what are the differences between K frame and L frame barrels?
Is there a thread difference and/or diameter difference?

Diameter is larger on the L-Frame, I believe that the TPI is the same.

Correct, all S&W barrel shanks are threaded at 36 TPI (threads per inch). The L frame barrel shanks got a larger diameter to eliminate forcing cone cracking seen on the K frames under a steady diet of magnums.

Edit Added: The frame & cylinder of the L frame is also larger to move the barrel so that the lower section of the forcing cone could be full diameter.

Diameters for Smith Barrels are..

J Frame, .500"-36 threads. K Frame, .540"-36 threads.L Frame, .562"-36 thread,N Frame, .670"-36 thread

Ken

Is that the OD of the barrel, or the OD of the threaded end?

__________________
Insert short witty words here

No reference materials at hand to verify the diameters are correct, but it is written like it's the OD of the threaded shank. It clearly isn't the diameter of the barrel forward of the frame, too small-in most cases.

Thank you, Mr. Moore. I guess it would take some machine work to "upgrade" a K frame to an L barrel.

__________________
Insert short witty words here

They are the OD of the threads straight from the armorers manual,Barrel diameter is different for all frame sizes and some are shaped different.

Ken

While you could machine an L frame barrel to fit a K frame, all you'd end up with is a K frame barrel shank. (Recutting the threads to full depth in the same grooves would be real interesting with that fine a thread pitch.) Since beefing this area up was the major reason for developing the L frame, this seems pointless. Trying to go the other way around isn't going to work. You can't turn a K frame into an L frame.

Aussie44 kindly shows that the L frame barrel shank is 0.022 inches larger. What that doesn't show is that the frame, frame window, cylinder diameter and yoke centerline to bore centerline distance of the L frame are larger than the K frame dimensions. This doesn't mean that you might not be able to find someone willing to try to turn a K frame into an L frame, it just means that you don't want to shoot it if they succeed.

Thanks folks!