| Subj: Any magic to MA-3SPA carb rebuilding
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| Date: 3/15/05
Have an MA-3SPA which purchased at Oshkosh that appears to be in good condition but has probably been sitting on a
hangar shelf for 10 or 15 years. I assume you would recommend getting a full rebuild kit for any carb like this one
that is in an unknown state? I rebuilt a few auto carbs back in the old days; if I can build a Corvair engine, is there
any magic to rebuilding an MA? Can't believe they get $600 plus a core for rebuilding one of these.
Thank you.
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| Rick Holland, Manual #5899, Castle Rock, CO, Building a Piet at 7000 MSL
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| Reply from WW:
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| The two most popular certified carbs used on the Corvair are the MA3-SPA and the Stromberg NAS-3. At first glance,
these are very simple, reliable, one-barrel carburetors. But there are other mitigating issues that make their
overhaul cost high if it's done in an FAA certified repair station. First, if you're going to do it yourself, you
should get the necessary parts from Chief Aircraft, and you absolutely must have and read the overhaul manual. Do not
buy the complete overhaul kit, but rather the specific parts that you need. In the case of an MA3, this usually totals
about $150. By doing careful work, you can put together a good carb that will serve you well. This is how we did the MA3
that is on our own 601. When you see $500 or $600 as an overhaul price, it is driven by several factors: The carbs must
be absolutely perfect when they leave a repair station, for they could be installed on a rental 150 the following week in
which somebody's 16-year-old kid is going to solo. Second, the cost of operating a repair station is much higher than most
people think. There's a lot of paperwork involved, training requirements, and in many cases, extremely high price insurance.
Fuel systems on aircraft have been involved in some of the largest lawsuits in aviation - a few fair, most not, but it's
a reality for those in this business. When an MA3 or Stromberg is really worn out and needs things like throttle shaft
bushings, it's time for a really experienced professional. My choice in this is to work directly with a repair station, and
my favorite one is D&G, 1505 N Front St., Niles, Michigan 49120-3933, run by Russ, phone (269) 684-4440. He completely overhauls the carburetor and includes many details
that other shops charge extra for. His price on an MA3, if you're going to have it set up for a Corvair, is $500. The
Stromberg price for a Corvair is $325. Although these may sound expensive in automotive terms, I believe that when you
consider all the factors, they represent a good value. Over the years, I've seen people mess with all types of
alternative carburetors, ground running them for months without ever gaining a lot of confidence in them. In many cases,
builders whose intention is to fly soon would be better served by an aircraft carburetor in which they can have immediate
confidence.
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| Subj: Oil Temp
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| Date: 3/11/05
Here are two pictures of my Corvair powered helicopter.
I changed oil and filter fron 20/50 to 10/40. This lowered the cold idle pressure 65+ to about 45 lbs.
The oil temp still goes past 320.
I checked for air blockage (like maybe a rag sucked in) but it is clear. The oil cooler is starting to leak now.
This is the second used oil cooler to blow up on me in less then 15 hours of run time.
Clark's has no new ones at this time.
The cylinder temp stays cool under 300 degrees.
Thanks for your reply. Any suggestions are appreciated.
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| Bill Gratriex, Susanville, Ca., #5494
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| Reply from WW:
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| Thank you for the photos. I prefer 10W30 and 10W40 over thicker oils because the Corvair was designed as a thin oil
system. The oil pressures you have are much, much higher than we use. Our 601 idles at 20 pounds of oil pressure when hot.
The system never goes over 42 pounds. If you have thick oil and high regulator spring pressure, it's possible, as you found
out, to overstress the oil cooler. I don't know what form of instrumentation you're using, but many oil pressure gauges
read low on start up. A mechanical gauge with a long line will not show you how high the pressure actually is. I've tested
a lot of Corvair oil coolers in excess of 100psi. Believe it or not, a high pressure and high volume pump setup with thick
oil can easily exceed this on a cold start without showing it on a mechanical gauge.
The main reason I don't recommend excessive oil pressure is that it is not required and it overstresses the system.
The engines we build use stock pumps, and only bump the oil pressure from a stock 37psi to 40-42psi. My primary thought
on an engine which has normal CHT but high oil temperature is that the oil cooler bypass is not seating. We test this
specifically when we rework builders' Oil Accessory Cases. If this doesn't seat, the engine
will bypass the cooler at all temperatures, and the oil will run hot. Your oil cooler should be a stock 12-plate cooler
in the stock location. We've had the best results with this simple setup. The mechanical work of raising the oil pressure
may contribute to its high temperature, and I suspect the high pressure is a factor in making some of your oil system
components function poorly. Your oil system cover could be removed and tested by us, but this is a far easier job before
the engine's built and installed. To field test your cooler bypass without major disassembly, you can make a block off plate
where the cooler's mount was bolted to the case. Get two mechanical oil pressure gauges, and put them into each of the
outlets from the case. Remove the distributor and run the oil pump with an electric drill driving an old distributor shaft.
If the cooler bypass is in good condition, there will be 7-8psi difference on the two gauges. On a complex issue like this,
feel free to call me on the hangar line, (386) 478-0396. We're there from 9 a.m. till 10 p.m. while we're prepping for the
beginning of airshow season.
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| Subj: Safety Shaft
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| Date: 3/10/05
Have finally found a good 110 engine, good crank (std.) and rebuildable
heads.
I am putting together all the mod. parts and kits. I have Clark's W.W.
gasket kit.
Have your Engine Assembly Video # 1. I have 2 questons: I see in The Corvair
Flyer that you
are using 7075 aluminum Safety Shafts now. Is it the same price, and also same thread (1-14)?
Is there a particular reason for using 1-14 thread?
Harold Davis, Manual #6361
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| Reply from WW:
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| After some experimentation, we have shifted to filling orders with 7075 aluminum Safety Shafts. For people plans
building this part, I still recommend using the 4130 specified in the Conversion Manual. The aluminum
is half the weight, but this is only a few ounces. The aluminum has to be 7075, as all other available grades are nowhere
near strong enough. The introductory price on this part is the same as the steel safety shaft. The 1-14 thread is our
flight proven choice for thread patterns.
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