1974 knocking

[911canuck]

Hello, first of all, I would like to thank many of you, since I’ve been finding answers on those threads to my different questions, while rebuilding my 1974 Targa. After the rebuilt (where I haven’t touch much of the engine, except changing gaskets and seals since compression numbers were good), on my opening drive I’ve heard a concerning knock coming from the engine left hand side. See videos under

https://onedrive.live.com/redir?resid=4EF82120187DFB26!167&authkey=!AAZ40RamEhW4E94&ithint=folder%2c

Checking with a stethoscope, did narrow down the source of the noise to cylinder #1, and I can say that there was a metallic ‘ping’ to it. Although it may seems hard to believe, I can testify that this noise was not always present. It was present at start-up, sometimes not. I did not run the engine for more than 5-10 minutes since this noise was making me nervous.

The list of possible causes I have been looking at are shown in the following fish bone diagram.



And the results of my investigations are summarized in the following table.
All comments are welcomed, is there other possible causes ?

[arry911]

Hi I see on section 10 you have no cam wear , does sound a bit like when my camshaft went because of a blocked oil feed pipe.

[Charles Freeborn]

The first thing I'd rule out is valve adjustment.
If it's pre-ignition, the first thing I'd check is timing and dwell. Pe-ignition will not likely be limited to one cylinder, and is generally worse under load.
Both of these things are part of a complete tune up. When you've got the valve covers off you can inspect for other valve train issues.
Exhaust leaks have a sound entirely their own, which I would not describe as a "knock or ping".
I couldn't get either of your movies to play on my Mac.
-C

[911canuck]

Thank you arry, I will double check and make sure the oil feed are cleaned.

[911canuck]

Thank you Charles, I will try to put them on Youtube

[arry911]

Originally Posted by 911canuck

Thank you arry, I will double check and make sure the oil feed are cleaned.

Have you turned the engine over by hand to see if it catches up on anything?:

[911canuck]

Yes I did... no signs of interference anywhere

[Bob Kontak]

How did you check the rocker / shaft radial play? I don't know how. Mine just had a bit of slop in the mid 90's when on the bench. Replaced all the rockers. Still have the old ones but not the same part as yours or I could spot you one for a test.

My spec book on the SC says six ten thousandths of an inch at the skinny end of the normal range. 1.4-ish thousandths on the fat end. With the wear limit being the same as yours - 3.1 thousandths.

Gotta be a reason why they have a wear limit. That rocker will wear fast now that it is banging on the shaft every two revolutions of the crank. The shaft will hold up longer than the bearing in the rocker.

All that said, I would pull rocker/shaft, check OD of the shaft and source another rocker.

BTW - Appreciate the rigor of your test work.

[911canuck]

Hi Bob,

How to measure radial play.... well playing with all rockers in my hands I felt a significant difference between intake rocker of cylinder #2 than the other ones.... so I've tried to put a number to it by making measurements of shaft movement with a dial gage:


Without claiming this to be exact... it gave me a confidence level that my feeling was right (about twice the play of the other ones). I did pushed a bit further to have an 'appreciation' of the resulting radial gap on the bushing... by doing a simple 2D lever calculation; and the result was in line with the wear limit stated in Mr. Dempsey book (How to rebuild and modify....)



Thanks for the offer, but I did pull the trigger yesterday and order a new one. I will check the OD of the shaft and keep a close eye on the other rockers... Intake #1 and Exhaust #3 seems to be the only ones within the tolerance range (according to my questionable calculation).

Thanks again

[911canuck]

pmax,

since everything is now apart.... I will make sure that upon rebuilt if the noise is still present I will look closely at this. I'm afraid that my heat exchanger flanges are a bit eaten up... so it may explain the improper seal. I will see.

Thanks for your comment.

[911canuck]

For the records....

The knocking was caused by exhaust leak. I did sand out the exhaust flanges to remove the groove and after putting everything together the noise was gone.