View Full Version : Harley Balance Factors

Bill Pedalino
03-17-2005, 06:29 PM
I am restoring a 1947 FL and a 1952 FL (both to factory specifications). The 1947 model year was known for vibrating 74" overheads (as well as blown head gaskets and broken cylinders).

Because I don't want a shaker, I will static balance these flywheels, but need to know what balance factor the factory used (for the reciprocating weight portion) and what it should be adjusted to for harmonic balance in the mid (crusing) RPM range.

I am thinking that a 55% reciprocating factor should be close for the panhead and the knuckle. Any suggestions ??

By the way, this bulletin Board is a GREAT addition to this web site !!

ON to OLEY !!

T. Cotten
03-17-2005, 09:01 PM
Bill P. wrote:
>>The 1947 model year was known for vibrating 74" overheads (as well as blown head gaskets and broken cylinders). <<

Somebody's gotta contest that notion.

But meanwhile, ...
The v-twin motor is very forgiving for wide variance of factors. Vibrations the rider feels are often chassis resonances, and not always curable by the carved-in-granite] 60% factor that has become accepted as the best internally .

There will always be vibes, but I wish you good ones.

A simplistic discussion of this can be found at http://virtualindian.org/1techflywheel.htm
under the "Theory" and "Balancing" topics. Although aimed at v-twins of a different angle, the laws of Nature and the practical techniques of static motorbalancing still apply.

A poor choice of factor might break motormounts, but other problems cause 'blown head gaskets and broken cylinders',.. even on "balanced" motors.

Bill Pedalino
03-17-2005, 09:41 PM
Thanks fo rthe reply.

I've balanced many motors and know that, considering the flywheel mass, small variences in the balance factor do not make a large difference in the median balance range - We've used 60% for S&S stroker wheels, but used 55% for stock Harley flywheels. I no longer work as a motorcycle mechanic (except on my own old iron) and haven't balanced a set of wheels in years, so I guess the acceptable factor is now 60%. Thank you - I appreciate the updated information!

Concerning the head gaskets, the 61 barrels were fine but the 74 cylinders were problematic as they were manufactured without the fire ring and certainly were known for blown head gaskets. Back in ancient history (1960's and 1970's) I worked as a Harley motor mechanic (actually for Eastern Motorcycle Parts, before he became a wholesale parts manufacturer). In that time I've repaired many knuckle FL head gaskets and have seen a few broken cylinders. The heavier re-pops are actually a welcome improvement to the original 74 cylinders.

03-18-2005, 04:04 AM
Bill -

I don't accept 60% as the norm for the heavy wheels, like used in the previous generation of Harleys. I still follow the 55% (and less) balance as the norm for these. But mostly, I feel that if care is taken to true the wheels to less than the book spec, along with this balance factor, your engine will be as smooth as you would like. Of course, the typical sprockets were 23T and 24T on the engine, and 22T on the transmission. That was because road conditions didn't allow for the speeds we travel, today. Those make a difference in what the bike will feel like at the handlebars and mirrors.


Bill Pedalino
03-18-2005, 07:46 AM
Thanks so much for the input Jack. Your position is consistant with the factors we used years ago - and with generally good results.

One odd recallection is that when balancing a number of factory flywheels at a given time using the same factor, some would require almost no drilling while some would be significantly off, even when using stock pistons. This seemed unnerving, as one would expect the factory Quality Controll to be within a tighter range. It also wasted a lot of time in second-guessing yourself by re-measuring and re-calculating just to make sure that you didn't mess up.

I would have to assume that the early knuckle FL's may have suffered from a wrongly-assumed balance factor. I was born at the end of the knucklehead era, so I can't speak from personal experience, but predessors whom I respect (one being a long-time New York Harley dealer) who pruchased new '46 and '47 FL's have often asserted that they did vibrate and that, for that reason, many preferred the EL.

Thanks again fo the input. I will most likely use the 52-55% balance factor for the pan and the knuckle (and most likely for my '64 CH).


03-18-2005, 02:38 PM
Bill -

Since you bring up the fact that you might be balancing your older Sportster, those wheels are skinny, and lighter. And, the typical factory balance factor for them was in the range of 50%. So were the WL's before them. I'm satisfied that it was to leave more meat on the wheels, as lower factors do this.

As far as the 'unnerving' goes, in the future, you can jot down some notes as to what all the typical weights of stock engine components would be for specific type engines. Then, when checking those type wheels, hang a bobweight on each and shim as needed to find the zero balance point (at this point, I'm usually matching the heavier wheel to the lightest). Now, you can reverse-calc what factory factor may have been. I believe you will find as I have, that the Big Twins were in the mid-50's range, and the WL/K/XL's were closer to the lower 50's.

Oddballs show up on occasion. My WLA has been in my possession since 1976, as a basket case. I don't know how long it had been a basket case before I owned it, but looked to have been in this condition a long time. When I started the rebuild of the engine, in recent times, that's when I found the almost-swiss cheese lineup of holes in the flywheels. But, with 0.020" OS pistons, and the weight of everything else, it came out to a 44% factor. No idea of the history behind this bike, or engine. But, I'm leaving it like that to see what happens.


Bill Pedalino
03-18-2005, 06:59 PM

You're a braver man that I. I would imagine that a 44% balance factor would do real well in acceleration. However, I would be curious as to where the sweet spot winds up.

I've balanced sportsters in the 52 to 55 range, all with good results. You're absolutely correct though- the 52% factor would probably serve a 'hot rod' bike like the '64 Sportster pretty well, as I don't plan on crusing with this bike at 60 mph for extended periods. Besides, I've had it forever, as it was my first Harley and I have an emotional attrachment to it - meaning that I will be babied !!

Thanks again!


03-18-2005, 07:10 PM
my 1947 FL with 12780 original miles doesnt shake,rattle or roll any more or less than any other HD from that era has,besides arent they suppose to?? If you want a smooth vibration free MC buy new! :)

T. Cotten
03-18-2005, 08:59 PM
Please detail what you mean by "fire ring", and how '47 FL cylinders were unique from '40-'46 F's and E's.

Many '47 Fls have survived in my region, but there has been no pattern of balancing complaints. (Or for any other vintage!)

'46 and '47 production that you referred to introduced the bullneck/offset fork which harmonically vibrates differently, especially at idle, than the generation of springers before them. Perhaps this visible difference was mistaken for a motor balance problem?

We all agree at least that there is no holygrail factor .

I'll even up the ante by asserting that the seat of the pants cannot detect a couple of percentage points difference. But that's a bet we shall never never be able to lay our hands down on. Even if we agree.

Bill Pedalino
03-20-2005, 09:21 AM
You're correct - I should define the term 'fire ring' This is what most local (Long Island) guys (or at least motor heads) often call the upper lip on the cylinder bore at the gasket surface. As you know the '41 through '47 FL cylinders don't have it, as the do other overhead cylinders.

I also agree with you that, given the mass (not weight) of the subject flywheels, a couple of percentage points will not matter much. I'm an Engineer and have studied the math of resonant harmonics, constructive reconance and impulse/momentum, so I know the theory well, or at least I used to - That was about 100 years or so ago and I probably couldn't calculate it now, even on a bet !! :confused:

So, I agree with your position. I just wanted to know if the Factory balance factor for the early 74's was and different than 61's or later 74's. I've Certainly had that question answered.

Your statement about bullneck frame angle is also an interesting point that I was unaware of. I guess it is possible that the slight change in fork geometry might cause a change in amplification. I guess I'll know after I set the balance factor at 55% and run the bike. But I'm a couple of years away from that point - and many trips to Harmony and Oley away from that as well. But it doesnt matter - these bikes are a labor of love, although intermittantly interrupted by monetary practicalities.

Thank you and the above members for your input and for helping me better-understand this topic.

03-20-2005, 07:38 PM
To change the direction just a touch, For you guys who have done a lot of balancing, where would you suggest someone to look to take the first giant step to actually start drilling and balancing? I've read with intrest the virtualindian discussion in the past and read this forum discussion, also I'm not afraid to tackle many mechanical situations on our beloved bikes, but this one just seems to have me a little psyched out for fear of the damage i imagine it could do. I have read the step by step process many times over but ,does it just seem higher when you climb up to the top of the high dive than it really is untill you take the 1st leap thanks signed scardy cat

T. Cotten
03-21-2005, 09:27 AM
Knuck cylinders without this?? (See attachment)

And 'scardy cat'!

The 'safest' approach is to hang the real assembly on knife-edges and back-calculate the existing factor.

Then decide if it even needs "fixing".

Often the factor can be juggled without risking drilling the flymass. Thinwalled wristpins can raise your factor significantly, and thickwalled ones lower it. Many forged piston designs have internal bosses for intentional lightening, particularly those of automotive design. And don't forget that existing holes can be stuffed!

T. Cotten
03-21-2005, 07:29 PM
Got to the shop and first thing checked Palmer's , and sure enough, he cites the 74" as having no "lip".

So now I must consider the implications that every 74" Knuck that I have had on the bench over the last quarter century had overbored 61" cylinders on it!
(Sure made getting the rear head on in the chassis easy, though.)

Yeah, the spigots were thin.
Consider also that all of the heads still had the counterbore for the spigots.

Bill Pedalino
03-21-2005, 07:51 PM
You're right - the heads on those '61 heads sure do go into the frame easier than the '74!!

And your also correct about the counter bore - I never even gave it a though - the heads fit '61 and 74 cylinders.

Tell you what - this bulletin board is great - even old dinasours like us can learn, re-learn or remember things !!


08-05-2005, 06:18 AM
gtsickle -

Along with Cotten's recommendation, if you have the means to do balancing, maybe you can pick up an inexpensive flywheel assembly (vintage, of course) from eBay for practice. The S&S Cycle site has some tech info on their balancing procedure, that you can download (in .PDF file format). Using this, you can build your confidence level to the point of tackling your own assemblies. Plus, you may be able to resell the purchase from eBay.

It is not that difficult to do. Just a bit time consuming. And, it needs to be. You will want to take care during this procedure in building. This is the rotating mass that dictates whether you enjoy the ride, or not. I back-calculate all assemblies that I rebuild. Never gave much thought to saving the data, as I did it for the customer's infomation, only. In recent times, I've decided to save this data, to compile for sharing with others. When I have enough variety of engine types and configurations, I will post them on my webpage.


Bill Pedalino
08-06-2005, 08:43 AM
Thanks for the reply and information Jack.

If you read some of my ealier threads you will note that I have been balancing 74 and 61 motors for many years (since the '70's). My inquiry was whether the Factory had changed the balance factors for certain years. This question was promulgated by the fact that, in 1946/1947 the street-opinion in my area was that the 74" OHV's were 'vibrators' while the 61's were not.

I have always used a 51-to-55% balance factor (depending on the model and application) with great success. I was just wondering if this factor was factory-developed of just the street-developed factor that yielded the best 'sweet spot' , developed by common practice.

Back calculating is fine, as long as you have the original pistons, rods, etc. Over the years however, MC pistons, Taiwan rods, aluminum bearing cages, etc. make this harder to do. Although, if the motor ran smoothly in your favorite RPM range, then the weights would seem to be ok and a back-calculation would probably yield an accurate balance factor.

By the way, S&S used to make a balancing set-up for retail sale. I never liked it though, as it was kind of crude, although it probably worked well. Mine is custom-machined and consists of rolling knife edges and custom ground shafts.

Thanks again for your reply!!

08-06-2005, 09:06 PM
Bill -

Actually, the reply was for 'gtsickle', and his question as to how to get started.


Steve Slocombe
10-03-2005, 12:52 PM
The early Hints and Tips books imply a 50% balance factor for all bikes, as the calculation shows weights being halved. I'm having my VL flywheel assemblies balanced to a 46% balance factor, which my balancing guy says he found in a notebook for early Harleys. It works well with this fairly square engine (typically 3.5" bore x 4" stroke). A customer has tried 42% and says it is even smoother at high revs. My balacer says the modern Harleys use 58-60% balance factor as they are tuned more for torque at low revs rather than top end performance.

10-03-2005, 04:24 PM
Steve -

60% is the number that I've heard for the late stuff, as they have skinny wheels, and this factor leaves some meat on them for torque. As to your balance factor, I've only seen it in print as to the 50% for the older stuff like yours. The lower factor would lighten the wheels, explaining the smoothness at higher revs. But, only experience from doing it proves it. I've never used a 42% factor. My WLA wheels reverse calculated to a 44% factor, though all the other typical H-D 45 wheels that I have done came out to be in the 50% range. I don't know the history on my wheels, but I'm leaving it at that, just to see how the engine feels. It's not that much trouble for me to change it later, if I don't like it.


T. Cotten
10-03-2005, 08:14 PM
Please be aware that the chassis design has direct influence on the best factor for a motor.

The late model cradle frame is significantly different from single-downtube models in how it dampens vibrations.

Matt Elvenkemper gave a good explanation of this at http://virtualindian.org/Flywheeltheory.htm

This helps to understand the wide variation in factors between early flattys and later OHV's.