Engine Knock Question

Sun May 11, 2014 8:00 am

I have a cub running cub that has an engine knock I can’t seem to figure out. I pulled the pistons, cleaned the head, cleaned and lapped the valves, bearing seem fine and the oil pressure is good (80%). I also pulled the spark plugs off one at a time to see if the noise went away but it didn’t. The only thing I can think of it that it might be a wrist pins? I can’t send it to the machine shop and am hoping not to have to do a complete overhaul because actually it runs good even with the knock. Any suggestions as to what I should do next? What are the common causes of an engine knock? Thanks for any and all advise. :mrgreen:

Re: Engine Knock Question

Sun May 11, 2014 8:16 am

When you pulled the pistons did you check the wrist pin fit? Could any wiggle be felt between the wrist pin and the connecting rod? When you say the bearings look good, what was the clearance measured with plasti-gage? We need more information.

Re: Engine Knock Question

Sun May 11, 2014 8:34 am

Ricky,
When I had the piston and connecting rods in my hands, there appeared to be no slop however, when I slid the connecting rod sideways on the wrist pin, or towards the piston wall, I did hear a slight tick. However, I wasn’t concerned because I thought the rod would stay in the center of the wrist pin while in motion and not migrate to the side.

I did not plasti-gage any of the bearings.

Something else I should have mentioned, that is, the knock takes a good minute or more to become pronounced enough to hear once you start the engine.

Re: Engine Knock Question

Sun May 11, 2014 9:28 am

It may be under the front cover, I have sen the idler gear make some strange sounds, try to locate the general location of the noise. you can use a broom handle if you dont have a stethoscope

Re: Engine Knock Question

Sun May 11, 2014 9:35 am

Does it only knock at idle or at all speeds? If only at idle, does it have a magneto? Did you check the valve adjustments?

Re: Engine Knock Question

Sun May 11, 2014 9:57 am

If it is high front, could be the timing idler. Mine needed a new bearing. Just be prepared if it is the idler...the replacement bearing is babbit material and must be finished to .001-.003 clearance. It must be scraped or carefully reamed, NOT HONED!!!

Re: Engine Knock Question

Sun May 11, 2014 10:29 am

Stethoscope. Length of hose or rubbing. One end to ear, other end to various parts of running engine. See if you can isolate the general area of the knock.

Re: Engine Knock Question

Sun May 11, 2014 10:40 am

Boss Hog wrote:It may be under the front cover, I have sen the idler gear make some strange sounds, try to locate the general location of the noise. you can use a broom handle if you dont have a stethoscope


I agree. Try to locate the source of the sound. A stethoscope is a great thing to have at a time like this. Often I can't get my stethoscope to seat well because there is not a large enough flat area. Then I try the broom handle method.

Re: Engine Knock Question

Sun May 11, 2014 11:08 am

Thanks for all the advice!
I put the stethoscope to it and found no obvious location. If I were to guess however, the noise is coming from the crank area. I guess it time to plastique the bearings.
I do have a mag on this machine by the way but found no loud noises coming from it or the front end.

Re: Engine Knock Question

Sun May 11, 2014 6:02 pm

As a stupid question does the knock stop if you press the clutch down?

Re: Engine Knock Question

Mon May 12, 2014 7:47 am

More information is needed. My guess is that the bearings may have been replaced somewhat recently (still could be a loose bearing) unless that oil pressure reading is only when it's cold. Usually when a rod knock first develops it's most noticeable when the engine is first started (before the oil pressure builds) and when the oil is hot.
Virginia Joe wrote:the knock takes a good minute or more to become pronounced enough to hear once you start the engine.
.
Virginia Joe wrote: If I were to guess however, the noise is coming from the crank area.
Virginia Joe wrote:because actually it runs good even with the knock
It will run ok with a rod knock for a while but the knock will get worse and potentially ruin the crank or at the very least require machining. If it is a rod knock I wouldn't risk running it without knowing for sure and fixing it. In addition to plasti-gauge I would check the rod journals for being out of round.

Re: Engine Knock Question - Solved

Sat May 17, 2014 5:00 am

Hello again, I have an update; the problem is apparently fixed. What I did was plasti-gauge the connecting rod bearings and noticed that #2 was way out of spec. Since the old bearings were Federal Mogul .010 oversized I able to cross reference to Clevite and ordered CB-310P-10 from CarQuest, These were the older wider type bearings. I’m usually a diehard NAPA guy but they really struggled identifying the bearings and Clevite was very helpful. Anyway, once installed, the noise disappeared. Thanks for everyone’s support. The machine is not like new but certainly better than it was.

PS if anyone has a trick or tool that makes installing that one oil pan bolt easier, under the flywheel, I’d love to near it. With my short, fat fingers, it’s nearly impossible. I ended up using a ½” open end wrench facing straight up and turning the bottom of the wrench with a screwdriver.

Re: Engine Knock Question

Sat May 17, 2014 9:05 am

Virginia Joe wrote:PS if anyone has a trick or tool that makes installing that one oil pan bolt easier, under the flywheel, I’d love to near it.

Joe this is what I use. 1/4" drive.
socket.jpg
You do not have the required permissions to view the files attached to this post.

Re: Engine Knock Question

Sat May 17, 2014 9:23 pm

Another possibly are wobble extensions
Image

Re: Engine Knock Question

Sun May 18, 2014 3:11 pm

I use what Bill uses plus I slot the bold head with a hacksaw so I can start the bolt with a screwdriver.