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Old 09-28-2015, 12:01 AM   #1
750Devo
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Default Hot rod knucklehead build

Disclaimer: There will be very few original knuck parts in this build. If that is not your thing you can stop looking now. I'm building with speed as the primary concern.

Just picked up a set of dual carb Flat head Power knuckleheads.



These are from when FHP was not owned by S&S. Supposedly these were the same set up used for the Evil Mother drag racer. Designed to use an evo style piston.

Ive have been slowly collecting parts with the intention of building a knuck capable of embarrassing modern sportbikes and harleys. Will still look the part of a traditional chopper. Follow along if that sort of thing interests you.

Last edited by 750Devo; 07-28-2017 at 05:00 PM. Reason: photobucket
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Old 09-28-2015, 12:16 AM   #2
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Default Re: Hot rod knucklehead build

Currently the cylinder bore is set up for 3 5/8" bore. Cylinder height is supposed to be for a 4 5/8" stroke. Would make a nice 96" motor.

I want to go bigger. I will most likely use a 3 13/16" bore cylinder.

I am also considering a 5" stroke.

If any of the members on here have any real life experience with a properly balanced 5" stroke motor I would love to hear about it.

I am aware of the short life span of the motor, but I don't have a problem doing regular rebuilds. Winter is long here in Canada, and the fun factor is high on my priority list.

My only concern is the vibration. My plan is to run the heavier Truett and osborn flywheels in an aftermarket 66-69 style set of cases. Just curious if any one out there has any experience running these 5" stroke motors that have actually taken the time to balance them properly.

S&s offers one in the shovelheads, how much worse can they be if built properly and geared on the higher side?
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Old 09-28-2015, 01:08 AM   #3
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Default

This should be fun. Although I can't help as I'm a limey guy, I'm watching.
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Old 09-28-2015, 03:36 AM   #4
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Default Re: Hot rod knucklehead build

Sounds interesting. What cam and carbs are you planning on using? Ignition? Do you have the cases yet? The reproduction Motortechnic knuckle cases look pretty good, no way original cases would handle that kind of horsepower!
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Old 09-28-2015, 04:09 AM   #5
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Default Re: Hot rod knucklehead build

750,
are ya friggin kidding me, hell ya I'm all in for this one.
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Old 09-28-2015, 05:12 AM   #6
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Default Re: Hot rod knucklehead build

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Originally Posted by 750Devo View Post
with the intention of building a knuck capable of embarrassing modern sportbikes
You either have no idea how fast a modern sportbike is or this Knuckle is going to be one hell of a ride. Either way, I am looking forward to watching it.
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Old 09-28-2015, 06:10 AM   #7
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Default Re: Hot rod knucklehead build

Oh hells yes!
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Old 09-28-2015, 06:23 AM   #8
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Default Re: Hot rod knucklehead build

Hell yeah!
But as a former sportbikerider, i have to tell ya. They do 100mph in first gear.
They run out of steam at 190Mph
0-60mph in 2.9sec 0-150mph in 10sec.
i say power adders an exotic fuels...
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Old 09-28-2015, 08:06 AM   #9
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Default Re: Hot rod knucklehead build

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Originally Posted by 750Devo View Post
Currently the cylinder bore is set up for 3 5/8" bore. Cylinder height is supposed to be for a 4 5/8" stroke. Would make a nice 96" motor.Limit rpm to 6200 and these last like stock. What length are those cylinders?

I want to go bigger. I will most likely use a 3 13/16" bore cylinder.
always talked about doing it but I never did one.
I am also considering a 5" stroke. 4 3/4 will last much better and balance better
If any of the members on here have any real life experience with a properly balanced 5" stroke motor I would love to hear about it.Static or dynamic mine were shakers

I am aware of the short life span of the motor, but I don't have a problem doing regular rebuilds. Winter is long here in Canada, and the fun factor is high on my priority list. High rpm and beating on them less then 4000 miles. I have a Shovel 4 3/4 that made 40000 miles but was stock bore and ran a stock cam.

My only concern is the vibration. My plan is to run the heavier Truett and osborn flywheels in an aftermarket 66-69 style set of cases. Just curious if any one out there has any experience running these 5" stroke motors that have actually taken the time to balance them properly.Never used heavy wheels in a stroker, but sounds like a good idea if you are trying to balance it out.

S&s offers one in the shovelheads, how much worse can they be if built properly and geared on the higher side?
Looks like a fun build.
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Old 09-28-2015, 04:59 PM   #10
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Default Re: Hot rod knucklehead build

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Sounds interesting. What cam and carbs are you planning on using? Ignition? Do you have the cases yet? The reproduction Motortechnic knuckle cases look pretty good, no way original cases would handle that kind of horsepower!
Haven't decided on a cam just yet. Either the hot lienweber or the S&s kn421.

Most likely use mikuni carbs, and a g5 magneto, although nothing set in stone just yet.

For cases it will either be std or S&S ALT/gen but open to other possibilites if deals surface.
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Old 09-28-2015, 05:02 PM   #11
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Default Re: Hot rod knucklehead build

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You either have no idea how fast a modern sportbike is or this Knuckle is going to be one hell of a ride. Either way, I am looking forward to watching it.

Haha you are correct on top speed, mostly I want to challenge them light to light.
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Old 09-28-2015, 05:54 PM   #12
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Default Re: Hot rod knucklehead build

G5 magneto would probably be your only option if you plan to kickstart this thing! The Leineweber cams work well in that sort of motor, I believe Anders at Flathead Power used to stock them. What compression ratio are you planning?

I like the Motortechnic cases because they look exactly like the originals, but far stronger material.
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Old 09-28-2015, 06:03 PM   #13
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Default Re: Hot rod knucklehead build

couple of adds to joe49s great post - before you decide on the cam - you need to do a valve / valve spring - rocker arm ratio analysis -- remember what you buy has to fit, when they the heads were made the cam choices were different then you have today

you need to know installed heights ( for correct spring pressures ) - you need to know the valve travel / bottom of the top retainer to the top of the valve guide and or the valve guides seal if you use adders you will need seals -- you need to know the cams Lobe lift for TDC valve to valve clearance if the rocker arms are not stock ratio -- you need to know the duration of the cam so you can make sure you take advantage of the static compression so you can kick this thing -- pump gas may not be something you will be able to buy

i am not going to tell you how to spend your money - but heavy wheels - freewheeling with that much motor in a oem kicker case is not going to happen for long

heavy wheels on a dyno take a lot longer to rev up verses light wheels - it will make more torque with heavy wheels BUT < this is the thing -- ( the speed of torque ) < that is what your after -- will be no match for lighter wheels with less torque

the heavy wheel motor will still be in second gear and the light wheel motor will be in third and going -- its all about rotating and reciprocating masses / and how it affects the speed of torque
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Old 09-28-2015, 06:44 PM   #14
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Default Re: Hot rod knucklehead build

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Originally Posted by little d View Post
750,
are ya friggin kidding me, hell ya I'm all in for this one.

X2 on this.

Edit... John said what I was trying to type and better details. Ill be watching this one for sure.


I will add this.. With the longer stroke and higher compression due to stroke and bore size and piston dome height, making more heat, valve to piston clearance when cold will be signifiacntly diffrent, hot.

I guess while typing this and thinking this out Im wondering what acceptable clearsnce would be. Myself on most I like around .055-.060".
I might be making a mute point here.
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Last edited by JAWS; 09-28-2015 at 06:56 PM.
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Old 09-28-2015, 08:58 PM   #15
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Default Re: Hot rod knucklehead build

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Originally Posted by joe49 View Post
Looks like a fun build.
Not sure of the cylinder length just yet. Should have them in my hands in the next week or so.

Thanks for your Input Joe!
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Old 09-28-2015, 09:13 PM   #16
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Default Re: Hot rod knucklehead build

Quote:
Originally Posted by joe49 View Post
Looks like a fun build.
Quote:
Originally Posted by johnjzjz View Post
couple of adds to joe49s great post - before you decide on the cam - you need to do a valve / valve spring - rocker arm ratio analysis -- remember what you buy has to fit, when they the heads were made the cam choices were different then you have today

you need to know installed heights ( for correct spring pressures ) - you need to know the valve travel / bottom of the top retainer to the top of the valve guide and or the valve guides seal if you use adders you will need seals -- you need to know the cams Lobe lift for TDC valve to valve clearance if the rocker arms are not stock ratio -- you need to know the duration of the cam so you can make sure you take advantage of the static compression so you can kick this thing -- pump gas may not be something you will be able to buy

i am not going to tell you how to spend your money - but heavy wheels - freewheeling with that much motor in a oem kicker case is not going to happen for long

heavy wheels on a dyno take a lot longer to rev up verses light wheels - it will make more torque with heavy wheels BUT < this is the thing -- ( the speed of torque ) < that is what your after -- will be no match for lighter wheels with less torque

the heavy wheel motor will still be in second gear and the light wheel motor will be in third and going -- its all about rotating and reciprocating masses / and how it affects the speed of torque
I have to confirm with S&S but supposedly the kn421 camshaft was designed specifically for these original FHP heads. Hopefully this takes some of the guess work out of the cam setup.

Biggest reason I am thinking of going heavier on the wheels is to decrease the vibration. Works well going heavier in the world of triumphs for vibration control. Going to use aftermarket cases so not worried about strength. This is def not a good setup for oem cases!

Still forming the final picture of what components to use so this is all good info! thanks
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Old 09-28-2015, 09:32 PM   #17
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Default Re: Hot rod knucklehead build

Many years ago I would take Shovel and Pan head castings down to Jerry branch for his bathtub combustion chamber modifications. They were for a friend of mine in Germany. He built ultra big bore short stroke, lightened crank motors and they sounded like sportbikes. Just food for thought.
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Old 09-28-2015, 09:56 PM   #18
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Default Re: Hot rod knucklehead build

Quote:
Originally Posted by 750Devo View Post
I have to confirm with S&S but supposedly the kn421 camshaft was designed specifically for these original FHP heads. Hopefully this takes some of the guess work out of the cam setup.

Biggest reason I am thinking of going heavier on the wheels is to decrease the vibration. Works well going heavier in the world of triumphs for vibration control. Going to use aftermarket cases so not worried about strength. This is def not a good setup for oem cases!

Still forming the final picture of what components to use so this is all good info! thanks
so your engine size is going to be around 103 inches ????

if i were you i would look hard for a local guy who had raced knuckels - they are not pans and shovels and need things the others dont - reason for local guy is you want to be very much involved and learn what its all about - i will also say you need a healthy wallet everything is on the very special

no matter who helps you - go hi tech - put EGT bungs in the exhaust as you are twin carb and at 1400 it will not if, it will burn up and it will be easy to lean it out / lean make more power but ends all the fun in a nano second

but keeping it alive is going to be the goal - i would adapt a curv control electronic ignition its been done - it will allow you a way of control the mag has no idea about - also a rev limit that can be used for shifting with a momentary ignition kill - you will also need a centrifugal lock up clutch and that will play into the the engine RPM kill - they make some very nice hats

lots more
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Old 09-29-2015, 10:24 AM   #19
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Default Re: Hot rod knucklehead build

Please everyone keep talking so i can read and soak this all in...

Lovin it
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Old 09-29-2015, 10:47 AM   #20
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i will also say you need a healthy wallet everything is on the very special
I have to disagree with that No way that is going to be enough money.
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