Replacement 305 for a 1961

[David R Leifheit]

Originally Posted by bobdylan

I would think that a knock that went away after it warmed up, could not be to serious. If it was a rod bearing I think it would get worse. How is your oil pressure? You will see a drop in pressure if you have a bad bearing. Have you checked compression? I would not pull to investigate until I knew what was going on.

I wouldn't take the chance (now).
I had a small knock. Didn't sound too bad so I let it go. Besides, I had been told the engine had been "rebuilt" fairly recently. The knock got a little louder over time, but never had a power issue and didn't seem to affect the vehicle at all. One day, at 55mph, the knock quit. So did the engine. The rod cap had finally fallen off and the connecting rod no longer was "U" shaped where it went around the crankshaft, it was almost a "T" shape. And the valves/inside of the head was embossed on the top of the piston. Never did get that one out of the engine before scrapping it (wish I hadn't, the block was still good).
A small tapping can be -anything- so if you have the opportunity to go through the engine to find it, do so. Even if that noise is the sole "excuse" for getting another one and learning about these engines.

[Jmclendon]

Originally Posted by bobdylan

I would think that a knock that went away after it warmed up, could not be to serious. If it was a rod bearing I think it would get worse. How is your oil pressure? You will see a drop in pressure if you have a bad bearing. Have you checked compression? I would not pull to investigate until I knew what was going on.

Once it warms up the only thing you hear is the valves happily tapping away. Oil pressure looks fine. It holds nice and steady.

I haven't done a compression test on it yet. I guess in part because other than the pre-warm up knocking it runs great. Over Christmas I was hauling about 1000 lbs (if you include my wife and the fat driver LOL) down the highway with no dramas at all. I did have to drop down to second once (I'm running a 3 spd manual), but that was on a rather steep incline off a scenic route.

I guess it's just one of those things where I know the sound can't be good news. So, I figure if I can find a quality replacement engine then the old man gets to stay on the road while I work out what's going on. If I can't find one, then I'll likely have to take it to someone for a rebuild, simply because I don't want him to sit around for ages while I learn what I'm doing and track down parts etc....

I really want to learn as much as I can, but I don't want to let the old boy fo fallow while I'm doing it. I'm greedy I guess LOL

[6066gmcguy]

As for a bolt in swap, any year 305, 351 or 401 would drop in place of the 305.
Finding one downunder mite be a bite harder then here in the states.

[Jmclendon]

My reason for concern.... Might have to turn the sound up. Sorry about the file size.



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[turbobill]

Maybe the "knock" is a valve a bit looser than the rest. Once the engine warms up, the clearance is reduced enough that it clatters no louder than any of the others.

[bobdylan]

Take a long rod , hold it to your ear, and against the engine in different places., see if it is the upper end or lower end.

[TJ's GMC]

Originally Posted by bobdylan

Take a long rod , hold it to your ear, and against the engine in different places., see if it is the upper end or lower end.

x2 to that notion

[snazzypig]

I agree that it could be a loose valve. The erratic tapping seems to point to something in the valve train.
Hopefully your stethoscope stick will find the source!

[bluestreak1854]

I have a 305e v6 that i would sell-- it would probably need work-- I completely rebuilt it several years ago -bored new 4 ring pistons- cam lifters etc. but it has been a few miles since then-- I took it out because it just quit one day and I haven't taken it apart to see what happened-- suspect timing chain or blown head gasket although it never got hot-- i also have a 478M out of an 8500 coe truck that has a allison automatic behind it -- it is a good running engine and tranny with only 8696 miles on it-- i have the radiator,shifter , and mounting if you are interested