Creaking Continental A75

[Tasca]

Dear all

I decide to change the VW based engine in my J3 replica, to an original Continental.
A friend of mine had one disassembled Continental A75 and lots of parts missing.
I’ve had every part check for cracks and magnafluxed, i.e, crankshaft, camshaft, cylinders and camshaft gear, all the rest was brand new from Fresno Airparts.
All the measures and tolerances accords to continental manuals and I had crankcase check for bearings alignment and it was refaced even there was no problem , physical and chemical cleaned and alodined, every little detail had been checked.
After the first flight with 1.5 hour I’ve noticed that when turning the propeller, it creaks and it is kind of heavy !
After cooling down to room temperature it turns free without creaking.
From room temperature to creaking condition takes only 4minutes of running time.
Since so I’ve disassembled and reassembled the engines 3 times looking for something wrong, but every time all dimensions were correct and I couldn’t find anything wrong.
The oil pressure is 30 psi in idle and 60 to 55 psi in flight, the temperature is under 140F, it was 3 flights of 20 minutes each.
I take cylinders off 2 times more looking for wrong gaps, material or marks and nothing wrong again.
Since it is an experimental aircraft and I have lot of experience in mechanical, I’m doing the job with the help of an A&P, but we can’t find any reason for this creaking as you can see in the video attached.
Someone had ever had a problem like this before ? some clue will be welcome !

please see video https://www.youtube.com/watch?v=_pMMVGEj0Ro

Thank you all


E. Tasca

[Eric Brown]

Would you happen to have 5 ring pistons installed in this engine ?

[Tasca]

Eric

yes there is 5 rings in this piston, it was another issue because there is no part number or top markings on it. I sent a post in this forum months ago asking for the correct position and it was installed according to this attached drawing.

thank you

[cub builder]

Yours seems to be abnormally loud, but I've seen this a number of times with small Continentals. It's usually ring chatter, and in the video, that's what it sounds like for your engine. With a 5 ring piston, the upper rings tend to be a bit dry. Make sure your rods are installed correctly in that the oil port on the bottom of the rod should be pointing towards the opposing cylinder which helps to lubricate and cool the opposing piston. The description of the rod installation in the overhaul manual is a bit ambiguous as it says how to install the rods with a reference towards up, but doesn't give the context of the engine position during assembly, so is often times misinterpreted. Most people now days overhaul on a vertical stand, but if you look in the manual, you'll see the photos are a horizontal stand, so the up reference in the overhaul manual refers to the top of the case as the engine would be mounted in the aircraft. I have found rods 2 and 4 to be installed backwards in about 30% of the small Continentals I have torn down, which leaves cylinders 1 and 3 a bit lean on oil, although I have never observed any damage from running with the incorrect installation.

Also, you didn't mention whether your cylinders are chrome or not. If the cylinders are chrome, you should have all iron rings. If the cylinders are steel, the top ring should be chrome.

If the things mentioned above are correct, then don't worry about it. It's not abnormal for a small Continental to do that and it doesn't happen while the engine is running.

-Cub Builder

[Tasca]

I'll check again the rods position, but I mount the crankshaft in a table with all rods on its working position and number facing up.
My cylinder are steel, but as I told there is no markings on rings, there is 2 compression ring each piston and both seems to be equal, are they ?
If it is dry on oil, is valid try to inject some oil trough spark plug hole and turn the engine to check if the creaking stop ?
it's a bit heavy there is a chance of locking in flight ?

thanks for your help

[Eric Brown]

The reason that I originally asked if you had 5 ring pistons is...... I had a problem with 5 ring pistons in an A65 a long time ago. Primarily overheating. I was told that the 5 ring pistons were for the A80 engines. Currently, Continental service bulletin M91-4 calls for a 3 ring piston for the A75. You can view or download CMI bulletins for free at continentalmotors.aero. It just may eliminate you concerns.

[Tasca]

Just to give all of you some info, that can be interesting in future.

I've made a test this weekend and after had run the engine for 5 minutes, as expected it was creaking, I took of the spark plugs and sprayed WD40 in the cylinders.

All the noise stopped and the propeller was free, the creaking problem is definitively connected with compression piston rings.

Now I have just find the exact problem, too tight or material.

thanks all for clues and information.

[cub builder]

Just to give all of you some info, that can be interesting in future.

I've made a test this weekend and after had run the engine for 5 minutes, as expected it was creaking, I took of the spark plugs and sprayed WD40 in the cylinders.

All the noise stopped and the propeller was free, the creaking problem is definitively connected with compression piston rings.

Now I have just find the exact problem, too tight or material.

thanks all for clues and information.

By your description of mounting the crankshaft horizontally and the rods in their working positions with the numbers up (#1 at the back of the crank and #4 closest to the prop), then it is put together correctly.

If the rings were too tight, the engine would have lost power, seized and scored the cylinders when you ran it. If that hasn't happened, it's not a problem.

If you had the wrong rings, you would have massive oil consumption and or seizing of the rings into the cylinders.

As Eric stated, the 5 ring piston is an A-80 piston, the A-75 piston is 4 ring, and the A-65 piston is a 3 ring piston.

What you are hearing is slow speed ring chatter as you pull it through. Kind of like spinning your finger on the edge of a moistened wine glass to hear it sing. It's not necessarily a problem. If the engine is assembled correctly, stop worrying about it and enjoy flying it. I have heard other small Continentals that make the same noise. It never hurt any of them. It's unusual, but not all that unusual and not necessarily detrimental.

FWIW, I'm not a big fan of using the A-80 5 ring pistons. There's just too much ring scraping going on, so the top ring runs lean on lubrication and much of the HP gain of that tiny compression bump gets lost to ring friction. But, some people use them and they do run OK.

-Cub Builder

[crusty old aviator]

All my old geezer mentors taught me that if you run A-80 pistons, you need to run drilled rods with them or you'll wear out your wrist pins. What say you, Cub Builder?

[cub builder]

All my old geezer mentors taught me that if you run A-80 pistons, you need to run drilled rods with them or you'll wear out your wrist pins. What say you, Cub Builder?

A-75 and A-80 both required drilled rods. Almost all A-65s had them drilled during the first overhaul. I've never run across one that wasn't.

-Cub Builder