, and it's not even close yet.

 

[ivan H]

Ha,,, understandable 2. Good luck with it, hope this 1 doesnt take as long 4 u 2 get 2 a reasonable state. Cheers

 

[BOYGOFAST]

INCORRECT,high speed balancing is done @ crankshaft counterweights.Few if any of these assembles need high speed balancing 11.1 or greater compression ratio's with forged rod and piston sets will endure abuse if balanced correctly this equates to PRECISE centers on the entire assembly as well .I've yet to hear of anyone building a machine or having one that will measure foot-lbs at the bearing surfaces that mount the crankshaft while spun at high speed with the connecting rod wrist pin and rings or ring load on the piece spinning but this is correct procedure as I describe it for those whom don't know or have never heard of Boy Go Fast.

 

[BigE]

Yea, I think most of us know who you are. Are you not selling on Ebay anymore ? I see Rose is kicking it big time.

 

[Huffydavidson]

hasty assembly is one of the biggest problems they just rushing drawn together and they don't care just get them out the door