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Roll the engine over until the rear piston is all the way to the top. Lay a straightedge across the top of the cylinder and measure wher it is. Run it all the way down and do the same. That number( difference between the two) is the total stroke. A stock '74 is 3-31/32nds. If your number is higher it is stroked.
Robbie
 

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Since he was asking about a 1961 FL I only gave him a number for 74 CI as 61 CI engines ended in '52 and the crank wont fit the later cases. 61 CI engines also have a smaller bore, 3-5/16 as opposed to the 3-7/16 of a 74 CI.
If someone truly wanted to though they could build a 61 CI engine out of any pan with a little machine work and small bore cylinders. Then you would be making a dog into a lazy dog!
Robbie
 

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I have built several 68CI engines using 61 flywheels and 74 barrels at 3.5 inch bore. They rev nicely and pull hard as well as being very smooth running. I like them well enough that I plan to build one as my next knuckle project, a '40s TT replica.
H-D themselves used such an engine in the '40s for some of their military projects.
Robbie
 
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