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Discussion Starter #1
I was asking this in another thread of mine and it really didn't belong there -- I was hijacking my own thread. :)

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A question for you motor building gurus. . . .For a stroker kit, what are the typical replacement parts after a different crank with "longer" throws (pins farther from centerline of crank) is installed? Here are the alternatives:

- (New pistons) Location of the piston pin within the new piston, or the height of new piston, is shortened.
- (New rods) Length of the rod is shortened.
- (New cylinders) Height of the cylinder is lengthened.

Which is it? Or is a combination of 2 or more?

(I'm just curious to know - my ongoing H-D education. :))
 

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Depending on how radical you get with the stroke, it is typically just the crank and pistons.

More radical strokes would require longer cylinders to accomodate.
 

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Discussion Starter #3
Thanks. I've read where low-RPM/lugging can damage a stroked engine. I think the stock H-D engines are "strokers" compared to most m/c engines -- this must be why lugging an H-D engine is really not a very good thing to do.
 

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way back when, we would put plates under the cylinders, use 80" wheels from a flattie....my how things have changed. now ya just order up what cha want:nod
 

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Discussion Starter #5
way back when, we would put plates under the cylinders, use 80" wheels from a flattie....my how things have changed. now ya just order up what cha want:nod
You would install the new crank and use the same pistons, rods, and cylinders -- with spacers under the cylinders? Did that affect the cylinder bolts/studs? How about the length of the push rods?
 

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I think the stock H-D engines are "strokers" compared to most m/c engines
They are.... That is where all the torque comes from: Stroke = Torque.

High horsepower/short stroke motors have to take advantage of higher RPMs and the torque multiplication effect of gear reduction to get any meaningful torque to the rear wheel.

It's bad to lug any motor, HD or not.
 

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You would install the new crank and use the same pistons, rods, and cylinders -- with spacers under the cylinders? Did that affect the cylinder bolts/studs? How about the length of the push rods?
you could get pistons with offset wrist pins, other s*it was custom, manifold, push rods etc
 

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My Pan was 4 3/4" stroke. My brother and I machined our own flywheels from steel. I used S&S pistons for that stroke and that was it. The pistons had the wrist pin moved way up into the oil ring groove and short skirts. You just had to make sure you had good clearance where the skirts would meet. Other than that, just like a stock rebuild.

If you were using the stock pistons, the old 80" flat head wheels were slightly smaller diameter than the 74 wheels so the bottom clearnace was taken care of, just had to add 1/8" "stroker plates under the cylinders. The studs were ok, but you had to watch in the intake area since the heads were now space slightly further out. Usually with the 1/8" plates it wasn't an issue but if you went to 4 1/2 stroke by opening up the 80 fly wheels for the larger 74 wrist pin, the 1/4" plates could cause issues at the manifold. Thicker "O" rings on the rubber band manifold usually did the trick. The 80" crank pin was stepped, the 74 was not, so you opened up the tapers for the bigger crank pin, you bored it offset to get the additional throw. Some people added an "oil scraper" to the cases, but S&S said it wasn't necessary,

It's been a while since I worked with this stuff so I may have forgotten some details. I editted this text to correct some nomenclature errors I made.
 

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Discussion Starter #9
Thank you for that Chris. There are parts of your post I don't understand and need to study what you said for a while. I do understand the intake ports diverging away with the taller cylinders (due to the plates). I hadn't thought of that.

Also thanks to Kainam and butch61.
 

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The skirts of the replacement pistons, because of the "high" wrist pin and stroked crank, actually extended down out of the cylinder some?
The skirts are really short, but yes, by virtue of the longer stroke the pistons do descend further down as well as up. S&S does a great job of designing their pistons, so it's more of a clearance "check" than for sure having to do any grinding. Shorter stroke engines are less of an issue.

I editted my text a little bit to correct some nomenclature errors I made.

Since MOST people will not be trying to use old 80" flat head wheels, most of what I was talking about is obsolete. S&S or Truett and Osborn stroker wheels, used with the proper stroker pistons for the engine being modified will pretty much just bolt together. Strokes up to 4 3/4 don't need stroker plates or longer cylinders anymore. It looks like S&S's stroke of choice these days seem to be 4 5/8, which yields 89 cid with a stock bore Evo.
 

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Discussion Starter #12
Thanks Chris. Here are the parts I still don't understand. I'm fairly "mechanical" but I'm not the machinist that my father was.

The 80" crank pin was stepped, the 74 was not, so you opened up the tapers for the bigger crank pin, you bored it offset to get the additional throw.
Stepped?


Some people added an "oil scraper" to the cases, but S&S said it wasn't necessary,
Oil scraper?
 

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Thanks Chris. Here are the parts I still don't understand. I'm fairly "mechanical" but I'm not the machinist that my father was.


Stepped?



Oil scraper?
The oil scraper first. On a 74 the flywheels were larger outside diameter than the 80". Inside the cases, the 74 flywheels came very close to the case insides and it was generally considered that the oil on the flywheels was "scraped" off by this close tolerance. When smaller diameter fly wheels were used, it was thought that the excess oil would fling off the flywheels into the cylinders to the point where the piston oil rings would be overflooded and excess oil consumption would occur. Some builders would weld a lip inside the cases just before the opening to the rear cylinder again with very close tolerance to the flywheels to "scrape" off this excess oil. It really wasn't necessary and S&S's 4 1/4 and 4 1/2 stroke flywheels were smaller OD than the stock Harley wheels (so longer piston skirts could be used without hitting the flywheels) and they never had over oiling problems.

Next the difference between 80 and 74 crank pins. The actual bearing size for the 80's and 74's were the same, so that section of crank pin were the same diameter. The 80's, however, had smaller diameter tapers on the crank pin and in the fly wheels. To make an 80cid 4 1/4 stroke flywheels into a 4 1/2 you would bore the fly wheel for the larger 74 crankpin, but you wouldn't bore it on the same center as the smaller hole, but offset as far as possible (an additional 1/8) to get 4 1/2 stroke. I have attached a sketch to illustrate. The smaller circle represents the original 80 crank pin and the larger circle shows how by offsetting the crank pin center is moved further out. I also included a sketch to show what a stepped crank pin is compared to a now standard crank pin.
 

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Discussion Starter #14
Thanks again Chris for the explanation. Much clearer now. :thumbsup

BTW, what tool (program) do you use to make those quick diagrams? That would be handy.
 

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Thanks again Chris for the explanation. Much clearer now. :thumbsup

BTW, what tool (program) do you use to make those quick diagrams? That would be handy.
AutoCAD Lt 2010. There are free drafting programs out there though.
 
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