Call me 'too complicated', but being a mechanical designer, and a drafter, I play with numbers, and like dimensions..

Here's what I do (and I havent had a problem yet!) jsut use simple math.. total distance from the firewall to prop, minus prop to front moutning hole, minus distance to rear moutning hole.. Will give you where the holes should be..

You know how far it is from the fire wall to the back of the spinner (or the backside of the prop) DIM 'A' [ 4.25" let's say..]

From the engine's data sheet, you know how far it is from teh backside of the prop. to the front moutning hole DIM 'B' [1.375"]

From the engine's data sheet, you know how far it is from teh front moutning hole to the rear (second) mounting hole DIM 'C' [1"]

You know the engine moutn will mate with the fire wall.

so, the front moutning hole on the engine mount should be drilled at A-B=
[4.25" - 1.375" = 2.875", and the second hole is 1" back from that: 1.875"]

Measured from the back of teh engine mount, measure off 1.875" and 2.875". then divide the beam into equal parts. at teh intersections, is where the hole should be drilled.

add a bit to ensure the prop will clear the cowl / has clearance.


(I've always found that clamping an enguine in place and trying to mark the holes will always lead to problems..)