M38 Engine Sound

MarkR1951M38 · Post by **MarkR1951M38** » Thu Jul 26, 2018 6:44 pm

I recently resurrected a '51 M38 with 12v and got to the point of moving it for the first time. Sorry for the fat ass belly, but it was 90 degrees.

First movement from a buildup from scratch. I rebuilt everything except the motor which I was told was rebuilt and ready to go. During my first movement/brake test, I noticed a noise at the end of the video. What do you think it is? The sound in question is when I get back in the garage and ready to shutdown....the thumping sound...disregard the belt squeal...I'll get that fixed.

https://www.youtube.com/watch?v=s7fe20uovZg

Post by **wesk** » Thu Jul 26, 2018 11:45 pm

That's easy. It's a rod knock! What was the oil pressure gauge reading?

MarkR1951M38 · Post by **MarkR1951M38** » Fri Jul 27, 2018 5:00 am

20 psi I think...wonder why it did it after returning into the garage and not going out.

I'll try it again this afternoon and double check the oil pressure. Thanks Wes.

I also need to purchase a new tach/dwell meter and get it on there to verify rpm and timing.

Post by **wesk** » Fri Jul 27, 2018 8:52 am

It had a milder version of that knock the instant you started it up and while you were backing out. The camera up front was in a better position to hear it then you were.

Good sense says pull the pan and physically check the rods before you do any more runups.

MarkR1951M38 · Post by **MarkR1951M38** » Fri Jul 27, 2018 9:01 am

Thanks Wes...good advice. I think I will instead of ruining the engine.

MarkR1951M38 · Post by **MarkR1951M38** » Fri Jul 27, 2018 2:36 pm

Wes...Would dwell at 43 and timing off contribute to the rod sound? I just purchased an electronic analyzer, hooked it up and discovered dwell at 43. Going to search for dwell angle now....I was thinking it was around 20.

MarkR1951M38 · Post by **MarkR1951M38** » Fri Jul 27, 2018 4:33 pm

I have dwell at 42, point gap at .020, and timing at 5 BTDC. Noise quieted down some but moving 100 yards the oil pressure went below 20 psi...was around 40 during run-up and initial movement. I think oil pressure is my culprit...needs to be up around 40. I'm going to drop oil pan and check rod caps for tightness, then I may move to the oil pump.

FYI...I purchased this engine with word from previous owner that it was rebuilt by an engine shop...this is my third issue with the engine (no bolts to mount flywheel, missing front pulley spacer). And to top it off, Wes informed me that the serial number indicates it was a Willy's station wagon engine...dang!

Mike_B · Post by **Mike_B** » Fri Jul 27, 2018 6:51 pm

You're loosing oil pressure because the bearings are loose/worn. Raising the oil pressure will not fix your rod knock problem.

Mike B

MarkR1951M38 · Post by **MarkR1951M38** » Fri Jul 27, 2018 7:26 pm

Mike_B wrote:You're loosing oil pressure because the bearings are loose/worn. Raising the oil pressure will not fix your rod knock problem.

Mike B

Well, thanks for the wonderful news.

Post by **wesk** » Fri Jul 27, 2018 9:44 pm

I would advise you to stop running the motor for timing checks and get the pan off and physically determine if you have a loose rod or not!!!!!!

MarkR1951M38 · Post by **MarkR1951M38** » Sat Jul 28, 2018 4:30 pm

Gettin' after it Wes...again, thanks for the advice.

This week I'll get-r-done thinking if only 3 quarts of oil come out and 1 quart in oil filter, then maybe low on oil. (hopefully) I'm not confident on the engine oil stick, yea it looks good, but is it accurate? Else, I'll pull the engine and replace crank and cam bearings and inspect the oil pump.

Ray101 · Post by **Ray101** » Mon Jul 30, 2018 9:59 am

Low oil level will not cause low oil pressure until it gets low enough for the pickup tube to suck air, and the it will be zero or fluctuate alot.
Better listen to Wes and pull the pan.

Post by **wesk** » Mon Jul 30, 2018 11:26 am

I'm not confident on the engine oil stick, yea it looks good, but is it accurate?

Two important things I have learned wrenching on and buying vehicles the last 60 years.

Establish baselines on fresh rebuilds and newly obtained vehicles.

Simple Things:

With a newly obtained vehicle I always put a full gas can in the trunk and drove it til it ran out of gas. From that point on I always new exactly when the gauge meant EMPTY. Of course be careful with the newer fuel injected cars that do not behave well if you run the pump dry.

With oil dipsticks it's similar, I always check the stick at the end of an oil change to see what it reads vs the quantity of oil I serviced. Then make a noticeable note somewhere if a correction factor is needed.

MarkR1951M38 · Post by **MarkR1951M38** » Mon Jul 30, 2018 12:35 pm

Good advice...makes sense. Thanks!

MarkR1951M38 · Post by **MarkR1951M38** » Thu Aug 02, 2018 10:51 am

Wes and others...I have removed the oil pan and all rod and main caps were tight. However, only 3.5 quarts of oil came out and what did come out was as black as coal. Maybe attributed to before the carburetor was rebuilt...it was running very rich before the carb rebuild.

When draining oil without draining the oil filter, should you get 4 full quarts? Guess I'll dump in 3.5 - 4 more quarts of clean oil and see how it runs before I just go and remove the engine.