Os .25 fsr
 

I searched and searched for a thread specific to this engine and didn't find one... So here's the start of one.

I got this engine from a guy that wasn't going to get around to replacing the old rusty bearings in it. He disassembled the engine some 20 years ago or something of that nature, stuck it in a bag and put it away. Faat forward to now... I put a new set of bearings in it and reassembled it... The only thing I wasn't sure of is which way the piston was installed originally. It had no cutouts or anything, just an arrow scratched into the top. I installed the piston so the arrow pointed at the liner locating pin and went with it. It had a decent pop to it cold, so I ran it that way. It's had a lot of use it seems - the piston seal isn't stellar, but I'm sure that will improve some as it kinda re-beds in. I got a diesel head (Davis) for dirt cheap NIP which is my intended purpose for the engine. I test ran the engine on APC props, 5% nitro 20% castor fuel, and McCoy MC59 plug.

8x6 - idle stable at 2,900 and peaked at 15,000.
9x6 - idle stable at 2,700 and peaked at 12,700.
10x6 - idle stable at 2,400 and peaked at 10,800. It didn't seem all that happy on this prop - spool up was a little slow, but it didn't sag and run hot. Probably will be the starting point for prop load on diesel. It seems like it should do a lot better than this though.. Maybe more nitro will wake it up?

I bet an engine of that size and vintage would like more nitro. Maybe at 15% would be good.

I have a pair of old Irvine 40 ABC engines that are very similar to the old OS 40 FSRs. I ran fuel ranging from 0 to 15% nitro and it performed well in all of them, with some degradation of performance with nitro drop. I have not used that engine in a long time and don't have numbers for you. But my point is that these engines run well with a large range of fuel, unlike some engines that can't handle low nitro or high nitro fuel.

As a glow engine, my 2 my OS 25 Fabc seemed to like the old Top Flite 9 x 5. I also used 5% nitro, 20% oil. I later was given a bunch of old Thunder Tiger parts - one was a muffler from a 40 that would fit on the 25. I gained 900 rpm with the same combination. I had a few other 25's of various makes that seemed to like a 9 x 6 better. I thought the power and handling of this engine was excellent. One interesting note about the engine is that the previous OS 25 r/c was rated a .4 hp, the OS 25 Fabc was rated at .8 hp. It was a big power upgrade at the time...

I just used the 5% because I wanted to burn it up so I can make fresh stuff. And I also feel it's a good baseline because I like to compare performance with the fuel used. I'm sure it will take more nitro, I just don't have any 10% or 15% mixed with all castor oil in the fuel. I wanted to have an idea if this engine is severely down on power or not. I'll find another muffler that fits. I used an old Thunder Tiger .25 muffler on it because it was a bolt-on fit and it was sitting on my bench. I'll see if I have another muffler that will fit without major modification and I'll also run the engine sans muffler to see what it will do. It's gonna get a Davis Diesel conversion head, so I like to have a good baseline on glow to compare to how it does on diesel.

The 10x6 will be my benchmark diesel prop as that's what I used on my K&B .20 that I'll compare this engine to and later the K&B .28.

QWk, just for reference the OS 25 FX spins a APC 9x6 at 14,000 on 20% nitro and I would guess around 13,000 on 5% nitro so your engine is right in the ball park. You can take out the head gasket and might pick up 200 or so on 5%.

I'd found a few references to the FX, but some were a fair bit higher - probably due to higher nitro. Since I don't have the stock muffler with this engine, I don't have a way to compare the TT muffler I have to the stock one. Nonetheless I'm glad it's running about where it should be. I'll test a couple more mufflers and see what it can do on glow fuel. I may make some port and timing modifications to up the torque to better suit it as a diesel. It's a pretty tame engine with milder timing and smallish carb (6.5mm).

As as for the head shim - without the shim the head clearance is pretty tight. I'd probably need to make a thinner shim. It has a .2mm shim currently. Without the shim the head clearance is .15mm (.0059").

my numbers are for a bone stock engine. We race an outlaw class here in cali every month with the GP spitfire and 25FX engines. all have to run a APC 9x6 prop. A few engines have turned up to 14,200 on a good day but most are in the 13,800 to 14,100 range. Now of course modified these can turn up to 17,000 on a 9x6.
Are you sure measured the head clearance correctly. .006 isnt enough at all. .016 is the lowest I have seen in this size of engine. .018-.020 is what I would think is ideal.

Without the head shim, the head clearance is .15mm. With the shim its .35mm or .0138".

I use my aircombat planes os fx .25 engines and my best little bit tuned runs apc 9x5 15%nitro 18%oil synt/castor 16000rpm.

The TT muffler is quite a bit larger than the stock muffler, and the outlet is larger. I found it ran a few hundred rpm better with the TT muffler than with the stock muffler, which I think is very restrictive. Didn't you do an adapter to run your Enya 30SS with a 40 size muffler? The Enya 30 has the same bolt spacing, so why not give your OS the same treatment?

I'm away from home, so don't have my figures in front of me. I remember liking the engine a lot, but from figures I've seen it is not nearly as powerful as the FX.

Jim

I used a TT .46 Pro muffler on the Enya .30 as well as a .46 Jettstream muffler.. I haven't tried the big muffler on the OS yet - I the TT .25 muffler fit without modification for a test run to see if the engine was gonna even run. Next test will be a .35 Jettstream muffler (stock bolt spacing for OS/ST .21-.32) and later the .46 size mufflers. I didn't get the stock muffler with the ..25FSR, so I used what was handy and fit. Part of the reason the FSR isn't as strong is likely due to the timing. The FX is timed hotter and likely has a bigger carb. Fear not, I will test different carbs and exhausts to see what my FSR can do - I need some data to compare for when I put the Davis head on it. I've been so busy, I haven't had a chance to run the FSR further.

G'day Tim,

Every OS I've seen has the arrow on the piston pointing to the exhaust port. Hopefully that's the way yours ended up?

Dave H

Greetings Dave H!

This engine didn't have a factory marking on the piston - it was a marking from the previous owner. The previous owner said he either marks the piston for the exhaust port or carburetor. In this case, it was the carburetor/front of engine.

The engine ran no differently with the piston installed in either direction. It's fairly worn - the piston seal isn't that great, so it won't idle super low. 2,800 is the brink of quitting. It doesn't hand start very well either. But it holds a needle setting and peaks at 11,200rpm on a 10x6 and 16,000rpm on a 8x6 (APC) using 10% nitro. Ran it with a Davis head and my own diesel mix and got 11,200 on the same 10x6 I ran on glow. Compression screw buried - contra not flush with chamber yet so it might give a little more power with a longer contra screw. Would not hand start on diesel either. I think it could use a new piston/liner (iron/steel).

That's interesting that OS didn't mark the piston, I've never seen an OS scneürle piston without the arrow so assumed they all had it.

I'm sure you're fully aware of the requirement for the con rod big end chamfer to face the crank shaft, and that baffled pistons, (pinned) ringed pistons, and pistons with holes and cut outs that match the transfer ports to be fitted the correct way. It's also advisable to keep lapped and ABX Pistons the same way round as they were run in for best compression seal.

Once the the engine is as worn as it sounds like yours is, I guess it doesn't matter as much though. You obviousely have found that Diesels need better comp seal to start than what a glow can get away with.

Davis deliberately makes the comp screw short enough that beginners won't be able to accidently have the piston hit the contra (very hard on the con rod). I replaced the screw on my head to get the comp high enough to run. I found that the contra was protruding well into the chamber. The piston was dark coloured under the bowl and squish band part of the head, but had a ring of clean metal under the contra. The contra must have been very close to the piston.

In theory the contra should be close to flush with the chamber. I replaced the head gasket with a thinner one. The contra is now in a much better position, and the original comp screw now works. I was surprised how much difference a small change in head gasket thickness made.

This wasn't an issue with most original Diesel engines because the contra is full diameter and ran in the bore, they were either dead flat with no bowl shape, or conical shaped to match a conical piston. You could set the contra too close to the piston but the engine wouldn't run that way (unless the compression seal was really low). The conversion heads mostly have the contra run in the head insert so it's possible for the contra to hit the piston, but the engine still be under compressed, if squish clearance is too high. A worn P/L makes this worse.

Dave H

The .60FSR I got at the same time as this engine has no marking on the piston either... Both engines are late 70s vintage - the .60 looks like it ran about a dozen tanks, crashed, and got disassembled and put in a bag. The guy I got it from said both engines had been disassembled and bagged for the last 20+ years...
The conrod was removed from the piston - the entire engine was disassembled completely - bearings removed, needles removed from carb, rod out of piston, etc. I know which way the rod needs to face - that was marked with an "R" for rear anyway. Piston was marked with just an arrow. Initially I assumed the arrow would face the rear since the rod was marked as such (I always look for the chamfered bushing on the rod to confirm its orientation), but after confirming with the previous owner, the arrow was supposed to point forward instead. I did run a tank or two through it with the piston flipped around (the piston had no other distinguishing markings or cutouts to signify which way it's supposed to face either), but later "put it right". It ran absolutely no differently. I do understand the importance of piston and rod orientation in regard to running in and keeping that the same way - with these engines - that wasn't easy to do.

I've replaced the comp screws on a couple of heads because they were too short - I'll do the same with this one. I installed the Davis head out of the package with the factory OS head shim as the other Dave has suggested. The Davis head came out of the bag 1/4 turn out from bottomed out, yet the instructions say the starting point should be one turn out from seated. I set the head to that setting (1 turn out) and it wouldn't start that way. I had to prime it heavy and increase compression pretty much to that 1/4 turn out before it would pop and eventually fire. I have some other engines that I'm gonna see if the bores are the same. If they are, I'll see if there are any Pistons that fit the liner better than the stock one. Can't hurt to try. Piston seal is paramount to a good low idle and hand starting. That is partly why I don't care for ringed diesels. They're not that easy to hand start either it seems.

Hi Tim,

I just reread my post.

I completely understood about your piston.

I'm sorry my post came across as trying to teach Grandma how to suck eggs, that's the opposite of what I was trying to say. What can I say, it was late at night.

It's good to see that you're still Dieselling away

Dave H

I never took your post that way. No worries. ;)

I found a new piston/liner for my .25 FSR on eBay, so once I receive it and get the engine running with the new P/L, I'll update here. I don't anticipate any real gains in power, but I suspect it will idle better/lower.

Finally finished breaking in the new piston/liner in this .25 FSR and got a peak reading after (8) 6min heat cycles. I let it run about 30 seconds at peak without any sagging and obtained 15,550rpm using an 8x6 APC using a Jett tuned muffler, MC59 plug, and 75/25 methanol/castor fuel. I used the same fuel for all of the break-in runs. The piston seal still stinks - not any better than it was before I ran it... It starts on glow by hand easily (back flipping). I'll run some nitro through it and give it some time to finish bedding in before I dieselize it.