Mysterious knocking

[Aledbette4]

I’ve got a 26 touring. I need some assistance in narrowing down a knock. The engine will knock when it is warmed up after driving a couple miles. It only does it when holding a set speed. If I open or close the throttle to speed up or slow down any it goes away. Then comes right back once maintains its speed. I’m not sure what else to describe or say to help. The T bible does a good job of describing and helping narrow down knocks but nothing sounds like what I’ve got going on. I’ll give any other input I can. Thank you in advance.

[Norman Kling]

Very likely a loose connecting rod bearing. One thing you could do before diving into the crankcase, would be to slightly retard or advance the spark lever and see if the knock continues. If it stops knocking, just run the spark at that position while driving. On a 26 engine it is quite easy to adjust the bearing clearances because you can easily get to rod number 4. If you don't already have it, I would suggest you purchase the book "Engine" from the club or one of the parts suppliers. It has details on how to trouble shoot knocks and also how to adjust the rod bearings.
Norm

[Les Schubert]

I think it is most likely a loose centre main. The “rattle” at moderate speed and moderate throttle has always turned out to be about.004” clearance in the centre main. I always remove the engine and do it upside down on the bench. That makes it much easier to put a dial indicator on the journal after the cap is removed and then push hard towards the block. This verifies that the wear hasn’t been caused by a bent pan. This can all be done with the pan and transmission cover in place. At the same time I would check the rods. You will likely find a bit of wear if engine is getting some miles on it. But the centre main will have most of the wear

[Norman Kling]

If the problem is the center main as Les suggests, the wear is usually in the portion of babbit in the block above the crankshaft because any sag in the transmission tends to bend the crankshaft upward at that point If you do find the looseness to be the center main attempts to tighten it by removing shims or filing the cap will just keep the pressure upward on the center main and not really fix the problem. You will need to pour the babbit in the block and correct problems in 4th main or bent crankcase so the crankshaft will run true. I have found that main bearing knocks tend to be heard as you accelerate or pull hills, rather than running at a constant speed, but rod knocks tend to be loudest when running at constant speed or slightly decelerating.
Just my input.
Norm

[Les Schubert]

I have on rare occasion encountered the situation Norm describes and perhaps the symptoms are as he describes. This is why I recommend the dial indicator test as it will rapidly confirm or eliminate that condition.
I have encountered numerous times the version I described. As I said, usually it will occur simultaneously with rods that can stand some attention. Personally I have never encountered a situation where the rods need attention that the centre main doesn’t also need more attention. Perhaps in California circumstances are different!

[Ruxstel24]

Sounds like a loose rod bearing to me...
You can also try raising the idle and if the noise is there on a fast idle, short one plug at a time and see if and which cylinder quiets up the knock.
Not a very difficult job to tighten if the Babbitt is still ok.

[John Dow]

I had a similar rattle and found it was "octane" knock caused by my Reeder aluminum high compression head. I went to 91 octane gas and used a little less advance. Rattle gone!