I just came into posession of an iron piston 25 FP that appears to be unrun. What break-in procedure works best on this version of the FP. I'm assuming it will need a bit more oil than the ABN version.

I would think that all the usual differences between an "old style" iron/steel engine versus the "new style" ABC/ABN breakin routines would apply. That is, your older iron/steel OS .25 FP would require fuel with a greater oil content (like 22-25%) and the oil would preferrably be at least 50% castor. Unlike an ABC/ABN engine (which is broken in at near full speed), the iron/steel engine break in routine would consist of numerous rich mixture, short runs (like 2-3 minutes) with a full cool down between runs. All this is based on my experience and is just my personal opinion. Others may have better info for you. Hope this helps.
DennisV

I figure it will have to be something like the break-in on a Fox 35. I didn't know if there was anything different between the two that might warrant special attention. Anyone have any experience with how long one of these usually take to break-in? Hopefully a bit quicker than a Fox.

Elwyn,

You got it break it in like a fox. 2 minute fat run and let cool in between. As soon as it can run leaned out for the two minutes with out sagging It's ready to use. I my experience it dosn't take as long to break in as a fox. I use powermaster GMA 29 10 on Iron and powermaster 22 10 on ABC/ABN.

Tight lines,

Tom

I believe the Fox is a special case and needs more oil than most (29%). I use 22%-25% oil for iron steel and ABC (or N) sleeve bearing engines. Larger BB, ABC engines can tolerate less oil. I would not go below 20%.
Castor is used as insurance in an iron/steel setup to prevent or lessen thermal runaway on a too-lean run. Iron/steel is much less tolerant of too-lean runs.
Since it is a new engine, I would use castor/synthetic mix to run it clean. Later, as the piston cylinder fit becomes excessive (worn out), you can switch to all-castor and regain some compression to extend the engine's life. As little as 3% synthetic will keep the engine clean.
The short runs followed by complete cool down is recommended for most iron/steel (as well as ring) engines.
Your choice. Excess oil will seldom harm an engine.

George

Marvin Denny is convinced that the Fox 35 needs the high castor content for cooling, not just lubrication. A high content synthetic will still burn the engine up if it's not castor. I guess it's kind of like anti-freeze - seems like it should be better, but in reality, it won't cool an engine as well as plain water. The Engineer in me says that must be because Castor has a higher specific heat.

I think part of it may be more of the synthetic burns up while a lot of the castor gets ejected taking a certain amount of heat along with it.

I've read that the extra oil IS for cooling. Fox .35's (not necessarily other Foxes) and red head McCoys seem to run better with higher oil content.
Not sure about the synthetic thing. It may depend on how you run it. A Fox .35 running in a 2x4 will probably benefit from synthetic also...perhaps not as much. My GUESS is that when you run it hot (where the synthetic flashes off) is where you would benefit more from all castor.
Duke Fox recommended Fox Superfuel (29% all castor) and he knew his engines. If Marvin Denny says all castor is best, I would go with all castor in a Fox .35...actually, I DO go with all castor in my Fox .35.

George

one thing I would suggest is use a smaller prop which will allow the engine to break in at operating RPM while being well lubed as well as cooled by unburned fuel. I use a 9 x 4 on Fox stunt 35's.

There is an interminable discussion of castor vs synthetic oil on the fuel forum. I fly with Sig Champion 10% nitro 20% oil, 50-50. On my Tower 40, which seems to me to run hot, the oil residue is all clear and colorless, not excessive. With my TD 09, there is oil all over the airplane and it is pink. I think the Tower burns the synthetic. However, the TD 09 runs cooler and the synthetic does not burn but is deposited on the airplane along with the castor.

Jim