R75 bottom end R100 top end?

[mattcfish]

wirewrkr, I said the R90 would fit right up.
I questioned the R100 parts. Mine (I have both an R75/5 and a '78 R100s) measures longer(Taller) on the outside, so I thought it would be good to check. My factory data sheet shows the stroke on the R75/7 as being the same as the R100, likewise the cam dimensions.
I don't have any factory data on a '74 /6 unfortunately, but assumptions can get expensive.
Both Haynes and Clymer imply the con-rod journals, etc. are the same but measurements prove it.

Going further, I found the clutch spring is stouter on the R100/7 (and s) than on the R75/7 STOCK clutch. It's the same diameter but made of thicker material and has a higher pressure rating. Probably not a problem unless you were going racing.

Stroke is the same on all /5,/6,/7 etc.
R100 rods are lighter and stronger than earlier rods, but they're all interchangable.
The cranks are all interchangable too. The counterweighting is different so vibration will be created at certain RPM's depending on which crank and piston combo is used. Cranks made after 9/75 (some earlier) should have the heavier 11mm flywheel bolts.

R90 jugs made before 10/75 will fit all /5 blocks. The blocks starting in Oct 75 were bored out to 99mm instead of 97.

[George Ryals]

Matt, stating the obvious here.....the longer rods with the spigotless cylinders keep the pistons higher in the cylinder at BDC.

[mattcfish]

Matt, stating the obvious here.....the longer rods with the spigotless cylinders keep the pistons higher in the cylinder at BDC.

Not sure about that. The pistons are much shorter and the rings actually overlap the pins. I bet the rings actually come down to the same point that stock pistons do. Am I wrong? The reason for longer rods is to create lesser angle with the piston during engine rotation. This picks up some HP that is lost with shorter rods that are moving at higher angles.

[George Ryals]

You are right, with the same stroke and longer rods the rings come down to the same point. With the wrist pin higher in the piston, the piston can be can be short enough to not need skirt support like the longer stock pistons. The bottom edge of the piston stays higher in the cylinder at BDC.

[mattcfish]

You are right, with the same stroke and longer rods the rings come down to the same point. With the wrist pin higher in the piston, the piston can be can be short enough to not need skirt support like the longer stock pistons. The bottom edge of the piston stays higher in the cylinder at BDC.

Cost of the pistons, rods, and cylinders would, of course, out weigh the cost of stripping and machining the block. Still, it is an alternative.