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Help chasing down engine knock

Posted: Sat Jul 13, 2019 12:29 pm
by Chad_Marcheese
Hi all,

Seeking some advice on chasing down a knock I have in #1 cylinder. I shorted the plugs and have found #1 cylinder to be the culprit. I have adjusted the rods once and have put maybe 20 miles on it since the initial adjustment two years ago. It is possible it may need another adjustment. Keep in mind this engine (stamped with a 1920 serial number) it is rather tired and things inside are not optimal by any means.

Two years ago, I did all cylinders, but #1 is still noisy. I sanded the original caps to the point I could add a few shims back in and adjusted accordingly. I do realize this makes a rod journal a bit out of round by sanding the cap. The engine runs good otherwise and is pretty smooth. It is firing on all cylinders, i just get a persistent knock, that seems to be most audible at idle (I can hear it driving, just not as bad, maybe due to normal wind noise and such. It sounds like a nasty rod knock.

So I took down the pan again this morning, didn't find much material in #1 dip, #2 and #3 had more than I would have liked to see---but again, its a tired old engine, it just kinda is what it is. Unlike the first time I adjusted them, this time all the rods including my #1 seem tight on the crank, that is to say I can not feel any up / down play on them and they move freely on the thrusts, side to side (front / back on the crank pin).

Could this be a piston slap issue instead that I am trying to chase? I am trying to figure this out without tearing this thing down. I have some of the parts and pieces to build another engine with a Scat crank, I just need to save up and have a good block rebabbited--and a camshaft too. So this one needs to last me a bit longer.

Re: Help chasing down engine knock

Posted: Sat Jul 13, 2019 12:53 pm
by Tim Williams
Piston slap is not out the relm of possibility. I have the same issue on my tired T engine.

Re: Help chasing down engine knock

Posted: Sat Jul 13, 2019 1:18 pm
by jab35
Chad: Did you measure the clearance? Plastigage? Or apply machinist dye on the surfaces, bolt up the caps and check surface contact on the rod bearings? If you cannot confirm piston slap, I'd recommend the dye or plastigage. FWIW, jb

Re: Help chasing down engine knock

Posted: Sat Jul 13, 2019 1:35 pm
by Erik Barrett
Bent rod can cause noise.

Re: Help chasing down engine knock

Posted: Sat Jul 13, 2019 4:49 pm
by Les Schubert
I have found that the centre main is almost always loose when the rods need adjusting. The characteristics is that it will rattle at light throttle at about 20 mph.
Another rattle that can sound like a rod is the timing gears. The difference is that there will be a “inconsistency “(not every rotation, but just a bit random).
I hope this helps

Re: Help chasing down engine knock

Posted: Sat Jul 13, 2019 7:32 pm
by Scott_Conger
To augment what Les said about center main, that can be checked by shorting out or holding coil vibrators down on coils for both #2 and #3 at the same time. If the knock ceases, it's center-main. Sometimes that can be fixed OK and sometimes it can lead to a broken crank if you just tighten up the bearing without doing some further diagnostics.

Feel free to send me an email for more info.

Re: Help chasing down engine knock

Posted: Sat Jul 13, 2019 7:43 pm
by Chad_Marcheese
I was out investigating the issue. Rechecked rods, 1-3 (3 dip pan so no #4 was checked). Those three rods checked in at .0012-.0015" via the plastigauge. Mind you nothing is truly round anymore in this engine, as I did check a few times in various spots of the cap. Obviously because I filed the caps, the clearances are wider toward the rod bolts.

So what else could I try---let me grab hold of a piston and see if it moves excessively in the bore---YUP! I think I found the answer. #1 and #2 bores seem to be REALLY loose on the piston clearance. Guess that is where the issue is. #3 piston showed no noticeable more movement than I would expect in the clearance.. I had to use a long prybar to even detect the rocking of it in the bore. I am surprised #2 isn't rattling as hard as #1, but maybe #1 is just that much more worn out. Compression is all above 45lbs on all cylinder hot. So I guess I just deal with the noise and try to get the parts I need to finish my engine---or find a spare.

I could tell you other horror stories about this ticking time bomb I have found in the past also, but I think I'll just keep the travel light and around the house and make sure my Haggerty emergency assistance card is in my wallet.