Rebuilt engine knock...

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staninlowerAL
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Re: Rebuilt engine knock...

Postby staninlowerAL » Sun Oct 11, 2015 5:08 am

SouthernOutlaw wrote:Have you checked the piston clearance. Piston skirt slap will sound like a problem wirh the crank shaft. Normally will start after a motor is warm to operating temp. I notice you dont hear it until after the motor has been running for 15 minutes or more.

This may be a stretch but what is the history of the pistons? I had a similar experience with a early 1990's GM 350 that had improperly sized aluminum pistons in a CI block from the factory. When the wear got to a certain point, the "slap" of the pistons in the bore was bad enough that it had to be rebuilt with new correctly sized pistons to stop the noise. In fact, GM had a "factory recall" on this problem.

SouthernOutlaw
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Re: Rebuilt engine knock...

Postby SouthernOutlaw » Sun Oct 11, 2015 12:56 pm

CraigKennedy wrote:Southern Outlaw, please explain,
Thanks craig

It comes from a piston and ring set installed in a cylinder that is either out of round, or to large for the piston. Most cases I have come across come from engines that have been honed out several times and the original piston used. I had it happen years ago when I rebuilt a motor in a mid 40's Jeep. Ordered a set of pistons for the over bored cylinders and one of the pistons was standard. I was much younger and in a hurry to get the engine done, and made a bunch of mistakes in the rebuild. The only way to check is to size the cylinder, and pistons. Most cases the noise will start when the engine warms up closer to operating temp.

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Re: Rebuilt engine knock...

Postby CraigKennedy » Sun Oct 11, 2015 8:04 pm

SouthernOutlaw, that does sound like the problem that I was having, But all the pistions are .30 over, and they are all stamped on top, and bore and pistons and rings all got checked out at the other machine shop after it acted up the 1st time

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Re: Rebuilt engine knock...

Postby CraigKennedy » Sun Oct 11, 2015 8:52 pm

Guys, I've been looking around on the website and its a long shot, but do you think its possible that carbon on the pistons are causing a misfiring? that is causing the knocking after it warms up?
Everything was clean when it was put back together, I didn't see any excessive carbon on the plugs.
any chance at all this could be my problem?
I'm running champion D21 plugs btw

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Glen
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Re: Rebuilt engine knock...

Postby Glen » Sun Oct 11, 2015 10:04 pm

Hi,
I wouldn't think the engine has carbon just after being rebuilt, you haven't hardly run it much, I think.
You should set the timing with a timing light, and be sure it is right.

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Re: Rebuilt engine knock...

Postby SouthernOutlaw » Mon Oct 12, 2015 11:09 am

Glen wrote:Hi,
I wouldn't think the engine has carbon just after being rebuilt, you haven't hardly run it much, I think.
You should set the timing with a timing light, and be sure it is right.

Ruling out every thing else I agree with Glen. To be honest after reading this thread the timing is really the only thing else I can think of right now. If the rod, crank bearings are good. Clean bore on cylinders. Correct piston, and ring size. That is where I would look next. I guess there is a small possibility that it could be in the camshaft bearings. Is there any way you can take a piece of 1/2" or 3/4 " PVC pipe, and hold one end next to you ear. Move the other around the engine while it is running to get a more exact location of where the knock is coming from. The knock will get louder the closer you get to it.

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Re: Rebuilt engine knock...

Postby Eugene » Mon Oct 12, 2015 1:35 pm

Eugene wrote:Ignition timing could cause a ping/knock. Timing could cause a knock if one cylinder were firing before TDC compression stroke.

Ignition timing is easy to check.

Try pulling one spark plug wire at a time when engine is knocking. If isolated to one cylinder, change the spark plug.

Could be a hot, red hot, protrusion igniting the fuel air mixture before TDC.

I previously read and understand that you had the engine mechanically checked by two different firms. If the problem is not ignition related, most likely it's a mechanical problem.
I have an excuse. CRS.

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Re: Rebuilt engine knock...

Postby Clark Thompson » Thu Oct 15, 2015 5:10 pm

Here is a idea to consider. I had a cub crankshaft turned at the machine shop. They had to pull the crank timing gear about 1/4 inch to the front to make clearance for the machining. When they pulled the gear forward they pulled so hard that it bent a tooth on the gear. . I pressed the gear back into place not knowing the tooth was bent and continued assembly. while test running the engine it had a knock that sounded like a hammer hitting the front of the engine. when I finally isolated the knock I found the bent tooth. replaced the gear and noise was gone. Just a thought!
Collector of Farmall cubs and cub cadets.Injoy helping people keep their cubs running. Years of experipnce.


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