The Stovebolt Site Forums The Shop Area Engine Shop Possible rod knock

Here is my situation:
61 apache 10 with 235.
Knocking sound if revved up.
Dont use oil or smoke. Oil pressure good, but drops at idle when engine is warm.
Knock disappears if spark wire for no 5 is removed. So problem is with no.5. Actually sounds very good without no.5!
Could this be a rod knock or a wrist pin problem?
Can i remove that piston no 5 from underneath after oil pan is removed? Can I replace just piston no.5?

Thanks for your ideas.

sound's like a rod bearing !

Hy 61lorrie, I not positive, but I don't think you can remove any pistons from the bottom. Yes you can replace just one piston, if you get the correct sized one. If you're going to install a different piston and new rings, you should cut the ridge off of the top of the cylinder, so the sharp edge of the new ring doesn't cut into the ridge, possibly breaking the ring or one of the ring lands on the piston, hope that helps.

Yes, it sounds like you have a rod knock. I don't think you can remove pistons from the bottom- unless the crankshaft is out of the way. And 'yes' to cutting the ridge from the cylinder to remove the piston, as 3B says.

If indeed you have a rod knock, the crankshaft is likely to be damaged, so you need to check that crankpin carefully with a micrometer.

Harvester

If the rod knocking is not really loud...say like an echo...it's nothing to worry about. 235's are notorious for this. An old timer I once knew who was a retired Chevy mechanic told me they just called it a "Chevy-knock" and they had quit trying to fix them. Your mileage may vary....Jim

I will disagree with ChetMan on that issue. It is true that the older Chev engines with babbitted rod bearings had a characteristic 'rod knock' that was often tolerated for a time, but even those could be tightened up to stop the knock. Those engines were just more tolerant of loosness in the rods- because of bearing thickness. With the insert bearings in the later engines, you don't have that tolerance. Tappet noise in the engines with solid lifters was often ignored- up to a point.

Harvester

I had a hi miler 216 with the same situation. I was able to pull thte rod and piston out from the bottom, I did have to cut the ridge to get the piston out. When you do put the piston in upside down a .002 feeler between sidewall of piston and cylinder should just slip a .003 will bind. When you pull the rod cap it should slide off easily if it is hard to get off, the hammering probably made it out of round on the big end. Need to check the wrist pin and wrist pin bushings for wear as the noise could be piston slap, rod bearing, most likely, and wrist pin noise.

P gossett

A knocking rod will give a loud metallic rattle when you give it a quick shot of gas and let off the throttle. It is loudest as the load drops off on the rod. If you have this it will not stay that way for long. The knock is the sound of a metal bearing being hammered.

I can't imagine taking a piston out from the bottom with a crankshaft in the way.

Thanks guys!
There seems to be concensus that I will have to remove the engine. I dont have the money for a complete overhaul so will have to look at just replacing this one piston and rod. But, will then have to remove the crank as well to have that checked. Jeeepers this is getting to be a daunting exercise! But I love driving the truck and the knock is hurting me each time. So it will have to be fixed.

There is usually no need to replace a piston because of a connecting rod knock, but you might need to grind the associated crankshaft journal. If the remainder of the engine is in good shape, repairing the rod/crankshaft will be sufficient for quite a few miles.

Harvester

well i would not pull the whole engine. while you cant pull the rod from the bottom and you sure could not get it back in even if you could. you have the option to pull the head and get it out from the top(best choice) or just replace the rod bearing from the bottom
you can plastigauge it it may work. philip

If you pull it apart in place then you probably can clean up the journal with some very fine emery cloth. I would try and buy a .001 and .002 undersize bearing and see which one fits the best. You may want to look at all the rods before fixing one. They may be fine or they may all be on the way out.

The crank isn't in the way, you pull the cap nuts at bdc then rotate to tdc and push the studs up through the cap rotate back to bdc and keep pushing up. The problem is the mess and drips. Working on your back is misery but if you are on a lift, this is just a modified head r and r. The fact that it is cylinder five is helpful as the oil pump and distributor are not in the surgical field. With this method you can diagnose and possibly resolve it for the least amount of labor and cost.

The noise you're describing sounds like a collapsed piston skirt. A loose rod will sometimes turn into a double knocking noise when the ignition is disabled, while a piston slap will almost disappear. You can drop the piston down far enough to change the rod from the bottom, but you won't be able to remove the piston that way.

Drop the oil pan, pull the head, and push the noisy piston out. If there's enough ring ridge to hang a fingernail at the top of the cylinder, ream it out before you try to remove the piston. It might be a very good idea to check the rod bearing clearance before going to the trouble of pulling the head and the piston. Anything over about 3 thousandths is too much clearance on the rod bearing.
Jerry