Look at the underside of an SFN/RFN piston, you will find that the aft pin boss has been relieved. This allows the rod to slide aft and clear the crankpin, FSR/VF pistons do not have this feature. It is often difficult to get the rod to slide back on used engines due to burned oil residue on the wristpin. The SFNs were introduced because the long stroke SF-H vibrated too much and were discontinued very quickly. My experience with carbs has shown that a 7D/DV with a .355" venturi on a .61 is the upper limit for good fuel draw, and thats at 14000+ rpm. The 6B carb has a .385" hole with a thin spraybar making it too big (depending on your standards) for most airplane applications, unless a pump is employed. Helicopters can use larger carbs because they turn more rpm and have fuel tanks mounted very close to the carb. For the most power, try to find an older .65DF cylinder/piston and head, they drop in .61 cases and are awsome with a pipe.

Got it. Thanks for the great info. I now see what you are talking about.

On the VF piston in a RF case you can put the conrod on the crankpin first, without the piston on it.
Next put a piston pin clip on the forward pin boss of the piston.
Place the piston and liner in the case with the piston sticking out the bottom of the liner so the pin boss is exposed.
Line up the aft pin boss of the piston with the rear exhaust port.
Line up the conrod with the pin boss.
Slide the piston pin in through the aft pin boss through the conrod.
Place pin clip on.
Slide liner the rest of the way down until it seats with guide pin.

I figured this out for the same reason you mentioned above about the burned oil residue on the pin. I didn't want to have to pry off the conrod from the crankpin possible ruining it (the conrod). So pulled the liner out until it past the exhaust port to gain access to the pin boss. Applied a little oil and heat from a heat gun. Used a small easyout on the piston pin and kept turning it to get it out.
Worked great and no damage to the bottom boss on the conrod. This probably why i didn't run into the the problem you were mentioning on the VF to RF case swap.

I was able to get a FSR piston and liner into a SF case. In order to get it on the crankpin I had to heat up the the conrod with a heat gun through the rear of the the case with out the liner. It finally went on. I had to do the same to get it off. Someone made this suggestion here in one of the forums about how to get the SF conrod off the SF crankpin. So I figure this must be SOP for getting the conrod off of single piece cases (no seperate front housing).

Bryan

Quote:

ORIGINAL: Roguedog

I was able to get a FSR piston and liner into a SF case. In order to get it on the crank-pin I had to heat up the the con-rod with a heat gun through the rear of the the case without the liner. It finally went on. I had to do the same to get it off. Someone made this suggestion here in one of the forums about how to get the SF con-rod off the SF crank-pin. So I figure this must be SOP for getting the con-rod off of single piece cases (no separate front housing).

Brian,


Since the FSR used the current 24x22 mm bore and stroke; and the newer SF is the 'Long Stroke' design, with 23x24 mm, the engine you now have is a 'square' 24x24 mm...

It is now a .66 cid; not a .61... And you will need to see that your engine still have the correct squish clearance and reasonable sleeve port timing numbers.

You may need to rework the head accordingly...


The FSR piston; being larger in diameter than that of the SF, may be slightly heavier, which will result in elevated vibration levels (compared to the SF).

You may also need to mill the piston's skirt, to clear the crankshaft counter-weight and the back-plate.

Also, make sure your milling, or the original design, do not allow 'sub-piston induction'...from the piston exposing the exhaust port to the bottom crankcase, as it goes further up than before... It is the worst power detriment, if you are using any type of muffler.

Good info DarZeelon. When I originally decided to investigate swapping possibilities the first thing I found out is exactly what you've mentioned. I placed a 61VF liner/piston in a RF case with the longer stroke crankshaft and conrod. I Found the formula for calculating the CID came up with the same 66cid you mention. I also ran into the problem you mention. The VF/FSR piston with the 24mm x 24mm bore/stroke now had the head protruding 2mm too deep into the liner. The head would need to be milled down at least 2mm for the piston to clear. So that's why I had to also use the short stroke crankshaft, conrod, piston and liner, and the head of the FSR/VF to make it work. It's a 24mm bore with a 22mm stroke.

I got cued into the idea by looking at the spec's for the 61 SF-N or 61 SFN Heli engine. OS lists it as having the same 24mm bore by 22mm stroke as the 61 FSR and 61 VF but it has a SF case. The really cool thing was that the head bolt holes on the SF-RF case are an exact match for the FSR/VF head.

I thought it being square as well, sorta like the Chevy 327 engine being square, that was really cool. A 66cid SFN would really be *itchen as well, but your correct about having to mill down the head to get it to work. I have a old FSR head that had been through a serious preignition problem. I thought I would get it milled down to try it. I also want to try kweasel's mod with the 65df piston/liner and head. I just checked and the 65 VF-DF hase 24.8mm bore and 22mm stroke.
Bryan

Sorry, but that's not correct. The reason the O.S. Max .61 SFN-H was introduced was because American heli pilots insisted on making the regular .61 SF-H run at higher RPM than it was originally designed for. The .61 SF-H was designed for FAI competition flying, and American flyers were more into 3-D type aerobatics with much higher head speeds. The result was an engine that was being over-revved and performing badly, with a mismatch between the engine and the purpose people were using. The result was that O.S. made a higher-RPM version, the SFN-H to satisfy the American market. The .61 SF-H worked very nicely for those flying FAI helis at the lower head speeds that type of flying demands...the lower head speeds make flying much smoother, but are not so good for the fast movements that 3D flyers demand.

Were the SFN and RFN models ever offered with pumps like the SF-P and RF-P?

It seems like an RFN-P with an aero crank would make a nice ballistic pattern engine.

David

Even an oversize carb cannot make up for the low exhaust timing that these engines have. Every other brands rear exhaust 60 made more power with a pipe than standard sleeve OS61s.

Well, the SX-H piston will not work in the FSR. The top of the SX-H piston is about 1.5mm or so lower (or the wrist pin is higher) than the FSR piston. Also, the bottom of the skirt hits the crank's counterweight (that would be easy to fix). I can sell the piston to recoup most of what I paid.

So, what would happen if a portion of the top of the case was milled down so that the sleeve dropped lower into the case? What all would be effected by doing that?

Or, if you machined the head so that it extended deeper into the sleeve? Wouldn't this be the same as raising the top edges of the ports?

Or, maybe half of each?

I really appreciate all of the great info that has been posted in this thread.

David

rainedave.

Bummer, I'm sorry that the piston didn't work. Feel bad about that. I was hoping that it would. I have several FSR's that need liners and pistons

I wouldn't touch the case it would change the port timing,but you could definitly mill down the the sealing edge of the head 1.5mm to compensate for the lower piston height if you have access to a lathe. Better yet, just get a 2" x 12" piece of aluminum round stock and make you own. Theres a 21Vf on ebay right now with what looks like a custom head.

I could take the piston off your hands if you like. PM me about it.

I'm looking for a mini lathe or mill for doing just what I was suggesting to you above. I figure that It would be a lot easier to make steel liners and ringed pistons than the ABC type. I could maybe get Bowman to make the rings.

I did find a guy in New York that sells a 2.5 gallon kit for hard chroming for about $600. http://platingsales.com/index.html There's also Caswells http://www.caswellplating.com Problem for me is that I live in California and Erin Brockovich pretty much screwed us out being able to too the hexavalent type of chrome (hard chrome). The only type of Chrome you can do here without serious regulations the Trivalent type. It's still chrome just not as hard. I could also get a Nickle plating setup which might not be so bad as it would be cheaper.

The guy I talked to there said that there was a simple way to remove the old nickle off our pealing liners using pool acid. the liners could be plated again. All pipe dreams for me at this point but was interesting to find out I could get a small chrome setup if I wanted. All you need is a 12volt battery or a 12volt 30 amp DC power supply plus the chroming setup of course.

Quote:

ORIGINAL: rainedave

So, what would happen if a portion of the top of the case was milled down so that the sleeve dropped lower into the case? What all would be effected by doing that?

Or, if you machined the head so that it extended deeper into the sleeve? Wouldn't this be the same as raising the top edges of the ports?

David,


First of all, in a tapered-bore engine, changing the relative positions of the piston and the sleeve by a significant amount, would bring the piston into the sleeve, up to places where it was never intended to venture..., or short of where is was...

While the first can be overcome, by evenly sanding/micro-machining the piston, to a very slightly smaller diameter, so that it begins to bind into the taper, the same number of degrees before TDC as before...

The second, i.e. raising the sleeve in the crankcase, would just result in a looser fitting piston, less power and a behavior generally attributed to a badly worn engine.

It is much easier, with a steel-sleeve, ringed piston engine, that has a parallel bore, where this has no effect at all.


It goes without saying that the position of the head will need to be changed, to re-achieve the squish clearance and the compression ratio.
This will have no effect whatsoever on the port timing...

Raising the sleeve will require that the circumference of the head; i.e. where it seals against the top of the sleeve/head shim, be milled, so the head's insert goes deeper into the top of the sleeve.

If the sleeve is 'dropped' deeper into the crankcase, the head will need to be shimmed up, to allow the engine to run...


Dropping the sleeve (and adjusting the head placement) will result in more modest sleeve-port timing numbers; later opening and earlier closing of all ports (no change to the blow-down period), which mean a lower RPM power range and allowing the engine to efficiently spin a larger propeller.

Raising it will make it a .61 pylon racing engine...

Thanks everyone. Roguedog, being able to have sleeves re-chromed at an affordable cost would be a great service.

Thanks Dar, it is a ringed setup I'm referring to. For argument's sake, the top surface of the SX-H piston at TDC is fixed and is about 1 to 1.5mm lower than the stock piston, because the connecting rod and crankshaft are unchanged.

If the sleeve was dropped down lower in the case - by the same distance that the piston is shorter - so that the squish clearance was the same as with the stock piston, how would this alter the timing? I must be overlooking something, because the the way I envision it, the top surface of the piston would be traveling the same exact distance over the same exact area of the sleeve as the original piston does.

On the other hand, I can visualize how keeping the sleeve in its original position, while lowering the head/squish band would have the same effect as raising the sleeve up (since the piston and head are lower at TDC, but the sleeve hasn't moved.

If I'm missing something obvious, let me know.

David

Quote:

ORIGINAL: rainedave

If the sleeve was dropped down lower in the case - by the same distance that the piston is shorter - so that the squish clearance was the same as with the stock piston, how would this alter the timing? I must be overlooking something, because the the way I envision it, the top surface of the piston would be traveling the same exact distance over the same exact area of the sleeve as the original piston does.

On the other hand, I can visualize how keeping the sleeve in its original position, while lowering the head/squish band would have the same effect as raising the sleeve up (since the piston and head are lower at TDC, but the sleeve hasn't moved.

Quote:

ORIGINAL: DarZeelon

Dropping the sleeve (and adjusting the head placement) will result in more modest sleeve-port timing numbers; later opening and earlier closing of all ports (no change to the blow-down period), which mean a lower RPM power range and allowing the engine to efficiently spin a larger propeller.

David,


This was written regarding a given 'stock' engine...

If the engine, like in your case, already has components that have lengths different from the original, it is up to you to make the calculations, to determine how much to raise, or to lower the sleeve, to achieve a near-stock condition. The above tells you which way to go.

I believe OS 'did their homework', regarding the design and competitiveness of their F3A engines, so 'stock' should be pretty good... I wish I could say the same about the break-in procedure, described in the manuals...

Thanks Dar. I suppose my questions are academic. I have no serious plans to alter the engine. I'm trying to learn what would do what, so to speak, from a theoretical point of view.

So, doesn't the location of the crank pin determine the amount of travel of given point on a piston (the top edge, for example)? If the piston is traveling up and down the same exact distance, then what matters is the relationship between the top edge of the piston and the locations of the ports in the sleeve, correct? Again, I'm just trying to get a handle on all this.

David

Hey David. You mentioned that you purchased a new connecting rod for that Piston in the PM you sent me. Was it an SX connecting rod or the 60-61 short stroke rod. Also I was thinking that the hanno/SF-RF connecting rod is slightly longer than the short stroke rod, but I don't know by how much. I have to dig into one of my SF's to see. If you have a SF-Rf rod lying around you could maybe use that to make up the difference.

Bryan

David,


It is all a matter of engine design...

The distance between the center-line of the crank-pin from the center-line if the crankshaft, determines the stroke (twice that distance).

It does not matter if the con-rod is long, or short, or if the piston is tall, or slipper style; the stroke will be the same.


And as to your assumptions regarding the porting; you are basically correct.

DarZeelon. I understand about the stroke length. If the pin boss on the 61SX piston is say 1.5mm higher than a 61 FSR/SFN piston boss and the SF/RF/hanno rod was 1.5mm larger that the FSR/SFN rod, wouldn't that place the SX piston in the right location?
Bryan

Quote:

ORIGINAL: Roguedog

...I understand about the stroke length. If the pin boss on the 61SX piston is say 1.5mm higher than a 61 FSR/SFN piston boss and the SF/RF/hanno rod was 1.5mm larger that the FSR/SFN rod, wouldn't that place the SX piston in the right location?

That is correct, Bryan.

It is more familiar in full size engines, BTW.


If you were ever interested in hot-rodding a Small Block Chevy engine, for instance;


The 350 cid engine has the standard rod length of 5.7" center-to-center.

An effective upgrade is to use longer rods of 6.0", and special pistons that have a 0.3" lower compression height.


As you guessed, this bring the piston to the same 'deck height' as before.

The advantages are; decreased angular forces (toward the side of the cylinder) on the piston and a lower rate of acceleration of the piston at TDC (ultimately allowing a higher RPM limit).


The pistons have to make do with less room for the three rings and the engine needs to be rebalanced, since the longer steel con-rods are heavier...

DarZeelon. I got out my calipers and drew both con rods side by side in cad.
I already know that the the pin bosses are the the same size;

6 mm for piston pin boss
and
6.5 mm for the crank pin boss

It turns out that the SF/RF/Hanno con rod is 42mm on center.
The FSR/SFN con rod is 40mm on center.

This could be good news for rainedave as it would make up 2 mm of space. The only missing part of the equation is the exact measurement between the the top of the SX piston pin boss and the top of the piston.

My FSR/SFN piston measures 7.3 mm between the top of the pin boss and the top of the piston. The SX piston would have to be 5.3 mm to for the SF/RF/Hanno rod to work, if my theory is right. What do you think?

Bryan,


The distance between the crankshaft center-line is comprised from half the stroke + con-rod on-center length + piston's compression height (from wrist-pin center).

It should work, but pre-fitting is imperative, to make sure nothing runs into anything else...

The SX-H piston measures 6mm from the top of the pin boss to the top of the piston. The wrist pin has a diameter of 5.98mm, so the piston's pin hole must be 6mm.

That means it is 9mm from the center of the wrist pin to the top of the piston.

Using the SF/RF conrod the SX-H piston would be 0.7mm higher than the FSR piston.

0.7mm is 0.0275" and I could probably use extra head shims to make up that thickness.

I will have to disassemble one of my SF's to get a concod. Unfortunately, I only have one FSR head shim.[]

David

Quote:

ORIGINAL: rainedave

0.7mm is 0.0275" and I could probably use extra head shims to make up that thickness.

Yup, David.


The entire squish clearance would be taken up by a 0.7 mm deeper piston penetration...

But shim-stock is a non-issue... You can use a pair of sharp scissors to cut them out of an aluminium soda-pop can... The OEM head-shims would be easier, but none better.

Since joints tend to leak, the thicker the can wall; the better, since less layers would be necessary.

Well, Webra Speed .61 head shims are a perfect fit and I have a few of those on hand. I think I'll give this a try later today.

I tached my stock .60 FSR again today after running in a new Bowman ring. My TNC tach registered a peak rpm of 12,742 and I had it backed off a few clicks holding steady around 12,620ish. This is quite a bit better than the 12,163 I got last week. This was with an APC 11x7 and Powermaster 5/22 GMA fuel. I got it to idle in the 1900's. It would quit after several seconds in the 1800's. I might try a 12x6 next to see how that does. All in all, I'm satisfied with it. If I can find a header I will try a pipe.

David