OS 25 F info

[rich gorrill]

I was checking out e-bay and saw an engine for sale. The box says 25F  SR underneath. Was this engine the model befor the FP? The crankcase looks the same as the early FP but it says OS MAX. This is a converted rc engine. The guy has a Super Tigre needle valve and as stunt venturi installed. Any info as to the age of this engine and it's use on a control line plane would be appreciated.

T.I.A.  Rich

[Dave_Trible]

Somebody will have better info I'm sure but as I recall I had two .46 FSR in the day and they were a ringed, ball bearing,  tad higher grade motor than the later FP.  They did have issues.  Mine chewed up rod end bushings.  Some were drilling oil holes in the rods I seem to recall.  But if not paying too much for it-under $40.00- I'd give it a go.

Dave

[Bill Little]

From what I remember,  my .25FSR was a ball bearing with a steel liner and iron piston.  A pretty "hot" engine for the day.  I felt it was hotter than the .25FP.

BIG Bear
RNMM/AMM

[rich gorrill]

Thanks guy's i thought this may be an early ringed engine but wasn't sure. I'll try to look it up in the OS Max timeline.

[Bill Little]

Hi Rich,

I am pretty sure the .25FSR was not ringed.

BIG Bear
RNMM/AMM

[Dave_Trible]

I think Bill is right.  I pulled up an old parts list for the engine and no ring is mentioned.  Looks like the head and wrist pin and a few other parts are the same as the FP.

Dave

[Elwyn Aud]

If it's the one I remember it fits the Fox 35 mounting holes exactly but the front end sticks out a bit further than a Fox.

[Andrew Hathaway]

The 25 FSR is a steel piston, schneurle ported, ball bearing engine that predates the FP by a few years.  Head to head vs the 25 FP the FSR makes a lot more power and weighs a little more.  If you compare the FSR liner to a FP, they're pretty close.  If you think of it as a BB FP you're probably not too far off.

Later OS offered the 25 F, an ABC version of the 25 FSR.  You can spot the ABC version by a slightly taller cylinder head, and the case will have an F under the 25, vs the 25FSR which just has the 25 marking.  The ABC version runs a little stronger, more consistent, and restarts easier when it's hot.

For CL use it depends what you want it to do.  OS did sell a CL version of the 25 FSR, but they're uncommon.  As a stunt engine it needs to be run fast with a low pitch prop.  I ran mine on a P-Force ARF with a pipe, 10-3 or 10-4 APC prop, ST 60 NVA and .250 venturi.  I've also run a couple on a Nemesis with a larger venturi, bladder and open exhaust at around 90mph.  The FP is probably a better choice for sport use. 

[rich gorrill]

Thank you Andrew, great information. I think I found out who to ask if I have any more OS Max questions.

Rich

[Clancy Arnold]

Rich
I own samples of all three of the OS engines.  The 25 FP, identified by the FP on the left side of the engine is a Plain sleeve bearing engine.  The 25 FSR is a laped piston ball bearing engine.  The only identification is that it does not say what it is!  The FSR was only on the box.  The 25 F is identical to the 25 FSR except it has an F below the 25 in the circle on the right side.  Some of the 25 F engines were sold with ABC piston / sleeve engines.
Clancy

[Steve Helmick]

There was also a .28F built on the same case. If you look in the exhaust stack and don't see "gold" (brass), it's an ABN or iron/steel piston/cylinder. That was a transitional time for OS, between iron/steel lapped, ringed, and ABC/ABN technology.

I was not using engines on my models during that time, so I don't recall if they were leading the charge or just trying to find cheaper ways, or simply avoiding the stringent EPA regs dealing with plating. You'd think that if they can chrome bumpers, they can chrome cylinders, but chroming the bore of a cylinder is a tricky process, as is honing the bore to fit. Apparently, ABN avoids a lot of those problems, and works pretty well for most (i.e., low performance) purposes.

FWIW, the Magnum XL .25/.28 are pretty equivalent to the OS .25F & .28F. Not as good in quality, but real ABC, with generally good P&L fits. It would be interesting to see if any parts interchange.  Steve

[phil c]

I've got an engine in the intermediate OS 25 series.  I believe they called it the 25SF.  It is the FP25 crankcase with added meat for ball bearings.  Except the the front bearing housing it is nearly indistinguishable from an FP.  Haven't run it, but it is an ounce heavier than an FP.
Looks like ABN construction.

[Andrew Hathaway]

The 25SF came out after OS discontinued the 25F.  The 25FSR became the 25F, and led to the 28F, and then the 32F.  All of the F series are "ABC" in OS terms.  The OS engine timeline shows the 25 FSR starting in 1977, 25F and 28F both ABC in 1982, 25FP steel in 1984, 32F ABC in 1988, and the 25 SF in 1989.  Generally speaking the OS timeline seems to run a few years ahead of what I saw at the hobby shop.  The 25FP I bought in 1993 was a steel piston engine, another I bought shortly after was ABC.  Of all the above, only the FP and 25FSR were offered as CL engines.  Another fun fact is that the 25 FSR and F engines can use FP venturis, but the 28 and 32 have a larger bored hole in the case.  

25 FSR-S


25 F  


28 F


25 FP (steel piston)


32 F


25 SF

[Steve Helmick]

A club member had an OS .28F that he wanted a venturi for. It had an offset throttle clamping drawbar like a ST G.21 .46. Not centered set screws, like the pictures on the OS website!  Steve