Strange knock after rebuild - only when hot

[bjones18]

I have a strange problem. I just rebuilt a seven bolt. I broke it in for ~20miles with repeated throttle on/offs on the highway. When arriving at home, I heard a slight knock.

Started up the next day, there was no knock, but after driving a while I got knock and valvetrain noise.

On jackstands: on start up there is no knock and a quiet valve train. When the oil gets hot I get knock (sounds like a rod) and valvetrain noise. When the engine is cold there is no knock. Listening with a short PVC pipe the knock is most apparent from the top of the bellhousing. Pulling plug wires... maybe #4 but not really definitive.

I am thinking an obstruction to oil flow may be the problem. but oil pressure doesn't spike. and why would rod knock go away when cooled down? The oil is super clean... no-shiny's.

If I understand correct, rod knock is exessive clearance in the bearing... why only when hot? The rod, crank, & bearing have similar temps & thermal expansions. Why rod knock & valvetrain noise only when heated up?

 

[wt71gas]

I have a strange problem. I just rebuilt a seven bolt. I broke it in for ~20miles with repeated throttle on/offs on the highway. When arriving at home, I heard a slight knock.

Started up the next day, there was no knock, but after driving a while I got knock and valvetrain noise.

On jackstands: on start up there is no knock and a quiet valve train. When the oil gets hot I get knock (sounds like a rod) and valvetrain noise. When the engine is cold there is no knock. Listening with a short PVC pipe the knock is most apparent from the top of the bellhousing. Pulling plug wires... maybe #4 but not really definitive.

I am thinking an obstruction to oil flow may be the problem. but oil pressure doesn't spike. and why would rod knock go away when cooled down? The oil is super clean... no-shiny's.

If I understand correct, rod knock is exessive clearance in the bearing... why only when hot? The rod, crank, & bearing have similar temps & thermal expansions. Why rod knock & valvetrain noise only when heated up?

When oil is cold it is a lot thicker and makes up some clearance issues. As the oil gets hotter it thins out and doesn'r provide as good of a cushion and that could explain that. Sometimes a thicker oil will help but if you have a fresh motor and you are trying to make some power I don't think you will wanna do that. What oil are you running and what did you break it in with?

 

[turborebel]

I don't think your rod bearings are the problem, but you could split the oil filter to make sure.

 

[Dream On]

Do you have an oil pressure gauge?