Carb Heat/Cabin Heat Repair on the M-18

Rejuvinating the heater coils in the exhaust system:

Comments submitted by Mike McCrath, N283DE:

Those of you with Continental-powered Mites (and probably the "L" crowd, too) may have a maintenance problem of which you are not aware. Seems the cabin heat and carb heat muffs on your exhaust systems contain heat coils -- just like those in a space heater -- that, after about 50 years more-or-less, tend to, well, give out. "Give out" can take the form of breaking, crumbling, or worse-case, simply turning to a fine white powder and exiting the scene via the nearest hole. That leaves you with a frost-bitten cockpit plus dangerously few BTUs to get the carb all toasty for those chilly fall morning final approaches. A quick glance into the heater intake port on the right side of your cowling will confirm whether or not your heater coils are still on the job. If you can see 'em and they're whole, they're working and you're home free. If that's the case, you can skip the rest of this message -- for now.. But if you don't see 'em, or if they appear to be broken, burned or otherwise incapacitated, read on.

Because there's a fix, and here's how it goes: first, you contact Century Spring Corporation, 222 East 16th Street, P.O. Box 15287, Los Angeles CA, 90015. Or simply give 'em a call at (800) 237-5225, and ask for #E-12 Extension Springs. They're 36" long each, and four should be more than enough. (Last I bought 'em, they were seven bucks each, so loans to finance this project are strictly optional.)

What you will receive in the mail are four lengths of coil-bound spaghetti that, at first glance, look more like they belong on an old back porch screen door than in your exhaust system. But fear not, you've not made a mistake. What you next need to do is remove the exhaust system (well, you or your A&P) and crack open the cabin heat and carb heat muffs. Then clean 'em up good with a wire brush, get the dregs of the old wires out of there, and be sure to save a length of that ashen-white old wire as a model. Now, you're going to need to stretch your newly purchased coil-bound springs to match the coil spacing on the old springs (That's where the "model" part comes in; see?). The good news is, these springs are very malleable, so they'll stay in place once stretched.

Now, if you were intemperate enough to throw away the pieces of your old springs at this point, or if there just wasn't enough left to get a measurement from, you should figure on stretching your new springs to roughly 3/32" spacing between coils; something more than 1/16" and less than 1/8", at any rate.

From that point on, it's simply a matter of wrapping them tightly around the exhaust within the muffs and closing everything up. With two people, that should take no more than three or four minutes; with one person, figure seven hours.
Now go flying! Barring any nasty cockpit air leaks, you'll now actually be able to fly your Mite in the winter without losing significant body extremities to the elements. Your feet will thank you. Your carburetor will thank you. Your loved ones will thank you.

Have a good flight!

Additional comments from Garry Gramman, N119C:

"I found another source for heater and carburetor heat coil springs which are a lot less expensive than Century in Los Angeles, whose price is about $7.00 each.

"Upon reading Mike McCrath's article in the Mitesite under "Maintenance" I went to LA and bought the springs I needed (I was one spring short), and I stopped at McMaster-Carr on the way home and found that they sell the identical spring in a package of 5 for $12.59. So I got my additional spring from McMaster-Carr this time and have several springs extra.

"The springs are 1/4" diameter, wire size .035" in 36" continuous length. They can be pre-stretched, and should be wrapped around the exhaust stack about 9 coils per inch. This conforms to the original Mooney prints. Part # is 9664K16."

McMaster-Carr Supply Company
9630 Norwalk Blvd.
Santa Fe Springs, CA 90670-2932
(562) 692-5911
www.mcmaster.com