Exhaust System Question

[VictorBravo]

Has anyone here ever seen a situation where you take an exhaust system off of a 65 HP Taylorcraft engine, and try to bolt it onto an 85 HP Taylorcraft engine... and it doesn't fit?

I had a nice aftermarket stainless Taylorcraft one piece exhaust on my BC-65 when I bought it. I recently bought an 85 HP engine, which had a plain steel exhaust on it. I removed the plain steel exhaust and tried to put the stainless one from the 65 onto the 85. The exhaust didn't fit. Not even close.

It appears that the distance between the left bank of cylinders and the right bank of cylinders is greater on the 85 than it is on the 65. It is almost as though someone shimmed the cylinders further away from the cylinder decks on the case. But there are no such spacers.

The 65HP stainless exhaust is about 3/8" to 1/2" too narrow... when I put one side onto the exhaust flange studs on one side, the exhaust flange studs on the other side are well over one hole-width outside the holes in the exhaust system flanges.

This appears to be a stock 85-12 engine, not a GPU.Holding the two exhaust systems together shows that the exhaust systems are different by 3/8 to 1/2 inch.

Anyone ever see this problem???? I'd rather fly with a nice clean stainless exhaust!

[Guest]

Re: Exhaust System Question

Hi Bill,

Does the c-85 have a longer stroke?

That could account for it.

Also the case could be thicker for strength.

Dave.

[Robert Lees]

Re: Exhaust System Question

Andy Duke (member of this parish) had the same situation when fitting the C85 to his Taylorcraft a few years ago.

The easiest solution we found was to deliver the engine + exhaust to a welding house and have them cut & shut the exhaust using the engine as the jig.

That's what I will also do when I fit a C85.

Rob

[Larry Lyons]

Re: Exhaust System Question

Or you could sell me your SS exhaust.
Larry

[VictorBravo]

Re: Exhaust System Question

I am indeed going to have it re- welded, using an old crankcase and some old cylinders as a jig. That is of course the right way to get it to fit.

The bigger question is this...

The one-piece stock exhaust system listed in the Aircraft Spruce catalog (and likely the Univair catalog) shows that it fits a 65 HP engine. Fair enough.

They don't show the same exhaust as being applicable to the 85 HP engine. Also fair enough.

When the 85 HP F-19 came out I'm guessing that there was a different exhaust design that was not the same as our beloved (be-hated) one piece T-craft style exhaust.

My first question is what exhaust was delivered new on the late 1940's era BC-12D-85 and BC-12D-4-85?

My second question is what exhaust system was used on all those dozens or even hundreds of 65 horse airplanes that were converted up to 85 horse engines under the Gilberti STC?

Forrest, if you're reading this... it might be one of the details that any new or upgraded version of the STC should address.

[VictorBravo]

Re: Exhaust System Question

Or you could sell me your SS exhaust.
Larry

Sorry, I want to use it on my 85 because it's so prreeettty

Get a hold of that guy in Florida, Sonny Poarch. HOWEVER :

SAFETY ALERT !!! In the research I was doing towards getting my 65 exhaust to fit the 85, I went to a local (high end, very technically competent) hot rod exhaust header shop to get them to simply make me a "slip joint". The guy said that because of the crossover design, the exhaust pulses from the left bank of cylinders and the pulses form the right bank of cylinders meet head-on and oppose each other in the crossover tube (underneath the heat muffs). Because of this "overpressure", any slip joint there would leak no matter how tight you tried to make it.

The only place to put this slip joint is in the cabin heat side of the exhaust, inside the heat muff. A leak there is a major safety issue and you CAN NOT risk it.

The only way that it appears you could make a safe two piece exhaust with the stock Taylorcraft design is to use welded flanges (like the ones where it bolts to the cylinder) and a proper exhaust gasket. But this, in turn, would be too big to fit inside the heat muff.

So I don't know HOW Sonny Poarch or anyone else is making two piece "stock" T-craft exhausts that do not leak into the cabin heat muff.

[Forrest Barber]

Re: Exhaust System Question

They are different of course. The STC ( Gilberti-Harer) shows the completley new exhaust for the upgrade to the equivalent of the Model 19 , it does not cross over parallel like the 65 hp,,,, The BC12D-85 did but was a different size. There never was an 85 HP F-19.
Go to ATC 696 then ATC 1A9 for the differences.
At the Feris factory we had a 65 hp & an 85 hp Cont. sitting on the floor upside down for fitting purposes before shipment.

[VictorBravo]

Re: Exhaust System Question

Thanks to a friend with a TIG welder, my stainless 65 HP exhaust now fits the 85 HP engine perfectly.

Of course, the heat muff now does not fit, and it is too far off to just tap into conformity with a hammer. So now it's a sheet metal inlay patch to do on the muff. What a pain to simply bot on a larger engine!

Anyone doing the engine upgrade, in all fairness I have to advise you to put on a different exhaust system... Aeronca, Luscombe, C-150, whatever.

[Forrest Barber]

Re: Exhaust System Question

OR the Taylorcraft Model 19 system. It even produces heat. It keeps the heat away from the gascolator too!

[VictorBravo]

Re: Exhaust System Question

My gascolator is on the firewall, away from the exhaust. My carb heat system will soon be using both sides of the heat muff. That will increase safety, reduce risk, and make a safer airplane, at the cost of cabin heat comfort.

I know this might be shocking to cold-weather operators, but the cold-weather operators have more places to land safely than this Los Angeles boy... and it doesn't get that cold here compared to many other places.

I might try to switch the cabin heat side to run the carb heat first, though. The carb heat side of the system allows a lot of the heated air to go overboard right past the SCAT tube fitting and out the bottom, likely "augmented" by the high speed flow out exhaust stack. The cabin heat side has a "dam" which forces all the heated air to flow out into the SCAT tube... none of it is lost.

I believe it was a mistake to design the original Taylorcraft exhaust the way it was done. Reading Chet's book I remember they were able to claim it's quieter, and of course there's only one hole in the cowling and one exhaust exit... so it was possibly cheaper to make. But neither of these advantages offsets the many significant problems, disadvantages and operational difficulties that come with the original design.

If Forrest is aware of some other reason why they used the one-piece system that is not obvious to me, it would be a delightfully esoteric exercise to hear about it. But I stand by the idea that this one-piece monstrosity should have been abandoned within a couple of years after it was tried and found to be problematic.

[Robert Lees]

Re: Exhaust System Question

I believe it was a mistake to design the original Taylorcraft exhaust the way it was done

I beg to differ. The one-piece exhaust works perfectly well (at least for 40-85hp), but I might be persuaded to agree that the original design did have its flaws:

It was mild steel. Of course, given manufacturing costs (and the then competition & the expected lifetime of the airframe back then) it's not unreasonable that the original design was "cheap & cheerful" mild steel. And I agree that the cabin heat is somewhat lacking, but at least it keeps the chill off.

But think of the design back then...it was easy to manufacture in one jig, and it was assembled to the engine before the engine was assembled to the airframe...quick and easy, especially when in 1946 Taylorcraft were making 30 aircraft a day.

It only really became "problematic" when folks like us decided to own these aeroplanes for longer than their "use-by" date. Some of us (myself included) put a bolted flange in the straight piece along the firewall, which at least meant that if the darn thing corroded away, you would only have to remove one side at a time for repair.

Then we learned about stainless exhausts, which pretty much last forever. I made the mistake of making my stainless one the same as my mild steel one, i.e. with a flange along the "back straight". It is this flange which is now the weak point on my whole exhaust system, because any leak is more likely here. No more corrosion or even cracks, but the darn bolted flange seal might only last 2 years before the seal goes.

Andy Duke (occasional poster of this parish) had his stainless exhaust welded up solid when he fitted the C85, and ne'er a leak has occurred in >1000 hours of operation since. And it still exits the cowl from one hole, gets the required degrees F carb heat drop, and even heats the cabin too. And it sounds better than a C-150 type exhaust.

So I shall be doing the same. It's not a monstrosity of an exhaust, Bill, given the design and commercial pressures back when it was designed. Later technology (and our willingness to throw money at our lovely aeroplanes, and the fact that we can look back at old designs with rose-tinted spectacles) allow us to improve upon those small weaknesses that were forced upon our 1930 and 1940 Engineers.

All the best,
Rob

[Forrest Barber]

Re: Exhaust System Question

I am back! Exhaust......... if you saw how these things were developed you would see all!
The main reason that we have the one piece and the other one piece and the even worse two piece was THAT was the way it was when it was certfied. A lot of things were not well thought out. I believe that the one piece for the A-65 is good, we can have the engine off in a half hour, i ahve one of the field that has been good since 1972..... i understand them. However in todays world , IF we were to rethink all this then we could change things , BUT it takes a lot of money to re certify.

[VictorBravo]

Re: Exhaust System Question

Forrest, when you (hopefully) take over the Gilberti/Harer STC, one of the MANY things that I suggest you address in more detail is the exhaust.

I had a TIG welder cut the exhaust, put a small sleeve over it, and weld it solid.

It took a few hours but the only way to make the heat muff fit was to cut it and patch it back together with the extra half inch width.

When you re-do the STC, I suggest a strongly worded sentence that forbids the upgrader to put in a slip joint down at the local muffler shop. It is very tempting to do, and would result in Carbon Monoxide in the cabin heat air.

Another thing I think I found out the hard way is that when you are putting the engine mount back on the firewall (with the engine and exhaust on the mount), I think the bolts through the engine mount rubber cones have to be loose (not tightened yet). I had mine snugged up, and we had a HELL of a time getting the bolts from the fuselage through the engine mount !