My buddy bought a used 225 to replace the tired one in his wagon. He and another friend fired it up outside of the car and deemed it OK. We got it in the car and found out it had a rod knock! Apparently they didn't listen very closely or couldn't hear it without the muffler hooked up.

We removed the engine and discovered that there was a gouge in the crank on one of the rod journals. It appeared that the engine was recently rebuilt and that someone was not very careful installing the rod & piston. It looked like one of the rod studs hit the crank when the piston was inserted.

So we had the crank turned, got new bearings and reassembled the lower end. All the journals were tested with Plastigauge. All the crank journals have .001" clearance. Five of the rod journals are .001, and one is .002. (Unfortunately, it has been several weeks and I don't remember which rod originally has the knock, and which journal was .002.)

When we re-installed the motor & fired it up, we still had a rod knock (on number 2)! Of course, my buddy does not have a lot of money to spend on this, so we didn't dis-assemble the motor, check everything else & recondition the rods. My guess is that the #2 rod is ovaled, and the Plastigauge showed it within tolerance, but when the engine was fired it up, the bearing was pushed into the ovaled area and opened up the gap.

Anyone got any theories for me?

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Chuck Rivers, Webmaster

Chuck, that would be a reasonable assumption. Usually any time you have a chewed up journal, even if the rod itself looks ok, its a good idea to go ahead and recon it just as a precaution. One indication of anything awry is how easily the rod cap is removed/installed on the rod. The cap should slide on easily over the rod bolts until the cap reaches the knurled portion of the bolts. As a general rule, even stock OEM rods are pretty tough, but when they get a little hot or get banged on in the journal area, they will tend to draw in at the cap and rod mating surface and need a little attention to correct.

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There's no such thing as too much cam....only not enough engine!

Oog. I wish I could think of something wrong with your theory on why it's still knocking, but I can't.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I had a damper split along the keyway once (right after an engine rebuild) and it sounded just like a rod knock. Long shot, but heck your damper.

Lou

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Home of Slant6-powered fun machines since 1988

Good idea. Check all that greebly* external stuff: damper, torque converter bolts, and...gosh, come to think of it the fuel pump's right near #2, isn't it.

*-Thanks, Reed.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yep, torque converter bolts or crank-flexplate bolts also sound very scary when they get loose.

Cheers,

Lou

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Home of Slant6-powered fun machines since 1988

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Casually looking for a Clifford hyperpak intake for cheap.

I forgot to mention that the sound goes away when we pull the wire off #2 plug. Gotta be a rod. Thanks for the replies.

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Chuck Rivers, Webmaster

…or a faulty piston.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yeah, piston slap or big clearance? Bad news, but might *only* require one piston to fix.

Lou

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Home of Slant6-powered fun machines since 1988

Seized wrist pin?

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

Going to pull #2 piston and take a look. I've heard piston slap before and it has a "lighter" sound. I will see if I can have the rod reconditioned and the wrist pin bushing replaced, as well as checking out the piston.

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Chuck Rivers, Webmaster