425 Nailhead big clunk when cranking

[64Rivbarnfind]

Need your help guys. My 425 was running great but I wanted to adjust the throttle linkage. So I did that but had to work the throttle several times. Then it was flooded and ran rough and backfired some. It runs now but has loud clunk or rod noise. I did compression test and all cylinders are 150+-5 psi. I dropped the oil pan (big job) and it still makes noise while cranking even with oil pan off. Seems to come from rear of engine. All rods are connected and do not look bent. I think I see some metal shavings near rear bearing. Could I have spun this bearing? If not that, any other ideas?
Thanks,
Jeff
64Rivbarnfind

[Dr. Frankenbuick]

Check the converter bolts


Sent from my iPhone using Tapatalk

[64Rivbarnfind]

The oil smells like gas.

[telriv]

After checking the converter bolts it's very possible if you did some NO LOAD revving a lifter pumped up & a valve hit a piston which more than likely caused a cracked piston. More than likely #8 piston. The ONLY way to tell is to remove the rear #8 & if you find nothing there remove #7. Of course if a bearing is bad/spun you will find that when you drop the cap.


Tom T.

[64Rivbarnfind]

OK Thanks Tom. But can a piston be cracked and still have good compression?

Jeff

After checking the converter bolts it's very possible if you did some NO LOAD revving a lifter pumped up & a valve hit a piston which more than likely caused a cracked piston. More than likely #8 piston. The ONLY way to tell is to remove the rear #8 & if you find nothing there remove #7. Of course if a bearing is bad/spun you will find that when you drop the cap.


Tom T.

[telriv]

Yes, the crack is usually BELOW the rings at the wrist pin area.


Tom T.

[64Rivbarnfind]

OK Tom thanks. I noticed that the Cyl 8 rod has more play in it than others. But have not taken anything off yet. I took a stethoscope thing used to find motor noises and found that the clunk was hugely loud on the starter. Then I removed all spark plugs and no clunk at all. If I put any spark plug back in, does not matter which one, the clunk is back. SO maybe I have a problem with the starter and some other issue like what you suggest. Have you seen a starter that makes a loud clunk but seems to spin the motor just fine?

[telriv]

Not really. Play in #8 rod. Remove the rod cap.

[64Rivbarnfind]

OK
I took rod cap off. Meant to do #8 but did #7 instead. Bearing was scored and so was crank journal. Not sure how bad since I dont do this very often. But I assume this means I should pull the motor, remove heads and fix those issues, check for cracked piston, and look at the other cylinders. Looking like a total teardown. Am I on the right track?
Thanks
Jeff

Not really. Play in #8 rod. Remove the rod cap.

[telriv]

Unfortunately, it sounds like yes.