Need Help Diagnosing Engine Knock


IHC in the early to mid-fifties.

Golden Jubilee
Golden Jubilee

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Post Mon Apr 19, 2021 3:19 pm

Re: Need Help Diagnosing Engine Knock

They look great. You can check the clearance with plastic-gauge, but I think that is not the problem.
If I had to guess I would be looking at piston slap, esp because it goes away when the cyl is unloaded. I'd still check the valve clearance and cam lift with a dial indicator before I pulled the head and popped the piston out. Piston clearance can be checked with the rings removed either with an inside mic on the bore and an outside mic on the piston or with special long feeler gauges for the job. A loose fitting piston pin should be able to be felt by hand with the piston and rod removed, and held in your hand. The fitment of the pin can be checked with an outside mic and a "T gauge" in the piston.
Pistons can b knurled if it is a loose fit in the bore and the clearance isn't so great as to require an over bore.
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Golden Jubilee
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Post Mon Apr 19, 2021 3:27 pm

Re: Need Help Diagnosing Engine Knock

I'm going back out to see if I can feel any slop in the wrist pin from underneath. Not to easy to get a good feel that way, but I really don't want to pull the head if I don't have to. I know the rod will slide easily back and forth on the wrist pin with the cap removed and freed from the crank, so at least it hasn't worn a whole lot. Will see if I can get a good eye on the cam while under there as well. If that all checks out, then next it's going to be getting good measurements on valve train and maybe pulling that lifter. Some piston slap I could live with for a while longer before I'd pull the piston. Will report back. I appreciate the opinions of you folks who clearly know better and more than I do.

Golden Jubilee
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Post Mon Apr 19, 2021 3:58 pm

Re: Need Help Diagnosing Engine Knock

If you can get at it from the bottom, you might be able to slip a feeler gauge between the piston and bore with the piston near the top, measure right and left side not fore and aft. It will not tell the whole tale as you will not be able to measure near the rings, but if the skirt is loose the rings will be also. It is the skirt knocking from side to side as the rod passes thru TDC that is piston slap.
You'll have to take the feeler gauge off the set and try them individually. not enough room to use them as a set
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Golden Jubilee
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Post Mon Apr 19, 2021 4:06 pm

Re: Need Help Diagnosing Engine Knock

I'll give that a try. Will look in the CTS to see what it's supposed to be with a feeler gauge ribbon, and see if I can get anything close with a regular feeler gauge. Here are some pictures of the cam. Again, I claim no expertise, but it doesn't look trashed to me. And I couldn't feel any play up and down on the wrist pin.
P1100177.JPG

P1100178.JPG

P1100187.JPG

P1100188.JPG
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Golden Jubilee
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Post Mon Apr 19, 2021 5:14 pm

Re: Need Help Diagnosing Engine Knock

Even with the feeler gauge off the set, not enough room to get at it and get a meaningful measurement. On new rebuild, 1/2" feeler gauge ribbon, .003" should fit with a 12 lb pull. I couldn't get a gauge in the side of the skirt at all. Not that there might not have been clearance, just not enough room. I was able to slide a .005 gauge at the back, but that doesn't tell much either. Having been at it searching google as Nikkinutshop suggests, the noise I have is different than any video clips I've found of piston slap. This is more of a hard metal hammering. I'd post a clip of it here, but the site won't allow it.

Rusty Driver
Rusty Driver

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Post Mon Apr 19, 2021 9:11 pm

Re: Need Help Diagnosing Engine Knock

I would first measure the crank journal to make sure it is round, then check the clearance with plastigage to see what the clearance actually is. Then move forward accordingly. The bearing doesn't look that great to me but pics can be deceiving.
You will soon find your problem.
Dennis

Golden Jubilee
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Post Tue Apr 20, 2021 9:06 am

Re: Need Help Diagnosing Engine Knock

The bearing in question has 80K miles on it, so will not look new. Little specks of dirt and a scratch or two will not cause a knock. If it was worn into the copper plate or thru the copper then it would be of concern.
An engine doesn't have to be "blueprinted" to run well for a long time.
Chevy survived a long time on dipper tubes and splash oiling.
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Golden Jubilee
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Post Tue Apr 20, 2021 1:38 pm

Re: Need Help Diagnosing Engine Knock

Here's the update. I put #3 cylinder on TDC and took the gap out of the rocker to valve. Turning the engine over with dial indicator on the top of the spring cap, I had .425 valve lift. Did the same with cylinder #6 and had .418. The .007 may be partly attributable to human error in setting the indicator, as there is no good place to hold the magnetic base, so may have had a little angle on one or the other. I took the measurement from the valve on the expectation that because of the rocker arm ratio, any variance in cam lift, lobe wear, etc, would be magnified by the approximately 1.5 to 1 rocker arm ratio. Now I'll see if I can get the pushrod out without removing cylinder head bolts. If anyone sees a problem so far, or advice, etc, I'm hoping for suggestions.
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Golden Jubilee
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Post Tue Apr 20, 2021 2:40 pm

Re: Need Help Diagnosing Engine Knock

And the answer is "no". Backing the adjustment all the way out with #6 on TDC, and trying to slide the rocker arm on the shaft to clear, there just isn't enough room to clear the top of the pushrod. SO, unless there are other good ideas, I may just have an unidentified knock for a while. Taking the rocker shaft loose would require taking cylinder head bolts out or loose, which would then require a head gasket replacement, which requires removing the head. And that will all probably get done this summer. The reason I wanted to find the cause now, is because I plan on taking this pickup on about a 5,000-mile road trip starting May 10. It makes no noise while running at speed or decelerating, and is worst at cold idle. My real concern was with connecting rod bearings and wrist pins, as I would prefer not to have a rod go through the side of the block that far from home. I am not as concerned about that now, thanks to the help of the members here.

Golden Jubilee
Golden Jubilee

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Post Tue Apr 20, 2021 4:35 pm

Re: Need Help Diagnosing Engine Knock

To my knowledge the rocker bolts are not head bolt, they just hold the rocker stands to the head. Regardless if you take the bolts out and don't disturb the head (break the gasket loose) you could re torque and life goes on.
I know the medium six rocker stand bolts don't hold the head on, I assume the small six is the same.
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