I just happened to be stumbling on here and figured I'd jump in.
What're my qualifications? Well, I coded the first crank balancing calculator that tells the people what everyone asks "How do I know how much material to drill, what size bits, how deep, how many holes, how many grams to go from one factor to a desired factor" I also can provide in detail math values for those interested and not make claims and when asked for proof/explanation I insist "bro just google it" Coding the calculator was an attempt to right the wrongs in MB society lol
The video is dangerous. I am aware of it and it's VERY inaccurate, almost can be classified as a work of fiction. First off, the "a crank has to be minimum 65% or it won't run" which is a quote from a video of his which is a statement pulled out of thin air. This false idea leads to what? People believing it needs to be 90% or 85%
·The Values in His Cheat Sheet are Impossible.
1) He claims a 12mm drill bit that is drilled 12mm deep will yield a 20g reduction. This is wrong and it is more like 10g. Density of steel is 7.85cm/3 but I'll fluff it in his favor to 8cm/3, it is impossible, including the other volume reductions provided.
Examples and Formula :
12mm drill bit radius = 6mm (0.6cm)
Area of 0.6cm radius =
1.13 cm2
20g ÷ 8cm/3 = 2.5cm2
2.5cm2 ÷ 1.13 cm2 = 2.21cm2
2.21cm2 ÷ 2 (holes) = 1.1cm
1.1cm to mm = 11mm
In order to remove 20g with 12mm drill bit, you must do two holes 11mm deep.
Through his cheat sheet, no, drilling 12mm deep w/ a 12mm bit will not remove 20g
By using a smaller bit diameter, we will need additional holes as it'll require more depth. Let's do only additional side holes (two more on each side and no center hole) with a 3/8" (9.5mm drill bit)
2.5cm2 ÷ 0.708cm2 = 3.53cm2
3.53cm2 ÷ 4 (holes) = 0.88cm2
0.88cm to mm = 8.8mm
Using a 3/8" bit to remove 20g, you would do four holes total (two on each side) 8.8mm deep
For it to be possible to remove 20g with a 12mm drill bit and drilling 12mm deep, the crank would need to have a density of 14.74/cm3 which is a density closer to uranium than steel.
2) 90% balance would be horrible. Cranks coming out of china are around 25%-30% (reciprocating mass plays a huge role, a crank that uses the reciprocating mass for a 47mm top end would have a higher factor than if you put a 52mm piston on it) I have seem them as low as 20% the more weight on the upper conrod, the more mass needs to be removed.
3) Balancing doesn’t make the engine smooth across all RPMs. For example, on my 372xp build (see photo), I used a crank at 44% balance, and it was perfect for cruising RPMs compared to a 53% balance. Balancing at 50%+ makes it smoother at high RPMs, not low. The common claim that "55% is best" is misleading; it depends on your typical RPM range. Balancing to 57% for cruising at 5,500 RPM is counterproductive.
3a) Anyone claiming "I balanced to 90%" didn't actually balance anything and drilled holes that someone else told you to do, therefor you don't know what you balanced it to. Sorry.
Going off of your comment on "any type of crank hi/low hole" that wouldn't be correlated. Whomever said repeated this, yikes.
1) Cranks cannot be low hole/high hole, only the pistons, and the pistons do not have a different weight based on low/high hole. Historically low hole pistons were paired with short rods (85mm) because a low hole piston on a long rod (89mm) would cause the piston crown to be way above the cylinder deck opening.
2) His cheat sheet is
way off for connector rods.
2a) He posts the total weight of the connector rod. This is once again VERY wrong. You only measure one side of it because its the reciprocating mass. You can't even manipulate the math to make that work.
2b) Doing a "mock balance factor formula"
·Reciprocating Mass = 140g
·Hung weight = 50g
·End of conrod = 38g
·Balance Factor = 49.5%
If I used 85g as he claims, it would now be 60%
People can argue "he's reliable, everyone does this, you're wrong," but that doesn't change how math works. I had to engage in a frustrating debate on YouTube because someone claimed the port timing formula for exhaust/transfer was (Opening degree ATDC x 2 = duration), which is mathematically impossible. Viewers kept repeating the incorrect formula without providing any numbers to prove or disprove it. I respect
@Wevil Kenevil for saying, "Here are the resources, let's figure this out"—a rare and commendable approach.
The calculator I referred to coding is listed below.
Any questions or clarification of what I have mentioned, please ask. Your question may be someone elses in ten years. This forum is a vault of information from 2008 that I still find valuable today.
Note : Google scripts won't open if you use an ad-blocker, I have no clue why but thanks big tech
