Cub smoke and knock

[fiddler]

So my cub clatters. I am not sure I would call it a knock. The engine has maybe 20 hours since a rebuild necessitated by the previous owner placing one of the main bearings in upside down so the part without the hole for the oil journal covered the oil journal. I was able to turn the crank and get oversize bearings. It runs beautifully but I am not sure how loud the valves are supposed to be. I checked all of the clearances with plastigauge and I can't find a reason for the clatter. I am not even sure that it is not valve clatter due to solid lifters. I think I hear the same clatter going in a few of the videos of cubs running, but most videos are not close enough to the engine to pick up valve clatter if that is what this is. So do these engines usually have valve clatter??

[Glen]

Hi,
From what I have seen, the Cub engines make sort of a light tapping sound, with the valve clearance set at .013".
You can probably hear it more on the left side of the engine, where the valves are.
It's more noticeable at slow engine speeds, like slow idle speed, about 500 RPM.

If you hear louder, knocking sounds, coming from low in the engine, that is probably a crankshaft bearing too loose.

[Gary S.]

I had an 1600 4 cylinder MG which are very similar that developed a knock after it got about 500 miles on it. Turned out the rods had been refaced so many times the side clearance was .020-.030 when no more that .012 was called for. Under load and deceleration was fine but cruising it would make noise. Ended up getting another crank that had not been reground and going through piles of rods looking for the widest bottom ends. The only thing I have ever been able to figure out was that the bearing shells at that point were wider than the rods so that when I used a feeler gauge to check clearance I was measuring between the bearing and the crank instead of the rod and the crank.They then wore down causing noise

[Gary Dotson]

Cub valve trains make very little noise, unless the valve clearance is well out of spec. If you're hearing concernable clattering, it's probably deeper in the engine.

[fiddler]

It has been a long time since the rebuild, but not a lot of hours running. I basically turned the engine over with the starter every month or so for the last 20 years. Now that I am retired I am going to re check the bearings with plasti-gauge. I don't remember... is it possible to get the main caps off without pulling the crankshaft? Seems to me I observed that it would be possible to roll in main bearings if one was so inclined.....

[tnestell]

I don't remember... is it possible to get the main caps off without pulling the crankshaft? Seems to me I observed that it would be possible to roll in main bearings if one was so inclined.....

Yes

[fiddler]

Thanks for your response! Well then I will re plasti gauge each bearing. I will start with a pan gasket and some oil... oh and plastigage of course..

[Glen]

Hi,
If you see a bearing that doesn't look good, try using the Plastigage on it with the crankshaft turned to 2 different positions, the 2nd check 90 degrees apart from the 1st check.
The crankshaft could have a flat place on it, that might not be seen using 1 check per journal.

Flat places on the crankshaft journals can be caused by running the engine with a worn bearing that was too loose.
If all the bearings look good, then you could check them all in 2 positions.

If you had the crankshaft at a shop, they should have found anything that was wrong with it.

[fiddler]

I did have the crankshaft turned 20 thousandths if memory serves. I will check the bearings when I have it open. When I originally rebuilt this I was still working and my job changed after that so I really could not get back to it. Anyway, it clattered and I was never sure if it was valves or the bottom. I think that I checked all of the bearings with plastigage once, but maybe I missed one. There was no forum when I rebuilt this so I was pretty well on my own.

[tst]

yes main bearings can be changed with crank in place, just insert a small low profile head screw or nail in the oil hole of the crank, rotate crank and the screw will catch the end of the bearing and push it right out

[SONNY]

Your title of post says smoke? ---thats bad rings, -- either broken or worn out. --- also could be plugged crankcase vent forcing oil past rings.
I have a 185 that I mow with every week and it been clattering for 8 years now!

[BigBill]

Note for the future,

I made a T shaped piece of steel to fit in the oil hole to spin out the upper main bearing half while the engine is still assembled in the car, tractor or truck. Even a angled T the oil hole is angled. The top of the tee was thinner than the thickness of the bearing. I put the tee in the oil hole and turn the crank till the bearing turned out.

I changed engine bearings while working for the amc/Jeep dealer in the 70’s. The cranks were ground .002” under sized. Every new car was knocking. It sounded like lifters but it wasn’t. I was changing main and rod bearings.