Engine Question

[1940plymouth]

I have had my '40 Plymouth out for about 100 miles this past week, the other day, Wednesday, it developed a knock, so I changed the oil and filter, added STP, like I always do, put a few more miles on her, the knock was still there. When my son got home, he is an experience auto mechanic, self taught and SUNY Morrisville educated, we went for a ride so he could listen, when we got home, he got under the hood to rev the engine with the carb linkage, he says it sounds like a rod knocking. I don't know the history of the engine other than it is a 1941 P-11 engine that hasn't given me any problems in the 21 years plus that I have had the car.

My Question is:

Can we replace the main bearings and rod bearings by dropping the oil pan and micing everything before I order the bearings?

I don't really want to pull the engine, but will if necessary.

I know just enough to be dangerous, that I would ask you for your expertise on this matter.

Thanks,

Bob

[1947PLEVY]

Bob:

Chances are if you have a rod knocking. The crank will have to come out and be replaced.. maybe even the connecting rod. John

[james curl]

I believe the engine specs say that a throw can be as much as .001 out of round and be OK. I tried that on my engine and after about thirty to fourty miles you could begin to hear a sound that I did not like when shifting through the gears. The new bearing had over 500 miles on it at the time. I just went ahead and pulled the engine and replaced the crank with one that I had that was already turned .010 on both thr rods and mains.

[Plymouthy Adams]

did you determine the bad cylinder by pulling the plug wire while running..rod knock should go away when the fire is removed..nothing to drive it..

[55 Fargo]

Bob, I would diagnose the engine in the manner Tim, has suggseted, to determine if it is infact a bad rod knock.

Did your Son isolate each cyl by pulling 1 spark plug wire at a time. Perhaps this is another noise, maybe fuel pump, or something else..........Fred

[55 Fargo]

From Crouse Automotive Mecahnics Vol 4, about 1962.

Connecting-Rod Noisesusually have a light pounding or knocking characteristics.

Most noticeable when engine is floating, not accelerating, or decelerating.

The sound is more noticieable, at medium speed as the acelerator is eased off.

To locate conecting rod noise, short out spark plugs one at a time.

A worn bearing,or crankpin, misaligned connecting rod,or excessive bearing clearance, can all make connecting rod noise.

So can inadequate oil, which is not your problem.........

[aero3113]

When I first got my 50 Plymouth It had a very bad knock. The PO tried to tell me it was a motor mount. That was a bunch of B.S. He had hardly any oil in the engine! Once I filled it up to the proper level the knock went away. A month or so later I changed the rod bearings,rings and pistons just to be safe.I put about 1000 miles on the car and has been quiet ever since.

[John Reddie]

Bob,

You should make sure it is a rod or bearing problem. How is the oil pressure? Try the spark plug wire procedure. These engines can make noises at times that can be serious and then not so serious. My '49 Chrysler started making a knocking sound which was diagnosed as a rod, or wristpin, or valve lifter, or loose timing chain hitting the cover. The oil pressure was perfect. Another mechanic told me that it could be a spark knock caused by a carbon piece glowing in one of the cylinders (he really said that). I was young and had hardly enough money to put food on the table so I gambled and used the car. Within several days the noise vanished and I ran the car another four years. What I am saying is to try to find the exact cause. Good luck to you.

John R

[Young Ed]

A few guys on here have mentioned their fuel pumps causing this type of noise.

[55 Fargo]

Bob, let us know what happened here, did you try and isolate the cylinder, or is the noise something else hopefully..........Fred

[Don Coatney]

If your engine is holding good oil pressure (ie, no change in oil pressure when the knocking started) I suspect you may have a wrist pin noise. Removal of the spark plug wire on the noisy cylinder will confirm what cylinder is giving you the problem. Drop the pan, remove the head, and remove the rod and piston from the engine for inspection.

If a bad wrist pin is found you can make repairs to that cylinder only, reassemble the engine and drive it.

If you are not comfortable with this then pull the engine and do a correct rebuild.

[1940plymouth]

I want to thank everyone for thier comments. I haven't looked yet as it is a wet, cold, very windy day up here. When the weather permits, I will back the car out of the garage and do the spark plug wire test first. The oil pressure isn't the best, but then again it hasn't ever been great, but there is no change in it.

Hopefully tomorrow I will be able to have some answers

[Niel Hoback]

Listen to the bell housing to make sure its not a loose flywheel, or maybe even the front hub.

Its been my experience that if its a rod bearing, it won't last long. The knock will get louder really fast. Then bad things happen fast. A loose main bearing can knock for a long time.

I drove home from Birmingham to Gary in a 60 Olds with a bad rear main, but it lasted quite a while after that.

[power_hungry]

I've had a couple engines that had rod bearing go bad (not flatties). I dropped the oil pan and replaced the all the rod bearings after I cleaned up the journals with fine emery cloth. Put on a new pan gasket and never had a another problem. The bad bearing was sloppy enough that you could tell just by moving it around on the journal.

[55 Fargo]

Bob, did you do any further testing to determine the engine noise?

[1940plymouth]

Yesterday my son and I let her run while I pulled each plug wire one at a time, the knock did go away at either time. I have a good friend that is very good with the old Chrysler and F... flatties, he is going to stop in one day this week.

Question, at this point, if it is the bearings, when I remove them, are they

numbered as to what size they are?

Thanks for all the help,

Bob

[55 Fargo]

Yesterday my son and I let her run while I pulled each plug wire one at a time' date=' the knock did go away at either time. I have a good friend that is very good with the old Chrysler and F... flatties, he is going to stop in one day this week.

Question, at this point, if it is the bearings, when I remove them, are they

numbered as to what size they are?

Thanks for all the help,

Bob[/quote']

Bob, when you rmoved each plug wire, did 1 cyl indicate a noise reduction?

I can't ascertain from what you wrote on what is happening.....

[1940plymouth]

I just came in from the garage, I ran the Plymouth, pulled each plug wire off one at a time, I didn't notice the knocking going away when I pulled each plug wire off. I let the engine idle as I removed each plug one at a time.

[Young Ed]

Check your fuel pump then.

[1940plymouth]

Ed,

How do I go about doing that?

[Young Ed]

Best way is with a mechanics stethescope to try and hear if the noise is coming from that area. I believe you can also use a long screwdriver or dowel to try and let the noise vibrate through that. Otherwise you'd have to swap it out and see if that cures it.

[Plymouthy Adams]

as the knock came on suddenly...do not overlook the fact of a carbon knock from dislodged build up...start it, get to temp, crack the throttle 1/2 open or so and pour a 12 oz bottle down the throat of the carb...keep the engine running should it start to die rev a bit more..do this out of doors...it gets nasty...will steam clean the combustion chambers..

[Johnny 5]

Best way is with a mechanics stethescope to try and hear if the noise is coming from that area. I believe you can also use a long screwdriver or dowel to try and let the noise vibrate through that. Otherwise you'd have to swap it out and see if that cures it.

An old broom handle works too.

[1940plymouth]

We dropped the oil pan this evening, cylinder four had a lot of play in it, but the other five appeared to be ok. The oil pan was rather clean, as was the oil pump screen, basically no slug to speak of. I am going to pull the head tomorrow. The number on the rod bearing is 2507-010, is that the part number saying that the crank shaft has been growned .10, or is it an original bearing?

Like Don said, it appears to be the wrist pin, but with pulling the head tomorrow, we will know for sure.

Thanks again,

Bob

[greg g]

I guess this means no coffee in Oneonta for a while............

good luck. Hope its an easy fix.