View Full Version : Engine mount height difference

04-03-2012, 08:02 AM
Can some one tell me the height difference in pan and Knuck motor mounts? Better yet what are the real measurments? Have 3 frames that are different. Thanks

04-03-2012, 08:16 AM
The front motor mount on a knuck frame is 3/8" higher than a pan.

I'd love to see photos of your frames.

04-03-2012, 09:09 AM
On the ground,
Knuckle front 4.375", rear 6.125"
Pan front 4", rear 6.125"
Other differences, post photos we can tell you.

04-03-2012, 12:20 PM
Egg-zackily right, Tom & Jerry. And never try to bolt either motor directly in the other's frame, or you'll snap off engine motor mounts when it runs, from the stresses created. (just throwin' that out there for anyone who hasn't heard of this before) The "shim" is a 3/8" thick steel plate, under the front of a Knuckle or Flathead in a Pan frame; or under the back, the other way around (with some serious butchering to the seatpost & backbone), to fit a Panhead motor into a Knuckle frame. There's a part shown with a -48 PN along with the Pan frame in the '49-era parts books: it's just a plate with two holes drilled through it that sits on the front frame motor mount, used to put Flathead motors in the new wishbone Panhead frames in 1948; the last year the Big Flat was offered as a catalog model. Top motor mount, "wishbone" front legs, keylock in rt. neck casting, toolbox bracket all different, too.

04-03-2012, 12:46 PM
And in the same vein, a knucklehead inner primary will not fit on a panhead motor in a panhead frame. Why? Since a knuckle motor sits a bit higher in the frame, there is no reason for the relief on the lower inside of the primary cover where the seatpost bottom strap is welded to the frame rail. You will see that relief on a panhead inner primary, but not a knuck.
You can use a pan inner primary on a knuck, but not a knuck on a pan.
(Go ahead and ask me how I know---- I've got a real cherry knuckle inner primary, now relegated to wall art...! ;) )

04-04-2012, 08:26 AM
Thanks, I am helping member Swiss who posted pics in the 36 section of his 36 in a 42 or 43 frame and the rear mount was broke at some point and brazed back on, the motor is very tight to the back bone, so wanted to measure the mounts and see if was in the right place, going to replace the mount someday! Pictures? Not that savvy.