Engine knock after carb change update 2

morgan in ar · Aug 21, 2019

I bought a mechanic stethoscope and narrowed the knock down to cylinder 7. I tried my bore scope again (it had grease on it last time), and the piston looked like the surface of the moon. It must be something really small in there, the piston has lots of little dimples, doesn't look really bad. I tried to run it with that plug out, hoping the compression would force the foriegn object out of the plug hole, but no luck. I can't see anything in there with the camera, either. The noise seems to keep time with the exhaust valve on that cylinder. I guess the head will come off this evening. I suspect that head will need a valve job now. If that piston is a total loss, is it possible to just replace that one?

Janicholson · Aug 21, 2019

Cratered top is not (usually) a death sentence for the piston. If a ringland is crushed it is not good. A solid dimple at the edge that probably shows up as a slight or more scar on the cylinder wall is an indicator. Jim

Geo-TH,In · Aug 21, 2019

I'm no mechanic, a shade tree at best.
I was told it's not a good idea to pull piston and expect the rings to seat.
Pull the head and look for the problem. Could it be an issue with either a damaged exhaust valve or valve seat.

Do a compression and leak down test before pulling head.

kencombs · Aug 21, 2019

Just a suggestion. When you start the head removal, try taking the exhaust manifold off first. Then take a look in the port at the valve. Long shot, but whatever it is may be stuck under the valve causing it to hit the piston. Again, long shot, but you may be able to loosen it and pull it out with the head on.

If you don't find anything, no loss as the manifold needs to come off anyway.

timcasbolt · Aug 21, 2019

(quoted from post at 11:50:34 08/21/19) I bought a mechanic stethoscope and narrowed the knock down to cylinder 7. I tried my bore scope again (it had grease on it last time), and the piston looked like the surface of the moon. It must be something really small in there, the piston has lots of little dimples, doesn't look really bad. I tried to run it with that plug out, hoping the compression would force the foriegn object out of the plug hole, but no luck. I can't see anything in there with the camera, either. The noise seems to keep time with the exhaust valve on that cylinder. I guess the head will come off this evening. I suspect that head will need a valve job now. If that piston is a total loss, [b:9fa32974dc] is it possible to just replace that one[/b:9fa32974dc]?

Yes it is. The purist will want to weigh the piston and make sure that the new one is the same weight, either by getting lucky or by getting one that is heavy and lightening it up with a die grinder. I had to replace a piston on a 327 once, and was able to find a used one with the rod still attached. New rings and bearings and slapped that baby in there like it was made for it. (it was)

Papaw8 · Aug 21, 2019

I haven't been keeping up with the forum and don't know what you are working on but I have had knocking after changing intake manifold on a v8 car. Was only a piece of carbon. I dribbled water out of a coke bottle down carb while running , no more knock.

miangus · Aug 21, 2019

It almost sounds like a throttle plate screw may have got in the cylinder. I got one out once on a farmall H with a flexible magnet. The kind on a bendable stalk.

David G · Aug 21, 2019

I would not replace a piston for small damage like that.

r.w.b. · Aug 21, 2019

Eventually id bet a piston cracks. But its yours do what you want. I got a 261 chevy block with a hole in the side.a wingnut fell in when carb was off.it started n was knocking.removed it with a flexible magnet after fishing for 3 hours seemed like.6 months later harvedting milo almost home from field it went to hammering .pushed clutch in n POW

Steve@Advance · Aug 21, 2019

Yes, you can just replace one piston.

Don't try to press it off and on the rod, take it to a shop that has the proper set up.

You can get a better call on it once the head is removed.

A couple dangers in running it, the top ring land may be crimped down on the top ring. That will lead to seal failure, and cylinder wall damage.

The skirt may be cracked or collapsed. At best an annoying knock, at worst a rod through the block!

With the piston out the cylinder can be honed to remove any scratches. A few minor ones won't hurt. Deep though are a problem.

Once the head is off, you can check the valve seal bu laying the head on it's side, ex port up, fill the port with diesel or some thin liquid, look for leakage around the valve. Flip it over, check the intake. If leaking you can replace the valve, lap it in. Be sure the seat didn't get dinged. If it is, to the machine shop.

That engine came with a steel shim head gasket. Don't go back with that, use a composition gasket.

ErnieD · Aug 21, 2019

Put a brake line air nozzle in the spark plug hole. With both intake and exhaust valve closed the air will take foreign matter out the plug hole.

ditchwitch · Aug 21, 2019

Thats funny and true .If you dont think water will clean a piston hole you've never seen a blown head gasket

4play · Aug 21, 2019

A rough or pitted locking piston surface can also be caused by pre-ignition. I'm wonder if the piston is oil, and carbon stained the about same, or is it cleaner than the others. Also consider this damage could be related to something that happened before you took possession

Engine knock after carb change update 2

morgan in ar

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Janicholson

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Geo-TH,In

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kencombs

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timcasbolt

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Papaw8

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miangus

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David G

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r.w.b.

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Steve@Advance

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ErnieD

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ditchwitch

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4play

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