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Old 07-24-2020, 01:32 AM   #1
7_Zero
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Default Help diagnosing engine knock

I realize it is difficult to diagnose through text, but I am hoping to get some ideas of things to check before digging deeper into it.

I recently purchased the car and the engine was supposedly rebuilt maybe 10 or 15 years ago. Based on some pitting in the valves I am wondering if it was more of a refresher than a complete rebuild. The previous two owners did not drive the car much and there are only 550 miles on the odometer, so I am assuming it only has that many miles since the ‘rebuild’ but we know what assuming does.

We drove the car around town for a couple weeks before taking it on a long trip and had no problems. On the trip however it started blowing radiator hoses (it has a pressurized radiator). After installing the third hose we watched for bubbles in the radiator and determined it to be a head gasket allowing combustion gasses to pressurize the cooling system. When we had the head off we decided to go with a 5.5:1 head, but unfortunately after the install the engine now has a knock.

The knock is audible at idle and when the engine is revved. You can hear it with the spark advanced or retarded. With or without the fan belt, clutch engaged or disengaged, and has no difference when disconnecting each of the spark plugs. Reversing the timing pin and putting pressure on the timing gear also yields no change.

Using a stethoscope, the sound is loudest at the front of the engine especially around the timing cover, timing pin and cam plunger area. Not sure if the knock has to do with the timing gear and cam, or if it is in the front main and only echoing through the timing cover? The engine had a two piece pulley that we originally thought was the cause so we replaced the pulley and also replaced the cam plunger and spring, but unfortunately it made no change.

Any suggestions on additional things to check, or is it just time to dig in to the bottom end?

Thank you for your help!
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Old 07-24-2020, 01:44 AM   #2
Kurt in NJ
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Default Re: Help diagnosing engine knock

Perhaps it has a larger bore and pistons hitting edges of head gasket, how did old head gasket compare to old one
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Old 07-24-2020, 05:08 AM   #3
Wick
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Default Re: Help diagnosing engine knock

Witch head gasket did you use?
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Old 07-24-2020, 07:33 AM   #4
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Default Re: Help diagnosing engine knock

Pull pan and check bearings,verify they are ok.
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Old 07-24-2020, 08:03 AM   #5
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Default Re: Help diagnosing engine knock

Did you verify piston pop up and gasket clearance? Pull head and check.
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Old 07-24-2020, 08:05 AM   #6
1931 flamingo
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Default Re: Help diagnosing engine knock

Disconnect the fan belt and try again, use a long screwdriver or broom stick as a stethascope.
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Old 07-24-2020, 08:44 AM   #7
30 Closed Cab PU
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Default Re: Help diagnosing engine knock

Perhaps possible the higher compression head and a new head gasket has increased compression to the point where it is time to check/adjust bearing clearances?
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Old 07-24-2020, 09:20 AM   #8
katy
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Default Re: Help diagnosing engine knock

Quote:
When we had the head off we decided to go with a 5.5:1 head, but unfortunately after the install the engine now has a knock.
It could be a piston hitting the head. The test is to put the head on the block w/out a gasket and turn the engine over by hand to see if any of the pistons hit the head.
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Old 07-24-2020, 10:38 AM   #9
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Default Re: Help diagnosing engine knock

Agree with Nosetime and katy, could be hitting the head. Use Prussian Blue to check popup, gasket and side interferance. Similiar thing happened to a friends car. He ran it and broke the crankshaft. Might also try just laying the head in place without bolting down and see if it moves on rotation.
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Old 07-24-2020, 12:16 PM   #10
Ernie Vitucci
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Default Re: Help diagnosing engine knock

Good Morning...Katy is most likely right. If the original head is nice and flat, put it back on the car with a fresh gasket and see if the knock goes away. Were you able to see that a head gasket failure was the cause of the exhaust in the radiator? If so, then your number one piston is most likely tapping the head. When you remove the 5.5 head you should be able to see the evidence on the piston and on the head. Or maybe on the edge of the gasket, but it is most likely the piston to head contact. Ernie in Arizona
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Old 07-24-2020, 04:06 PM   #11
Dave N.
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Default Re: Help diagnosing engine knock

I can only offer my life lesson for my engine knock, but raise the distributor up from the new head about an 1/16 to 1/8 of an inch and than tighten it up again.
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Old 07-24-2020, 06:24 PM   #12
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Default Re: Help diagnosing engine knock

When I took delivery of my 1928 Tudor from the US, I didn't know the engine had had a "freshen up". The job they did on it was pretty lousy and when I put a 5,5 head on it, I too got a knock from it (sound familiar?). I stripped the engine and found 0.005" over sized gudgeon pins had been installed. This was done by either reaming or honing the bushes to suit, only they went too far and they were loose, causing the knock. A new set of bushes properly installed all is good now. It may be worth your while checking them.
BTW, the bronze they use in those small end bushes is nearly impossible to ream - terrible stuff.
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Old 07-24-2020, 08:12 PM   #13
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Default Re: Help diagnosing engine knock

Back about 50 years back I had a similar knock--constant. Turned out a chunk came off the timing gear. It sounded just like a rod knock but was consant.
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Old 07-24-2020, 08:26 PM   #14
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Default Re: Help diagnosing engine knock

I vote for checking the piston/head clearance. I had a similar knock and traced it down using an accelerometer and an oscilloscope - the knock lined up perfectly with the exhaust stroke of #3. At that point, I realized that the recently skimmed head had gotten one skim too many. The engine turned over freely by hand - the knock only occurred when the piston was flung up while running and took up the slack. Once apart (for the 3rd time) the mark on the head was beeeaaarly visible. A little relief cutting in the head solved the problem.
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Old 07-25-2020, 12:40 AM   #15
TerryH
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Default Re: Help diagnosing engine knock

I also had a faint knocking sound that none of my Model A friends or I could identify. It ended up being a fiber timing gear going bad, which eventually failed. New gear installed and the noise went away.
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Old 07-25-2020, 06:22 AM   #16
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Default Re: Help diagnosing engine knock

The engine in my car was "rebuiltl" back in the 60's but never run. When I started running it recently, it seemed quite noisy to me. At first I suspected an oil pump problem but I removed the plug on the side of the block and there is lots of oil flow. The engine has .125 oversize pistons. Would the gaskets available then have accommodated this oversize? Can I tell if the pistons are hitting the gasket or head without removing the head. The head was probably planed at the time as well. I am going to remove pan to check rods and also check valves for proper clearance. To me it is more of a "clatter" than a knock. All suggestions are appreciated.
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