| Drbrown | -- 12-18-2013 @ 6:10 PM |
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After starting my '47 V8 (59AB) and before the engine is warmed-up, I hear an engine knocking sound, most pronounced btwn 600 and say 1000 rpm. It is quieter at idle - 500 rpm and seems to blend into other engine noises above 1000 rpm. When the engine is at running temperature I do not hear it. Because of its proxcimity to the cylinder heads I suspect its most likely a worn pistion pin and maybe less so a worn rod bearing. The engine was rebuilt about 33,000 miles ago under another owner. My question: I have seen comments in other posts about disconnecting the spark plug wires one by one to locate the particular cylinder (have not done that yet). What other tests can I do to better identify the location and cause ?
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| supereal | -- 12-18-2013 @ 9:49 PM |
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Without actually hearing the sound, it isn't possible to narrow the cause. If it is a tapping sound, it may be a valve with excess clearance between the stem and the lifter. That tends to lessen as the engine heats up. A bad rod or main bearing is usually a knock, rather than a tap. If it is pronounced, it requires some disassembly to locate before it can cause extensive damage. Often, a loose or damaged bearing is accompanied by low oil pressure. It can also be caused by a fuel pump or other accessory. Some time ago, a customer brought an engine to us that had a bad knock. We found that someone had sleeved one cylinder inproperly, and coolant had entered the oil pan, causing one rod bearing to fail. As you can see, locating the source of an unusual noise can be tricky. This message was edited by supereal on 12-18-13 @ 9:51 PM
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| Drbrown | -- 12-18-2013 @ 11:17 PM |
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I pretty sure it is not valve train noise. Compared to the sound of rapid higher-pitched valve action sounds .... what I'm concerned about is a lower pitched steady and slower "flat" sounding knock that is occurring at a slower rate. I can also hear that typical valve lifter "tapping" sound (more as you say) when they are cold. And as far as it being related to the fuel pump lifter rod, I think that kind of sound would be at a higher tone, like the valve tappets. As soon as the outdoor temp's improve (would love to have a garage) I will do the spark plug disconnect test and get a stethiscope too. Perhaps a compression test might reveal something when combined with other info. I'll keep up my minimal driving of the car unless the concern gets noticably worse. Thanks for your thoughts. Will post more later.
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| TomO | -- 12-19-2013 @ 7:29 AM |
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To isolate the sound, you can use a stethoscope or a long screwdriver held up to your ear. The fuel pump can make a sound that is similar to a bearing knock. A valve guide that is moving due to a broken keeper can make a sound like you describe, but it usually doesn't disappear with temperature. Your problem could also be piston slap as that sound does go away as the parts warm up and the clearances tighten up. Disconnecting the plug wire to the cylinder that is noisy, can make the piston slap less noticeable. Another area to investigate is the timing gear. The metal timing gears can make a knocking sound that will go away when the engine is at operating temperature. Tom
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| supereal | -- 12-19-2013 @ 7:38 AM |
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Let. Us know what you find. Rod noise is usually described as a "hammering" sound that gets louder as engine speed increases. A week ago we had a late model truck come into the shop with a heavy hammering sound. Turned out to be a bolt in the cylinder! The owner set out to replace the intake gasket, but dropped one of the bolts down into the valve port. We saved the block and the head, but had to sleeve the cylinder. A new valve and piston was required. There wasn't much doubt about that noise! That DIY job turned into a very expensive mess.
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| supereal | -- 12-19-2013 @ 7:39 AM |
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Let. Us know what you find. Rod noise is usually described as a "hammering" sound that gets louder as engine speed increases. A week ago we had a late model truck come into the shop with a heavy hammering sound. Turned out to be a bolt in the cylinder! The owner set out to replace the intake gasket, but dropped one of the bolts down into the valve port. We saved the block and the head, but had to sleeve the cylinder. A new valve and piston was required. There wasn't much doubt about that noise! That DIY job turned into a very expensive mess.
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| oldford2 | -- 12-19-2013 @ 2:27 PM |
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I think it is just piston slap. If it diminishes when the engine warms up, don't worry just drive it. John
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| Drbrown | -- 12-20-2013 @ 9:49 PM |
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Oldford2 .... I have an intuition you're right about piston slap (hopeful thinking?). I've only rebuilt a flathead once (60 years ago) and don't remember checking the pistons or pins by measurements. Luckly that rebuild came out very well, at least until a fuel oil tank truck later tried to drive thru the passenger side door. I don't know who rebuilt this engine but I can see how a non-professional could overlook piston & pin conditions. I think it is not a "hammering" sound (i.e. metal against metal) and it does go away when at running temp, so maybe the engine can survive until the next valve job. Thanks all for your input. Will advise.
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| shogun1940 | -- 12-23-2013 @ 7:08 PM |
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Main bearings make noise on start up at low speeds and go away when you speed up. Rod noise peaks on a quick rev up and down at the peak rpm. Pins and pistons double click. Use a heater hose,wood dowl or steth ascope to find the noise location. If that does not work drop the oil pan and do a leak down test by pumping oil in at the pressure sending unit . the loose bearing if it is one will have the most oil coming out of it. Use plasigage to see what the clearance is and then shim it with brass shims.. good luck and Merry Christmass
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