OS 20FP/25FP drop in for an OS 25 MAX?
 

I purchased a used Sig Wonder with a very old and worn out OS 25 MAX with a strap-on muffler.

The engine just needs to be put down, and I was looking for a replacement with an old OS 20FP or 25FP, however I don't know if the one of the FP series will be a drop in replacement? Was hoping someone could confirm, as well as confirm if the 20FP and 25FP have the same dimensions?

The current 25 MAX is PLENTY fast, and I wouldn't mind slowing things down a bit.

Thanks!

I don't have the older OS .25 to check, but the hole spacing on the FP .20 and .25 seems to be the same as well as the LA .25. The case width of the FP and LA .25 is the same but the case width of the FP .20 is .017" narrower than the .25s. I measured about 1.415" wide bolt spacing, .558" fore and aft bolt spacing and the case width is 1.137" for the .25s and 1.120 for the .20. You could convert it to mm and round it up or down to make it more accurate than with my old eyes and egg shaped holes in my motors. My guess is that they are all the same if they are plain bearings. Ball bearings are likely different. At any rate, the FP .20 and .25 would bolt up the same to each other with the .20 wobbling a bit between the bearers. I have a .15 in my Wonder and it is plenty fast, but have thought of putting in a plain bearing .25.

Thanks for the info - good to hear you have a Wonder also.
I'll be on the lookout for an old 20-25 FP series and I don't want to deal with ball bearings.

Do you know what the difference is between the FP series and the LA series? In many cases I've found that the older OS motors are higher quality - not sure about FP/LA.

-Skeezix

Hi!
The FP/LA .25 OS engines are more powerfull/better than the old (-70 and and -80) OS .25 cross scavanged engine with iron piston /cylinder.

The FP and LA are both nickel plated. I have had a couple peel, but most others are fine. The LA is a more finely engineered motor than the FP. Mostly it was for cost cutting and weight. The LA has 4 head bolts, and a plastic backplate which breaks or gets brittle and breaks even without a crash. The head is squared off, as well as the fins, for weight savings. There is also one less boost port which seems to make no difference in performance. I use both, maybe prefer the LA. I have .10, .15, .25, .40 and .46 LAs, and a few FP .15, .25? and .40s. One FP .40 has peeled very badly, but still runs great for some reason, and an LA .40 that has peeled. All were used with who knows what history. Really the peeling thing is overblown. I have dozens of OS motors that are fine, but thought it is worth mentioning.

I have the older 25 Max and also the 25 FP. The holes looks like they line up, though I didn't measure; I think the FP would fit fine. It sticks out farther by a little, maybe 3/16th inch, but unless you have a custom cowl it shouldn't cause a problem with the fit. The FP is about 1.5 oz heavier than the older type, besides sticking out more, so you might have a balance issue.

I don't see any reason to buy a 20 FP. They are exactly the same size and weight as the 25 FP. The 25FP is a very nice starting and running engine that makes more power than the 20. They stopped making a 20 when the LA version came out.

The older 25 with the strap-on muffler is a really nice, light engine that makes plenty of power for a Wonder. Why not just replace it with a newer one from eBay?

Jim