I have an OS FS-91 II Surpass that has worked well for me. Last year, the engine quit while I was flying the plane, and I landed the RV-4 without a problem. I checked out what had happened, and it turns out that the threads in the muffler were stripped. The muffler had fallen off, but it was being held onto the plane by the pressure tap tubing. I used some JB weld and glued the muffler to the exhaust header. It worked fine for an hour or two of flying, but the engine again quit, and the same thing had happened.

A couple of questions:
1. How hot does a glow fuel model engine get?
2. Is there something better than using JB weld?

Right now, I have drilled the muffler and header, and I've placed a 4-40 screw through to hold it on.

If I had bought the engine with the pump, this wouldn't be anything but a noise issue.

Thanks

They don't make much more noise without a muffler.

You have previously determined that the muffler gets too hot for JB Weld

Thanks.

I was actually wondering about the actual temperature of a glow engine because I have a friend who can get me a bit of industrial adhesive that might work. Like most engineers, he wants to know such things as actual working temperature range, some weird vibration number, metals bonded, etc. I figure if I give him the high temperature, lots of vibration, aluminum and steel, he can help me out without giving me too much kidding.

This particular engine, without the muffler, is especially quiet after takeoff, since it isn't running any more.

In case you don't get any real numbers, for 25 bux plus shipping you can get a Duratrax temp guage and actually measure it.
[link=http://tinyurl.com/69ebfa]Duratrax link at Tower[/link]

Just buy a new muffler and a new header. Start fresh.

Use GREEN PERMANENT Loctite to assemble it. There are 2 types of Green Loctite. You want SLEEVE AND BEARING RETAINER. I think it's stock # 603 (don't quote me on that)

Clean the NEW parts with a good solvent, like brake cleaner and a plastic brush. Clean the engine threads as well. Keep in mind that Loctite sticks to greasy parts like Crisco cooking oil sticks to a door knob (i.e. not at all)

Assemble with the Green Loctite and don't EVER plan on taking it apart again unless you heat it to over 500F with a propane torch. It's PERMANENT if you get ALL the threads clean and assemble it tightly. Give it at least 2 or 3 days to fully cure. It's anaerobic. It cures in the ABSENCE of oxygen.

I agree thats its probably best to just start with a new muffler.

Personally I dont like to permanently glue the muffler. I think if you use teflon tape or rtv the mufflers stay on fine and you can remove and reposition when necessary. I just make sure that the threads on the header go as far as possible into the muffler and the engine.

Thanks for the tip on the Duratrax.

Thanks for the other tips, but I've already done my makeshift repair.

After seeing how much a new muffler and header cost (~$56), I decided to fiddle with my old one. I degreased the header and the nonexistent threads on the muffler. I had already drilled the muffler and header, and I used the holes for a 4-40 screw and double nut + Loctite. Then I tightened the header's retainer screw (+ Loctite) down against the muffler. No vibration now! And finally, I used a 19 g retention wire.

On the engine side, I degreased and used high temp Loctite.

When it falls off next time, I get a new muffler/header and follow your suggestions.

The next 4-str engine I get will have the pump.

I hope your remedy is successful. The culprit here is vibration, not temperature. You might look for ways to anchor the muffler so as to minimize vibration. Good luck.

Try using what motorcycles and snowmobiles have been using for years, a compression spring under the head of the bolt.

I use Associated clutch springs found in the RC Car section, good and stout, fit perfect under 3mm bolts, a 4-40 would need a washer. There's about maybe four winds to them, really heavy gauge wire diameter. They will exert as much force as a tightened bolt but will allow expansion and contraction and soak up the vibration.

Not cheap though, about four bucks a crack but I think it will cure your problem to your liking. Still use removable loctite though on the bolts, but at least they can be taken out if needed OR make studs out of a bolts for the engine, which take the wear, not the block threads, and use a locknut and then the spring. The studs can be replaced. Easier on the block threads if you find yourself removing the muffler or whatever frequently. You'll see that in race engines that get taken down constantly.

Looks like I got here a little late, but thought I would throw it in, might help someone along the way.

JetpackThanks. Unfortunately, the header on this OS engine screws into the head of the engine, rather than being secured by a couple of bolts. There is a retention nut that screws down against the head which is supposed to keep it from coming loose.

I've discussed this with several members of my club, and, besides being sure that the retention nuts are secured on both ends of the header, the main suggestion is that I should get rid of the exhaust diverter. Apparently it adds enough weight to the muffler that it worsens the problem. I find that thread-stripping on the muffler side is very common with this engine.

And, lastly, my Rube Goldberg repair worked for one flight. When I put the plane back on the stand after a nice flight, I found the muffler rattling around again. The 4-40 screw had acted as a saw, cutting all the way thru the muffler side. The safety wire worked. So I'll be forking over $64 for a new muffler.

Weep.

Get a muffler for a Magnum 91 4S for your OS 91. The Magnum muffler is cheaper than the OS, but will work exactly the same. The Magnum is a copy of the OS, and the threads are the same.

http://www.hobbypeople.net/gallery/284084.asp

My local hobby shop guru keeps trying to sell me Magnum engines instead of OS. Are the engines just as good? When I look at this muffler/header, it's obvious that they're close to being identicalin appearance at least. I am told that the guy who started Magnum used to work at OS.

I like Magnum engines so far. I got 2.

I like OS engines too, but I refuse to pay for them. There's motors available that are just as good and cost 50% less.

I agree

Hi - similar problem but not the same. I have stripped one of the muffler mounting threads on the head of a OS 40fsr. Any suggestions on a simple repair? I want to be able to mount the new Pitts-type exhaust I just bought.

Thanks.

Could you drill through that side and then run a longer bolt through and use a couple of washers on the backside. I have done that with ST engines. You have to tighten them after the first couple of runs to make sure they stay on.

Thanks Gringo Flyer.

Doesn't look like it is an option on this engine, quite an old OS40 FSR, the muffler bolt holes look like they are in line with the internal ports and would probably drill through into the cylinder.

Done some research on the web and looks like the best option is to fit a 'helicoil' into the hole. I've ordered an 'Armacoil' thread repair kit, M3 size, which comes with helicoils and correct size drill, tap and installing tool. Hopefully should work.

Cheers.

norm

That might work...if you didn't pull all of the threads right out of the hole...if so, then you might need to go the next size up?



FWIW...to the O.P. I have seen temps of well over 400 degrees at the exh. outlet of a 4 stroke. (I have one of those Dura Trax temp thingies too... )

Maybe you could build-up the internal dia. of the muffler w/ aluma-braze and re-thread it? Probably cost more for the tap than a new muffler though?

Got the Armacoil m3  thread repair kit in mail on Friday and installed today.</p>

Reasonably easy considering I have never used a thread tap before - all worked first try and now have good positive thread.</p>

Norm</p>