5HP Crank for Stroker

gregp07

Member
What is BS part number for flathead crank..wanting to use a flathead crank for stroker motor. Seems to be alot 5HP cranks out there but not sure of part # for 3/4 output.
 
The Part number is 495645 but they are no longer available from Briggs.
I'd suggest the ARC billet crank. Part number 6580
As usual Jimbo is right. I have a ton of flathead cranks even a couple of the induction hardened ones. But I am building a stroker and using the Chrome molly billet crank from ARC.
 

spincycle

Premium User
Ive got a small block animal with a flathead crank in it, super stout piece, no problems with it and have ran it 2 seasons... other than it tends to want to push a lot of oil out into the overflow, last time i ran it, it pushed the oil seal out on the flywheel side...is that because of the shape of the lobes of the stock crank and shape of the inside of the case affecting the circulation of the oil and approx 8000 rpms, or something else? 🤔. Btw- the motor has 2 vents on the side cover and 2 vents on the valve cover...
 

delray

Member
Ive got a small block animal with a flathead crank in it, super stout piece, no problems with it and have ran it 2 seasons... other than it tends to want to push a lot of oil out into the overflow, last time i ran it, it pushed the oil seal out on the flywheel side...is that because of the shape of the lobes of the stock crank and shape of the inside of the case affecting the circulation of the oil and approx 8000 rpms, or something else? 🤔. Btw- the motor has 2 vents on the side cover and 2 vents on the valve cover...
think you may have answer the magical myth how strong the flathead 5hp crank is. 2 season and no problems with the crank. seems to be a lot of put down on the stock crank. alot of 'maybe's' it can't do this or that...etc.. but no real hard evidence that it's a big problem to run.
 

Jimbo

A trial w/o witnesses is like racin w/o tech
Oh man there are so many variables.
How much HP, what RPM you are running it at, how hard the clutch hits and since there are no new cranks anymore how old is the one you are installing and how much clearance do the bearings have.
I haven't used a cast crank for a long time because they scare the hell out of me. I also don't use the 6580C because i haven't found them to be any better than the cast briggs crank.
If you build a 2.690" bore a good low mileage Briggs crank will probably last but if you are building a 3" bore or bigger with a good head and carb and want to spin it at 9K I'd suggest you don't put that thing on a minibike unless you want to get a job as a unich.
 

delray

Member
Oh man there are so many variables.
How much HP, what RPM you are running it at, how hard the clutch hits and since there are no new cranks anymore how old is the one you are installing and how much clearance do the bearings have.
I haven't used a cast crank for a long time because they scare the hell out of me. I also don't use the 6580C because i haven't found them to be any better than the cast briggs crank.
If you build a 2.690" bore a good low mileage Briggs crank will probably last but if you are building a 3" bore or bigger with a good head and carb and want to spin it at 9K I'd suggest you don't put that thing on a minibike unless you want to get a job as a unich.
yes I agree too if a person was to build a 3inch bore and run a billet head,big cam ...etc methanol gas then your asking for it.
5hp crank in a animal block with a small bore and a stock animal head and just spinning only 8000 rpm's...it's just going for the ride.
 
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This has turned out to be as divisive as who is the best engine builder or best tire prep.
Bottom line
If you have the budget, billet crank
If you don't, used flathead crank.
Everyone will be happy. I have a lot of used flathead crank if you need one. to use in a animal.
 

spincycle

Premium User
yes I agree too if a person was to build a 3inch bore and run a billet head,big cam ...etc methanol gas then your asking for it.
5hp crank in a animal block with a small bore and a stock animal head and just spinning only 8000 rpm's...it's just going for the ride.
mine has had some head port work done with bigger valves/dual springs with an alky tilly on it, I don't dare go that big with the bore/piston....mind you, I run mine on mainly small ovals with tight corners.....and lastly, no, I do not have the budget.... ;)
 

flattop1

Dawg 89
The why of it must be pressure . The outlet on the vent tank has to flow more then the lines that connect too it .
You could pin punch the side cover , thats a 50 / 50 fix .
Loctite stud and bearing seal .
A pvc valve on the vent tank might help as its drawing in as much as its pushing out . Add in cylinder pressue. It might be quite a bit .
It would be interesting to connect a manometer to one of the vent lines .
 

spincycle

Premium User
The why of it must be pressure . The outlet on the vent tank has to flow more then the lines that connect too it .
You could pin punch the side cover , thats a 50 / 50 fix .
Loctite stud and bearing seal .
A pvc valve on the vent tank might help as its drawing in as much as its pushing out . Add in cylinder pressue. It might be quite a bit .
It would be interesting to connect a manometer to one of the vent lines .
didn't think of the pvc valve...ive got the vent tank vented with the size tubing/fittings as the incoming line...not at all familiar with the "pin punch the side cover" thing....
 

flattop1

Dawg 89
Use a prick punch too uspet the aluminum on the od of the side cover were the seal goes so it has a better grip .
I would use locktite stud and bearing lock or red first .
 

Jimbo

A trial w/o witnesses is like racin w/o tech
What did i miss?
How did we get into crankcase pressure and side cover gaskets?

Liquid gasket is superior to a paper gasket in many instances.
 
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flattop1

Dawg 89
Now there may be the problem .
Reason i suggested a manometer or blow by gauge . I have never tested one so have no idea of the pressure in the crank case . A yard stick some fuel line and a fluid you have a very sensitive manometer . From water too mercury .
The fluid weight might need to be adjusted to keep it in the tube or the length .
Could a crank case pressure test show this condition ? Same as a 2 stroke test .
 

Jimbo

A trial w/o witnesses is like racin w/o tech
What did i miss?
How did we get from cranks to side cover gaskets?
 

spincycle

Premium User
What did i miss?
How did we get into crankcase pressure and side cover gaskets?

Liquid gasket is superior to a paper gasket in many instances.
Because i mentioned ive run a mod animal with flathead crank in it that ive had no problems other than it pushes alot of oil out to the overflow tank, and mist recently, it pushed the oil seal out on the flywheel side of the engine case....were trying to diagnose it and looking for possible solutions to keep it from happening moving forward...😉
 
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