Could any one of you 4-cycle guru's help out a person new to them. I have a .45 2-cycle size spacewalker what size 4-cycle should I put on it?

This type of question shows up quite often in the engine forum. The answer is usually dependent on what type of performance you want. If you want something that putt-putts around, a .56 four cycle could be ok. If you're looking for shorter takeoffs, higher speeds, and more vertical, suggestions go to .70 or even higher. Another factor is whether the plane has light or heavy wing loading.

I typically multiply the two-stroke displacement by 1.4 to get a workable four-stroke displacement.

Example: An airplane recommends a .46 2-stroke

46 * 1.4 = 64.4 So, I might choose a Saito .65 for it.

This doesn't work for all aircraft types (high-performance aircraft, Pattern, etc..) but it works fairly well with sport models.

Awesome thanks for the great info guy's. I see a 65 or 70 4-c in my near future

Actually the factor should be 1.5 not 1.4, .45 x 1.5 = .675. Right between .65 and .70. However a .46 x 1.5 = .69, very close to a .70.

I think the x1.5 ratio is about right too.

What Spacewalker?
I tried to fit an OS 70 in my Seagull and the mount was too narrow (you would need a new mount and blind nuts which are hard to get at on that model)
I believe the Saito is a bit narrower and should fit the stock mount.
Of course if it is another model, like Great Planes, the preceeding information is quite useless

Try the Saito .62

It's the same physical dimensions as the 50 and 56 so it's lighter and more compact than the old .65 yet it packs more punch.

It's actually lighter than most 46 two strokes when you include the weight of the muffler on both motors.