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O.S. 61 Four Stroke? - 5/3/2010 7:21 PM   
cpeisher



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So i've been on the lookout for a four stroke engine for my GP 40 Cub Kit im starting to assemble next week.  I found this ad in the classifieds this morning. 

http://www.rcuniverse.com/market/item.cfm?itemid=630597

I went ahead and made an offer on it.  However, i've been unable to locate any information on the O.S. 61 Four Stroke.  They don't list the manual on their website.

http://www.osengines.com/faq/service-faq.html#q5

First, i'd like to get some opinions or experiences from people on this engine and how it will perform in my Cub.

Second, does anyone know where i can find a manual for it?




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RE: O.S. 61 Four Stroke? - 5/3/2010 7:51 PM   
baronbrian



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Status: offline 		Great engines, just 20 years old.would be a very good choice for an easy flying .40 like your cub. have seen an even older version (no rocker arm cover!) on a goldberg cub (larger and heavier the the GP .40 cub) flew well, but it was a little underpowered with the addition of floats. if it turns over easily meaning that the bearings are good, it would be a sweet engine for your project. check the valve clearances when you get the engine and every 200 flights after that. The exhaust valve in particular has a tendency to gain more of a gap between the rocker arm and the valve over time, limiting how far the exhaust valve will open and cutting power up to several hundred RPM at full power. try a 12x6 prop first. sorry, no instructions. look for the o.s. .70 surpass instructions (not the II version) and use those as a reference for operating and valve clearances. hope this helps!

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RE: O.S. 61 Four Stroke? - 5/3/2010 7:59 PM   
w8ye



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Status: online 		I rebuilt a 61 a couple years ago for a friend. I had been around the old open rocker 60 on a LT-40 and didn't have much faith in the 61.

Why the 61 turned out real nice. I thought it had pretty good performance being as it was only a 61. Seemed like a 13 X 6 at a lower rpm was more fitting for it. It ran much stronger than the old 60.


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RE: O.S. 61 Four Stroke? - 5/3/2010 8:02 PM   
cpeisher



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Status: offline 		One of the things i was probably gonig to do was add floats to the cub.  Do you think it would underpower on the GP model with floats if i use the 61 FS?


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RE: O.S. 61 Four Stroke? - 5/3/2010 8:07 PM   
w8ye



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Status: online 		The 61 has more power than is needed for a land plane 1/5th Cub. You will be flying around on low throttle all the time.

I don't know about the float plane. May have a lot to do with how you trim up the floats, Like the incidence relative to the plane and their position front to rear as to how the step matches with the CG..




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RE: O.S. 61 Four Stroke? - 5/3/2010 8:24 PM   
cpeisher



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Hmmm....well i don't want to have to chase it around at idle either, so now i'm a little worried that i might have too much engine.  Especially if im on water....

Regarding the floats, i did a good bit of reading on the proper way to set the gc, step etc...so i hope that all pays off when i put these on!  Thanks!




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RE: O.S. 61 Four Stroke? - 5/3/2010 8:43 PM   
w8ye



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Status: online 		I think it should be a good combination for you


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RE: O.S. 61 Four Stroke? - 5/4/2010 12:44 AM   
cpeisher



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Status: offline 		Great thanks!!


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RE: O.S. 61 Four Stroke? - 5/4/2010 4:12 AM   
MILDBILL-RCU


 

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Status: offline 		cpeisher, I have 4 of these little engines & everyone of them are wonderful. They are not "power house's"of power, but just wonderful, dependable, & predictable little gems. Air bleed idle carbs & very easy to set & forget. I havent had to touch carb settings in 20 years. just a click or 2 for summer running if I last flew in winter. I have the manuals for them & would scan them & e-mail them to you if that would help, just let me know. Bill

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RE: O.S. 61 Four Stroke? - 5/4/2010 4:34 AM   
cpeisher



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Status: offline 		I would very much love to have a pdf copy of the manual.  Thank You!


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RE: O.S. 61 Four Stroke? - 5/5/2010 11:21 PM   
ejmorgan



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Status: offline 		OK, here is a PDF for the O.S. FS-61 engine.

Ed

Attachments
Click to see the file in new window.Attachment.pdf


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RE: O.S. 61 Four Stroke? - 5/6/2010 8:47 AM   
submikester


 

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Status: offline 		I have had two of these and they were both good engines. I did decide to part with them but only in favor of a couple of more modern 70s. The 61 if my memory serves is actually the same size as the 91s of today. In fact if I recall correctly the connecting rod is the same - I had to replace one on one of them.

The 70s are not the same size as the 91s - though they look it. They are in fact a bit smaller.

I had one with a little dent on the intake of the carb and decided that since I could not repair it or find a spare intake manifold I would let them go. One thing that I always thought was a little strange was that the intake manifold produced a lot of condensation and would always seem quite cool to the touch. It was interesting.

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RE: O.S. 61 Four Stroke? - 5/6/2010 3:09 PM   
rcdude7


 

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Status: offline 		The condensation comes from the cold temps in the manifold caused by fuel going from a liquid to a vapor state. It is the same in all engines, but I think it is more noticable on the chrome plated intake manifolds. I had a OS 120 surpass II and it would really sweat alot.

I have two of the .61FS engines also, they run nice, but power is comparable to my .52 surpass. If I'm not mistaken this is the same engine that maynard hill (hope that is correct) used to power his model across the atlantic to Ireland a few years back.

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RE: O.S. 61 Four Stroke? - 5/6/2010 3:45 PM   
overbuild


 

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Status: offline 		I have a O/S 61 bought from Indy RC in 1983 it has been through several sets of bearings and at least six models.Per other posters comments the engine has been a excellent running motor .The last three years it has been in a Four Star 60 and for my style of flying it is all I need to power the plane.I use Master Air Screw 12x6 wide props for best performance.

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RE: O.S. 61 Four Stroke? - 5/6/2010 11:04 PM   
cpeisher



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Status: offline 		Thanks for all the input, i really feel much better about my purchase.  Now i just need to hurry up and build this cub :-)

< Message edited by cpeisher -- 6/7/2010 6:41 PM >


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RE: O.S. 61 Four Stroke? - 6/7/2010 6:50 PM   
cpeisher



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Status: offline 		Well, i finally got around to putting the engine on the test stand over the weekend.  I got very lucky because even though i tightened the prop nut pretty well, the nut, thrust washer, and prop came off at full throttle.  It went flying away from the engine and at an angle down and away from me, instead of up and into my face.  I found both the prop and the thrust washer, but the nut made it all the way to the lawn, and i never saw it fly off.  Do i have to order a special nut from OS, or can i get something from Lowes ro Home Depot?  I think it's a 5/16" thread correct?  Thanks again for your help.


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RE: O.S. 61 Four Stroke? - 6/7/2010 7:07 PM   
w8ye



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Status: online 		You are running it too lean if it backfired


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RE: O.S. 61 Four Stroke? - 6/7/2010 8:32 PM   
cpeisher



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Status: offline 		Oh, it didn't back fire, it just vibrated right off at full throttle...i think it was vibrating pretty abd because i drilled the prop out on a drill press and didn't balance the prop.  So i think the prop was pretty far out of balace causing it to vibrate pretty bad.


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RE: O.S. 61 Four Stroke? - 6/7/2010 10:56 PM   
MILDBILL-RCU


 

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Status: offline 		That will definatly make it vibrate. Crank thread size is 5/16 fine thread. Any nut from hardware store will be fine. Most hobby shops should carry the double locking style nuts in that size if you wanted to go that route. If you can find an old surpass .70 engine for cheap, or for parts, then you can get about 400-500 more rpms from these .61's by just putting the .70 camshart into it. reset the valves & your off & flying again with a little more power. (O.S. might still offer a new cam for the .70, but I havent checked in years.) It will not compaire with the power of the .70, but in gives a nice boost to this great little engine. Bill

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RE: O.S. 61 Four Stroke? - 6/7/2010 11:15 PM   
w8ye



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Status: online 		The cams for the OS 70 and the OS 91 both Surpass I & II have all the same dimensions and will interchange


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