I own several OS 25LA engines and no FP 25s. My question is, will the back plate from the FP 25 fit the LA 25? ....Gene

Thank you. So the FP back plate fits but doesnt enter the LA case as far as the plastic LA back plate does. Does the extra case volume create any adverse conditions if i use the FP back plate on the LA?....gene

Thank you. So the FP back plate fits but doesnt enter the LA case as far as the plastic LA back plate does. Does the extra case volume create any adverse conditions if i use the FP back plate on the LA?....gene

   Hello Gene;
     You do have a valid question. and old "Two Stroke 101"  performance tip to get better performance out of an engine, and can also work with 4 strokes, is called "stuffing the crank case " and maybe some other nick names. That trick is to fill every void you can inside the cranks case in order to fill up excess crank case volume and this in turn increases crank case vacuum and thus increases fuel draw. it in itself is not a big over all answer to a lot of problems but when added to some other details does contribute to a better running engine. This was one of the reason for the Fox Stuffer Back Plate for coming into existence. If you wanted to adjust the FP back plate and see if this makes any difference, just add enough  metal to the face of the back plate to take up the difference. A flat plate of aluminum gut to a circle the correct size, then JB Welded to the back plate should do the job. Double check the clearance and if satisfied, you could add a rivet to the assembly right in dead center so the crank pin can rotate around it and seal it with a bit more JB Weld or silicone, and you know it won't go any where. You can make up a spare back plate and take it to the flying field, and put up several flights before  you change it, then several more after, and you should have your answer and not hurt a thing.
    The older metal bracket for holding the old style rear needle valve is very nice for the reasons you want it for. The newer LA series went back to a metal bracket but uses the new/old style O-Ring type needle. If you have the original LA series back plate with the molded in needle valve holder, that can be cut off and the back filed flat and just use the metal bracket or make what you need out of aluminum angle from the hardware store. Lots of options here for you.
   Type at you later, 
   Dan McEntee

            I noticed on my newer LA cases that this wasn't  a issue. I only found this to be the situation with my early LA version. I purchased mine from Tower Hobbies when they were first made available. I just made mention of this due to not knowing what you have.

This was one of the reason for the Fox Stuffer Back Plate for coming into existence.

The stuffer backplate was not intended to stuff the crankcase - it was deeper, and anodized,  to keep the conrod from getting pushed off the back of the crankpin and wearing out the crankpin and conrod big end prematurely in Foxberg Racing. From the man himself:

https://stunthanger.com/smf/engine-set-up-tips/fox-35-stunt-mods/msg34899/#msg34899

  Any "stuffing" is provides is merely a side effect, and it is far from clear that stuffing the crankcase has the performance benefits claimed for it.

     Brett