The International Cessna^® 170 Association

1954 170B w/C145

Well, last week was annual time. Again I have cylinders 3 and 4 with less than stellar performance during the leak test. The other four all show in the low 70s, but these two are barely making it over the 60 mark. Tracking the sound reveals the familiar hiss to be coming out of the exhaust stacks and the exhaust valves are surely the source of the trouble. My AI is not seriously concerned and signed me off for another year. He made his usual statement about the baffling design and the fact that there was insufficient cooling for these center cylinders and that exhaust valves took the brunt of the abuse.

Wow!! Here we are, some 50 plus years later and we are still putting up with this problem??? I can’t believe it! There must be a better way! I suggested that we just pull the two center cylinders off and touch up the valves with a little valve grinding compound and lapping procedure, like we did in my old English motorcycle days. He quickly dispelled that thought with “If you are going to pull them, you’re just as well off to put on a couple of new or rebuilt cylinders”. Why?? Everything else is okay and we have less than 500 hours on a major!

That brings up two questions:

One, any thoughts on just touching up the valves??

And two, any thought on improving the cooling of these two center cylinders??
In looking in from the front, at the airflow, as it goes through the engine, it is obvious that those center cylinders are not being treated fairly. The front ones, 5 & 6, get plenty of air, and the back ones, 1&2, surely get enough, as all of the upper pressurized air must get out the bottom some how and must pass over them, but what about those poor #3 & #4 cylinders? There has got to be a better way.

Help!!

Tom

With only 500 hours, are you sure it's the seats? If they were lapped in correctly at overhaul, I would think they would be OK.
Have you checked for sticking due to build up of lead on the stems? I understand that can be a problem if you use 100LL.
There is a procedure, which I've never done, for cleaning the stems and guides without pulling the cylinders. I'm sure one of the other 170rs with more experience will chime in on that.
Do you use TCP? I find with TCP and MMO, along with agressive leaning, I have no problems with valves and plugs using 100LL.
Just a thought.

We have only had our 170 for 3 years now. I was having some valve sticking issues when we first got it. You know, that familiar morning sickness that comes in the form a slight shudder, until everything get warmed up. I soon switched to the same auto fuel mixture that I am using in my Cub. I mix it with 2 ounces of pure synthetic 2-cycle oil to every 5 gallons of auto fuel. The Cub Club suggested it as it performed better than MMO on tests that they ran; less separation and better lubrication of the exhaust valve guides. This has worked flawlessly for us in our Cub for the past 10 years and over 1000 hours now. And, the 170 runs quite smooth now. These valves aren’t sticking. I just think that my troubles are due to the lack of cooling on these center cylinders and a fix is needed.

Tom

You guys just had to pull the pin from that grenade didn't you!
Standby!!!!!
David

Tom-
Before you do anything else, go to the TCM website and read (or make a copy of) SB 03-3. This sets forth TCM's current suggestions for performing a compression check. There's more than the following, so it's a "must read," but a borescope exam of the valves and cylinder walls are mandatory for every compression check. If there's a leak, the borescope exam will reveal if there's a valve leak while running (i.e., a burned or burning valve). Often low compression means nothing because a valve that leaks slightly at the 80 psi compression test won't leak at all while running at 800 psi.

Teledyne Continental Motors says way too many cylinders are being removed for repair because of perceived low compression.

It's unfortunate, but many A & P's haven't read this service bulletin, and perform the compression test the "old" way, and don't own or use a borescope. Owners need to read and insist their techs. follow this bulletin.

On my C-145, almost every annual a different cylinder is a little low . All the others will be in the 70's and one will be in the 60's. Supposedly it has a lot to do with how well the engine is warmed up and unknown other factors. Apparently it is acceptable to have a cylinder as low as 40 on our Continentals. My last annual all the cylinders were near 74 +-

Just to expand upon Kmac's post, the above Service Bulletin says the compression tester must be used, first, with a master orifice tool to see how much leakage is permitted, before testing the cylinders. That permitted leakage may well be in the low 40's. What happens next, if cylinder leakage is excessive, is described in the bulletin but is NOT to remove the cylinder for repair.

I urge all who want to protect both their airplane and checkbook from uninformed techs., to read SB03-3. It's one of the best ever written by TCM.

Tom

First there is a lot of truth to your IAs attitude to "do nothing". He has determined to his satisfaction the engine is within standards and airworthy.

You can't legally change the cooling baffles and pulling the cylinders evokes inspections that you and he may not be able to do correctly and that is why he says once you pull them you might as well replace them with new or overhauled or have them overhauled all by someone with the knowledge and tools to do it right.

For example it's probably more that the valve seat but all so the guide is a bit worn not allowing the valve to seat just right. Are you going to replace the guide? Then you probably need to cut the valve and seat. Do you have that equipment?

Now if you decide your airplane looks a lot like your ''old english motorcycle" then you can probably imagine the valve seats might respond like your motorcycle valves did using the same techniques. But of course you would only imagine it because your probably not licensed to do it.

If it were me and I was worried about it I might try to find a bore scope and look at the valves and seats to try and determine if this is normal wear or the valve an seat is really getting burnt to the point you should do something. I would also check how loose the valve is in the valve guide.

You will either come away form this inspection feeling OK with the cylinders or ready to pull and replace them.

BTW I haven't tracked this or payed much attention so you may be right but I haven't notice my center cylinder valves having more of a failure rate than others. And I thought it was the back cylinders that ran the hottest.

N2868C wrote:... any thought on improving the cooling of these two center cylinders??
In looking in from the front, at the airflow, as it goes through the engine, it is obvious that those center cylinders are not being treated fairly. The front ones, 5 & 6, get plenty of air, and the back ones, 1&2, surely get enough, as all of the upper pressurized air must get out the bottom some how and must pass over them, but what about those poor #3 & #4 cylinders? There has got to be a better way.

Help!!

Tom

It's not the air going OVER the cylinders that cools them, it's the air going AROUND them that counts. The air at the rear baffle (blast tubes and cockpit ventilation) is for all practical purposes the same temperature as the air entering the front of the cowl. I'd suggest taking the IPC and compare the baffle parts page for 1953 and later (Figure 53, page 92 in the B-model IPC) part by part to your installation. If there are any parts missing or damaged, air can get by without picking up heat from the cylinders. Make sure the inner cylinder and head baffles (figure 53, indices 32, 35, and 36) fit tight against the cylinders and heads to maximize airflow between individual fins. Also make sure the seals against the inside of the cowl fit well to prevent loss of cooling air.

If you're having trouble with exhaust valves, pay particular attention to the condition and fit of the 0550117 baffles (figure 53, index 31). These small parts, when properly installed, direct cooling air through the fins around the exhaust port on the cylinder head. If you want, you can seal that baffle to the intake side, but make sure air can still flow through the exhaust port fins on the adjacent cylinder.

Some of the best money I spent on my overhaul last year was a complete new set of baffles and seals from Airframes, Inc. in Big Lake, Alaska. Even on climbs out of 100+ surface temps this summer, CHT's are all under 400F (#'s 5 and 6 barely break 300). Cruise CHT's are 25-50 degrees cooler than that.

Hope this helps, Miles

Thanks guys!! This 170 group has to be one of the best type club support groups out there. I now have a few things to do. First is to purchase a differential pressure tester with a master orifice. I found the E2M for $120. Next, I will begin a search for a suitable bore scope. This will be new territory for me. Also, I will follow this up with a close inspection of the baffling, as compared to the parts diagrams that were mentioned.

I really like this old C-145. It runs smooth and sounds great. And since we mostly use it as a 2 place and baggage airplane, it has plenty of engine for us. I just want to keep it healthy and not have to pull cylinders with monotonous regularity.

Thanks again

Tom

I fly behind an IO540 most of the time. We just finished the annual on it and the compression numbers were in the 60s - engine has 1300 hard hours on it. My IA says Continental has changed it's position on compression numbers and has stated 60's are acceptable.

Tried the TCM site and SB03-3 is not available...so it says. Anyplace else one might pick it up?

Thanks,

I found it here: http://www.tcmlink.com/pdf2/SB03-3.pdf

John

I agree with your IA also, Tom. Fly it another 10-20 hours and check it again if it makes you nervous, but otherwise I'd fly it, run it hard and lean, until the next inspection is due and see what develops.
Miles and Bruce and the other guys are right on the mark, in my opinion.
(And I won't toss the grenade, but don't agree with unapproved additives. How will you determine they aren't the cause of trouble themseves? Using unapproved additives throws a whole new variable into the mix, as well as being illegal.)

I agree with what George said above.
There is one thing often missed tho when this SB is cited as continuing with low compression cylinders, and that is:

"TCM requires a cylinder borescope inspection be accomplished in conjunction with the
differential pressure test."

I have not done this either, but after the above procedure mentioned by George didn't work with one of my cylinders my IA boroscoped it and found a valve seat that had worked itself loose probably due to a stuck valve.