M38 Engine Sound

[Balvar24]

Bearing clearance is what you're looking for.

[wesk]

Being a quart low on oil doesn't give you a rod knock.

Black oil doesn't always mean rich running engine.

Black oil can be a few things. How much operating time does the oil have on it?

Now that the pan is off the next step is to remove bearing caps one at a time and plasti-gauge them to check clearances. Not just rods. Rods & mains need to be checked.

[MarkR1951M38]

Another question that may be related...does anyone run NON-ethanol in the L-134 engines? That's my current gas now. Would an oil or gas additive help?

After pulling the pan, the underside of the motor does look like it was rebuilt as the previous engine owner stated so I'm real hesitant to think it's the bearings. Back in the early 70's we had rod knock if the octane wasn't high enough for some of the muscle cars we had. But I would think 87 is high enough for this little non-race engine.

[Xamon]

just because it was rebuilt does not mean it was done correctly. If the crank has been reground (or not when it needs it) and they did not install the correct oversize bearings you will have too much gap.

Wes knows these engines inside out, if he says check something I do suggest listening. He will lead you to the answer quickly without throwing unnecessary parts at the problem.

[wesk]

Would an oil or gas additive help?

No. That will not help a mechanical bearing knock. These engines will function on an octane rating of 60.

Back in the early 70's we had rod knock if the octane wasn't high enough

That is called spark knock or pinging and sounds similar but is not as deep sounding a knock as a bearing knock. However, if ignition timing is far enough off it can cause a spark knock that is very close to a bearing knock in pitch.

I have dwell at 42, point gap at .020, and timing at 5 BTDC.

But you have already posted you have the correct timing.

That leaves cam timing which can also introduce a knock under load.

If you are overly reluctant to inspect bearings then possibly you can remove the crank pulley and timing gear cover and get #1 TDC and check cam timing dot alignment on the crank & cam gears.

[MarkR1951M38]

Thanks Wes, Since I have the pan off, I'll pull the rod caps off and inspect bearings maybe this weekend or next. As for the cam timing, I had to remove and reinstall the gear set a couple times during the build, so I was extremely careful to make sure #1 TDC and dots were aligned, but I may pull the cover just for verification like you said.

[wesk]

I'll pull the rod caps off and inspect bearings

Don't forget the plasti-gauge!

[MarkR1951M38]

Don't forget the plasti-gauge!

In TM9- 1803A, I'm seeing 0.0005 to 0.001 inch (.001 max limit) on connecting rod bearings with torque of 50-55 ft lbs.

Would you say this is correct?

[wesk]

In TM9- 1803A, I'm seeing 0.0005 to 0.001 inch (.001 max limit) on connecting rod bearings with torque of 50-55 ft lbs.

Would you say this is correct?

Yes & No. The correct reference is the M38 TM 9-1804A and max limit there is .001 however the max wear limit is .002.

[MarkR1951M38]

Awesome...I looked in 1804 but must have overlooked it...I'll review again. BTW...I'm not going to check bearings and pull gear cover until temps are under 80. So it will be a week or so. 90's and humidity are real killers here in AL.

[wesk]

Having spent 13 years in Louisiana working on Air Force planes on the parking ramp I know exactly what that feels like!

[MarkR1951M38]

Uh...a Keesler Kid?

[wesk]

Northwest! Barksdale 2nd BW SAC.

[MarkR1951M38]

Yea, that's about the same latitude as me...you know what I mean. Hot, #@$%@ hot.

[Stiknruddr]

Taxied by the alert area Christmas tree many times looking at the Buffs sporting nukes. One way mission