Pre-war (up to 1942) '39 Flathead hot knock

[kidclerk]

Sorry for such a long post and it being my first one.  I'm the proud owner of a 1939 60 special and 1949 Series 61.  Been working some kinks out of the suspension and engine on the '39 and can't seem to pinpoint the lower engine knock.  Here is a list of what I have going on.

Upon starting the engine, she purrs like a Singer sewing machine.  It can idle and be revved up sitting in the garage for 20 minutes and sound wonderful.  When I take it out for a five mile drive and return, I have a knock that sounds like it is coming from the engine's belly.  Keep thinking it's a rod until I read more message boards and then second guess myself.  When I take off the #1 cylinder plug wire, the knock disappears.  I read one time that this would not eliminate rod knock sound, only change the sound.  The knock seems to not be the same rhythm all the time.  It appears to come and go in a matter of a few seconds of a cycle. Does not appear to get worse upon increase RPM.  Can't really hear it going down the road.

What we've tried:  higher octane gas, different spark plugs, heavier oil (Lucas 20W50 high zinc content).  Upon installing a new pan gasket my mechanic saw no signs of sludge.  He said the engine was very clean. We have not yet checked the timing but we did adjust it slightly to get the starter to spin more rapidly on cold startup.

I don't know if I have a rod issue, detonation, or something else. The engine is believed to have been rebuilt a few years ago based on all the spare parts I have and how clean it is inside.  This does not appear to be coming from the top part of the engine so we have not thought about pulling the heads off.  Would a rod bearing be accessible without pulling the engine?  What other diagnostics can we try at this point?  Why doesn't this happen when the engine is hot while in my shed idling or being revved up and only after a few minutes under load of drive time?

Any and all suggestions are welcome, and thanks!

[James Landi]

Have you used a vacuum gauge on your engine... it can diagnose MANY issues, and is easy to set up and with a little reading can help you out immensely  (including timing). Could be timing... does your vacuum advance hold vacuum... when vacuum is applied does the stator plate move?  How about the spring weights on the distributor... do they spring back when you move the rotor? oes the stator plate have any lateral lash?  How about the timing with a light?   Is the carburetor "dialed in" with the right float level... could there be an internal vacuum leak that's causing your engine to run LEAN when under load or a vacuum leak on the intake manifold... lean on gas causes engines to run hot under load. |How about using a laser to check for hot spots on the cylinder heads?  Just some ideas... hope this helps you, James

[kidclerk]

Thank you, James.  We will give these a try.  I want to rule out everything else before we tear into the engine assuming it's a rod.

KDC

[KenZ]

The Fuel Pump can make a knocking sound when the vacuum supply is disconnected or maybe leaking. Good Luck, KenZ   

[Warren Rauch #4286]

 What are the engine temperatures in your garage and on the road?  I'm guessing they go up after the drive.If it is  temperature dependent  ,like a crack. Maybe a piston?

Warren

[James Landi]

One more suggestion... if it were my car, I'd use 30 weight  high detergent oil and NOT multi grade oil.   Your engine was not designed for multi grade, and so if you have an oil pressure gauge, you'll watch, in horror as your oil pressure goes down to near zero with multi grade oils as your engine heats up.   

[Tom Beaver]

I had a 38 engine that had a strange knock which was loudest if listening from under the engine.  If I shorted the spark plugs one at a time I found that when I shorted the #7 cylinder the knock went away.  The result was that the wrist pin bushing on the #7 rod was worn out.  In fact all of the wrist pin bushings were out of spec, and #7 was the worst.  Whoever rebuilt the engine neglected to replace the wrist pin bushings.

I would suggest that you pull the #1 rod and piston and check the the wrist pin bushing clearance.

Tom Beaver