The Evil Manifold Leak.

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The Evil Manifold Leak.


Post by DuoDave » Mon Apr 06, 2009 2:19 am

Okay, we all know by now that we should carefully pressure test our inlet manifold for air leaks to ensure easy starting and even running, but how about when these bikes were originally assembled at the factory? You can bet that the manifolds and carbs were just quickly bolted in place and then the bike would be sent down the line. As Cotten has pointed out, if these bikes had been hard to start when new, HD would never have sold so many of them and police departments would never have bought them. So how did the factory get it right?

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Re: The Evil Manifold Leak.


Post by john HD » Mon Apr 06, 2009 4:23 am


ok, i'll venture two guesses.

1 when you put dozens of motors together a day a person assembling them would get pretty good at it. if the bikes wern't starting at the end of the assembly line someone would have heard about it.

2 back in the day gasoline did not disintergrate the o rings. maybe it actually swelled them to make them seal better. but not to the point of extruding the o rings into the intake like my last set!

plumbers nut manifolds would have to be someone elses guess as i only have a bike with o rings!

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Re: The Evil Manifold Leak.


Post by VT » Mon Apr 06, 2009 11:32 am

The dealers were expected to fix their customers problems. H-D® provided replacement parts or re-designed replacement parts. I think is was sort of a Krispy-Kreme donut franchaise sort of thing. The dealers were and still are supported by the factory.

If you want to take a virtual tour of Milwaukee and the way things were run at the factory - get this book. It is the very best history book you will ever read about any thing." onclick=";return false; ... 268&sr=1-1
Last edited by VT on Mon Apr 06, 2009 11:57 am, edited 1 time in total.

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Re: The Evil Manifold Leak.


Post by steinauge » Mon Apr 06, 2009 11:47 am

I imagine a lack of 40-60 years of metal fatigue and wear had something to do with it too.I know that I worked on a lot of late 60s and endless 70s and 80s HDs when they were new and manifold leaks were a daily problem.

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Re: The Evil Manifold Leak.


Post by FlatHeadSix » Mon Apr 06, 2009 1:11 pm

I think it boils down to just 2 things: the materials and the people.

After reading all the biographical and historical stuff by Jerry Hatfield and Herb Wagner and others about the early days it is pretty clear that the original Bill Harley was almost fanatic about close tolerances. The designs that he and his engineering department put out were good but they only worked well in production because the machine shop made the parts exactly the way they were supposed to be out of materials that were exactly what the specifications said they should be. The engines were assembled all day, every day, by the same people, who I'm sure developed a feel and more than a few tricks to get them assembled right, and most of all they probably cared about what they were doing.

The plumber intakes go all the way back to the beginning. The original parts were designed, machined, and assembled in the same factory. All the pieces were intended to work together as a unit. The piles of parts in the bins that the assemblers had available were all good parts to begin with, they were all exactly the same because of the close tolerance, and they were made to fit together with each other, if the assembler didn't like the way it felt or fit he just reached in the box for a different one, he had a handy supply of substitutes.

The roller bearings that harley used extensively are a good example of this. In most of the applications the bearings were "plug fit" on the original assemblies by hand using a selection of various oversizes in increments of .001", the bearings were machined that close! I remember reading somewhere that the guys that raced Indians preferred to use Harley bearings in their engines because the quality and tolerance was so much better than anything else available.

We are trying to achieve the same thing today on our bikes using an intake manifold from Taiwan Tedd, a set of nuts from Colony, some brass washers from India and intake nipples that have seen years of abuse. We're mixing apples and bananas, you just can't expect the same results!



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Re: The Evil Manifold Leak.


Post by Cotten » Mon Apr 06, 2009 1:22 pm

Let's try considering the historical perspective:

In the early decades of the 20th Century, you were lucky if you could find a stretch of road long and smooth enough to maintain speeds that a tiny leak could use to destroy a motor.

And as VT pointed out, the Dealer would have a supply of fresh annealed seals so you wouldn't have to risk re-installing work-hardened ones every time you de-coked the motor. If the manifold was worn, you could just get another off the shelf. (It must be noted that H-D produced the brass-ferrule'd manifold design until 1973!)

When Eisenhower's highway system came to be, the demand for speed and horsepower became universal. Cooler-running OHV's soon pushed the flatties into oblivion, with only the little utility 45" Servi maintaining a dwindling market.
The cooler motors, and WW2's advancements in rubber tech, allowed for the money-saving O-ring design, making regular maintenance easier, albeit necessary much more often.

In the "Happy Days" of the rest of the Century, motorcycles became much less a utility machine, and morphed into the jolliest toys that Western Civilization has ever known. But over this grand era, the average rider slowly lost his dirty fingernails in favor of polishing chrome and clear-coat.

And the MOCO eventually phased out the need for expensive top-quality career service technicians at their Dealerships, by engineering the machines to require only remove-and-replace components.

Thus, as Steinauge points out, machines that had been played with for generations suffered for it.
Today, we must resort to pressuretesting because there is no other way of telling how many worms are in the can.

After all, how else will you quickly determine if a previous owner perforated a pan cover screw into the intake port?
That's one worm at least that the Factory never had to worry about!

And Flathead Mike points out that our greatest hopes are also our greatest fears, as we now are at the mercy of the Aftermarket.


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