The International Cessna 170 Association

Hi Ya'll,

Just completed annual and #6 cylinder measures 52 PSI. The rest are in the 70s.

Should this be replaced, and if so, which is the best way to go? Overhauled cylinder, new, etc?

Thanks, Mark

Hold the phone. First watch a few videos about cylinders by Mike Busch of Savvy Aviation before you turn your mechanic loose replacing cylinders.

Here’s one to start: https://youtu.be/dSklruQiPz8

Thanks Pete good information

More info about the cylinder is needed. Is it leaking at the exhaust valve or the piston rings? Exhaust leaks can be improved in some cases without removing the cylinder by lapping the valve with the cylinder in place.

At the very least I'd run it 10 hours and check the compression again.

Just spoke with mechanic he says that the leak is by the rings. I’ll probably fly it awhile and check the compression again when completely up to running temp. I was out of town during this annual. Thanks

Just a couple of further comments.

Leaking past the rings rarely results in catastrophic failure. It usually results in higher oil consumption, and a dirtier belly. What is your normal oil consumption and what are you presently experiencing?

It can also but not necessarily lead to lower power output. However, if you are making max rpms on the ground as specified for static tests and showing max rpms in flight, you are probably making full power as well.

I would fly it for a full 5 hours or so and then check the compression. And I would not worry about it in the meantime.

When I first got the plane I ran it with the oil level around 7-8 quarts and got 4-5 hours per quart but I now run it with the oil level between 5 and 6 quarts and get about 8 hours per quart. I haven’t really noticed any reduction in power. What rpm should I get on the ground? Thanks for all your help.

You need to get a copy of the FAA TCDS (Type certificate Data Sheet) for your particular model. You can find this by googling FAA TCDS A-799. If you are a member, you can find iy in the members only section under Technical Documents. You can also check on your Airplane Flight Manual which you should have in your plane. If not you can also get a copy of that from the Technical Documents section.

Typically max static rpm is 2320 to 2330 rpm depending on the prop. Max rpm is 2700 in the air.

BTW, your oil consumption does not appear to be unreasonable. I would guess most of us never fill above 7 qts. I never do even if I am starting a long leg of a cross country flight.

Thanks Pete I would have looked but the owners manual is in the plane and the mechanic has all the other data sheets etc. I get about 2300 rpm when I start the take off roll.

Don’t confuse the Airplane Flight Manual (one page front and back) with the Owners Manual (50 to 70 pages or so). You need to have a copy of the AFM in the plane. The TCDS is not required to be in the plane but it’s a very handy reference and you should keep one around the house or hangar . IIRC, The current document is dtd about 2015 and is easy to access via the net.

BTW, Bruce suggested flying about 10 hours. I think anytime between 5 to 10 hours would be fine.

And just to add to Pete's comments, assuming your mechanic followed the procedures in TCM M-0, for the compression check, 52/80 is a completely satisfactory number and a serviceable cylinder lacking evidence of some other issue according to TCM.

While 52/80 is, in most of our experience I"m sure, low for a C-145/0-300 cylinder and an indicator to look closer, we are very happy to get cylinders at 52/80 on the IO-550s we check daily. Heck if we get them in the 60s we figure their bad cause the cylinder is full of oil and if we get them in the 70s we figure our compression tester is bad.

Thanks y’all, lots of good advice on here. Right now the plan is to check compression again in about 5 to 10 hours, as suggested.

Next week I’ll be asking about how to produce better flying weather.

I fully agree with Bruce. Read TCM M-0, pp. 6-104 to 6-115. The thinking on compression tests has changed in the last couple of decades. Now, it's not considered as reliable an indicator of cylinder condition as in the past. Plus, now you are supposed to compare the compression readings to the reading you get through a "master orifice" (which can be in the 40's), not an arbitrary fixed number like 60 psi used in the past. Now in many cases when you get a low reading, you run the engine at cruise power setting and re-test. I don't know how many times that I've found that re-testing after running the engine resulted in a better compression reading. Also, given that you can now buy reasonably-priced borescopes, M-O requires that you borescope the cylinder before making a final judgement call. I certainly wouldn't remove a cylinder without first borescoping it.

Blow by the rings on a single compression test is not necessarily an indication of a bad cylinder. The rings rotate in use and sometimes the end gaps in the rings all line up and allow some blow-by. That's another reason to run the engine at cruise (to allow the rings to rotate and remove the lined-up gaps, if any) and re-test.

Just some thoughts.

Thanks Gene appreciate your advice