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

[tcom2027]

Ninety percent, cracked piston, wrist pin bushing or a clip that came out is the source. Too bad you fired the parts cannon at it.

Why didn't the mechanic suggest checking the rod bearing while the pan was off? At least you know that the crank was good. 

It's not difficult to remove the pan and left side cylinder head. There is a ton of room underneath and you can see how easy it is to get at the left side head. Just for fun I looked up the factory and Motor Age shop times for the operations.

R&R left head.               1.0 hrs   
R&R oil pan                  1.4 hrs
Renew Piston/piston pin
(include hone bore If nec.)  1.5 hrs
           
                 Total time:  4.9 hrs

Twenty years go when I was sixty I could make that time easily and have even without a hoist.Jack up the car,drain water and oil.While draining start on the head with a half inch drive impact,set the head aside,clean up the coolant mess on the floor and attack the pan with a three eighths impact. Do what I had to do underneath, clean head and block, check head for straightness and cracks, (seldom found)  prepare head gasket for assembly,replace a piston and hone,etc. and reassemble.

Now, that was on a clean fairly fresh engine which is not the case virtually all of the time. On those occasions head studs can break and often do requiring additional work and time. Same for pulled threads which is common requiring coiling. Those jobs are strictly time and materials and can add a lot of time to the job. If what the OP says is true and it sounded like a "Singer sewing machine" and the looked like it were fresh and the undercarriage didn't look like a toxic waste site I'd quote five and a half hours plus parts, with the understanding there still could be unforeseen problems.

I hope it turns out to be the fuel pump push rod, but that, because it runs off the cam can have its own unique set of problems.

Hope this helps and good luck. When these engines are right there is nothing like them. 



 

[kidclerk]

Thank you all for the suggestions.  Hopefully we will tear into some of these in the coming weeks.  And Tony...no cannon fired just yet.  The spare parts are all what came with the car when I bought it.  I'm eliminating the cheap things first! 

KDC

[tcom2027]

Best way to approach it. Let us know what you find. HAving and keeping known to be good parts is always a good thing.

tc