Hi, noticed tower is stocking OS 91s VRDF small head again

Yup.... They've been in stock since January.

Craig

wonder if they are any tuffer.I went thru about 3 pistons and sleeves in less than a year.That why I had to try turbines.Power and reliability so far.

Three pistons and sleeves? What kind a fuel are/were you usings in that thing Strong?

Joey.

STRONG is right. I use to change piston & sleeve every other flight also muffler header screws. It must be some thing i am doing wrong.

Must be I used to get a couple hundred flights on mine between overhauls.

Whats an OS.91 anyway???

tim

(Redelman) with 20 Discussion in a jet forum, I think you know what a O.S 91 D/F engine is

You should get a couple of hundred flights between rebuilds if your using the proper fuel and making sure you always see a smoke trail in flight (not too lean).

Back in the good old days before we had fuel with 23% oil I had a very small sport plane (9 3/4#) with an OS.77 in it. I decided I wanted more power, so I installed a brand new OS.91. I melted a hole in the piston on the 3rd flight. When trying to figure out what happend (besides getting it too lean) I discovered that my engine cover cap was so close to the top of the cylinder head, that there was very little air flow over the top of it. Expensive lesson learned.

I've got about 30 flights on my f 15. The piston looks a little scuffed but it still has good compression and still runs strong.
Does anybody have a good suggestion on how to keep the manifold bolts tight. Mine come loose after every couple of flights,even with red loctite...

Russ

Not sure which bolts you mean, but red loctite has always worked well for my head bolts and exhaust header bolts. The bolts must be clean and dry when the loctite is used. Also the bolt holes need to be oil free.

If it's the head bolts that are loosening, then you might try some replacement bolts from Jet Model Products. They are better quality bolts. Heat is what makes red loctite let go, so you may be running a little lean.

Manifold bolts,Use a small drill and drill a hole threw the bolts, tie wire with 020 from top bolt to bottem bolt like they do in full size you are slightly out of balance for some reason, this is causing the bolts to loosen up

go here to see how good luck
http://marrc.nova.org/html_docs/rrs....ep.html#wiring

Great idea jettset99. Looks like that problem is solved. Do you balance your fans? The dynamax manual says not to. They say it is very close out of the box.

Thanks for the info.

Russ

Ramtec no Byron yes, It could be in the motor also.

I never really had any long time luck with the OS-91.I used wildcat 5% nitro 23% oil.I would tune and make sure it was not lean before every flit and even with mixture control it would burn up a sleeve in no time flat.The rossi 105 is the best choice for duct fan,IMO it's a real power house and very tuff.The only down side is they are very hard to set up,but once you do that thing has unreal power.Loves heat and high RPM's......Also the top gun f-15 is a great flying jet every one around here love the way it look in the sky.

My O.S. 91 was blowing the seals on my header, eating glow plugs like candy, and breaking the header bolts every two flights.
I checked the balance of my rotor way outa whack! So I just took the dremel started grinding on the hub of my dynamax...problems solved!

What a hobby
Joey.

"My O.S. 91 was blowing the seals on my header, eating glow plugs like candy, and breaking the header bolts every two flights."

Sounds like a hot engine to me. Maybe too much nitro?
JA

I never could get much longevity out of the stock OS piston and sleeve. They are NOT ABC but are ABN (Aluminum piston, brass sleeve and nickel plated) Nickel is not as hard as chrome and was prone to peeling. Also, take a new OS .91 out of the box and turn it by hand---You will notice that is is quite easy to turn and doesn't have much of that "ABC pinch" at top dead center as compared to other engines. This is great for a quick break-in on a new engine but terrible for longevity. Now, take a another popular engine such as a BVM .91/.96 and turn it over by hand right out of the box. You will notice that there is a much more pronounced "ABC pinch" at TDC. This requires a much longer break-in period but will reward you with a much better piston to sleeve seal and will last much longer than the OS. I ran both engines on the same fuel (Wild Cat 5%) and had much better life from the BVM engines. When you wear out the stock OS piston/sleeve don't go back stock---For about the same price you can get a high performance Dave Shadel ABC or AAC piston and sleeve that is good and tight. But, for the same money as the new OS .91 and a replacement piston and sleeve you could have bought a BVM engine. Also, the BVM engines have a one piece crankcase which provides a more ridgid engine (very important in a high revving engine) and better balancing. Yes, BVM engines cost more but are worth every penny. Better resale value too. A used BVM engine can bring as much as as a new OS.

Kevin

Kevin-

At what rpm is the OS flown at without burning up the piston? (Provided I am use in-flight mixture control and Wild Cat 5% fuel.) I am not interested in top speed but more safe operation of the engine and longer life.

Glenn

Guys, mine blue plugs on 3 straight flights until I found a hairline crack on the tuned pipe so don't forget to check it as it cused engine to go lean and blow plug. (charred).
Wildcat 5% MC-9's best combo.

I bought the Weston after seeing and reading about it. About ready to try it again.

I've had nothing but stellar performance and longevity out of the O.S. .91 on Performance Plus 10%. Whether it's been in Ramtecs, Dynamaxes and Viojetts, I've never had a problem (well, except when I crashed em).