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bing kunzig
07-26-2005, 11:22 AM
I recently purchased a nice '57 Silver Hawk with about 5000 miles on the restoration. After it is warmed up, there is a distinct knocking sound that keeps pace with the RPMs. Doesn't have that lifter type sound. What are the likely causes?

Roscomacaw
07-26-2005, 12:22 PM
There's the off chance it's the fuel pump. There's a chance it's a flex plate going bad (is it an automatic?) - but it's it's a dull knocking sound, it might be a connecting rod bearing OR a broken piston. What's the oil pressure look like at speed? Has it changed of late?[}:)]

Miscreant at large.

1957 Transtar 1/2ton
1960 Larkvertible V8
1958 Provincial wagon
1953 Commander coupe
1957 President 2-dr
1955 President State
1951 Champion Biz cpe
1963 Daytona project FS

bing kunzig
07-26-2005, 04:17 PM
It has a new fuel pump. It's automatic. Oil pressure shows 60 when driving, 40 at idle.

whacker
07-26-2005, 05:53 PM
The symptoms you describe are those of a blown head gasket between the internal cylinders, such as between 1&3 or 3&5 or 2&4 or 4&6. What sounds like a knock is the hot gas going from one cylinder to the adjoining one. Oil pressure will stay OK, as long as you don't breech an oil passage and you won't see any coolant in the oil or oil in the coolant. Get a cheapo stethescope and listen between the cylinders on both heads to determine which side it is on, and which cylinders are affected. All that should be required if you get to it right away is a new head gasket (check for flatness before you reassemble). If you let it go too long, you will be looking at doing both heads and probably planing them both as well.

Roscomacaw
07-26-2005, 06:03 PM
whacker - what you say may be true. But you'd think he'd be telling us that this thing is running rough if that were the case.[}:)]

Bing, can you run a compression check? [?]

Miscreant at large.

1957 Transtar 1/2ton
1960 Larkvertible V8
1958 Provincial wagon
1953 Commander coupe
1957 President 2-dr
1955 President State
1951 Champion Biz cpe
1963 Daytona project FS

bing kunzig
07-26-2005, 06:14 PM
I think Mr Biggs is right in that a blown head gasket usually results in a vehicle running roughly. This thing is VERY smooth. Yes, I can do a compression check. What might the results tell me?

whacker
07-26-2005, 07:26 PM
No, this kind of blown gasket doesn't cause the engine to run rough. It will cause it to burn oil tho. All the gas is still kept within the heads and cylinders. It is usually caused by not re-torqueing the head bolts, or not torqueing them enough, or not getting sealing compound between the leaking cylinders. A compression test will definatly determine if I am right. You will find two cylinders next to each other with very low, but equal compression.

whacker
07-26-2005, 11:30 PM
I've been thinking about this, and the possibility exsists that I could be wrong (hard to believe, I know). Let's look at this logically. Here are the clues we have: engine has a recent (5000 mi) rebuild. Sound (knock) occurs after warm up. Sound keeps pace with engine RPMs. Doesn't sound like typical lifter noise. Oil pressure is good and stays good. Here are the assumptions: engine doesn't seem to loose power. Engine isn't overheating. Gas consumption is normal. Not a noticable vibration.

OK. The sound must be engine related, so we eliminate brakes, U-joints, transmission, other running gear. What happens when the engine warms up? The thermostat opens. The oil, if it is single viscosity, thins out, if it is multi-vis it thickens. The rubber belts stretch.
Just because it dosn't have the typical lifter sound doesn't mean it isn't the lifters - it could still be the lifters.
It could be that the water pump or the fan flexes or rotates off center when the thermostat opens.
It could be that the distributor has worn bearings that allows it to tap when the oil thins.
It could be the generator is loose on the bracket and rattles when the belts expand.

It could also be a blown head gasket between two adjacent cylinders.

bing kunzig
07-27-2005, 10:07 AM
Good enough. I will do the compression check tomorrow and post the results.

65cruiser
07-27-2005, 11:26 AM
My engine mechanical knowledge isn't that great, but why could it not be a rod bearing? I know that sound can go away when under load, but it would increase with rpm--right?[:I]

________________________
Mark Anderson
http://home.alltel.net/anderm
1965 Studebaker Cruiser

oldcarfart
07-27-2005, 04:22 PM
check the torque converter bolts and engine oil level, then drive it until it blows up and the issue will self identify, you guys are too serious, just enjoy! this is a hobby, remember? the damn thing may last forever!