Technical Knock knock

Fired up the blown 350 Chevy in my son,s 48 Anglia. We built this engine 3 years ago. It fired right up after priming the oil etc and we ran it for 20mins to break the cam in. Just before we shut it off we heard a knocking noise coming from around no5 or 7 cylinder. We let it cool fired up again and after about 10 mins the knocking was back. It's not coming from the big end its further up, valve train is quiet. We removed the head on that side and took out numbers 5 & 7 pistons. Only thing we saw was a bit of rust near the bottom of the bore on number 5.

 

Last similar knock I had to that ended up being a cracked piston skirt on a TRW forged piston.

 

Hi managed to remeber my password and log in - As dad (junkyardgenius) said above ..


when the motor is cold sounds fine as warms up we got a knocking.. got the old screw driver to the ear .. pinpointed noise to number 5 or 7 … I reset all valves … same again quiet when cold .. once warm a knock … not a really bad knock but a knock .. thought could be a wrist pin knocking … just pulled no 5 and no 7 .. where the noise was … at the bottom of bore number 5 there is some rust .. now this engine we put together 3 year ago had been kept dry and turned over every month … question is could this rust at the bottom of the bore be causing my knock once it all gets warm? Or would you lean towards a wrist pin prob? Pins feel tight in rods when out but they are obviously cold …

Compression is good and Within 10psi on all cylinders ..
the rust is at bottom of the bore below where the rings get to but the skirt of the piston does go this far
it was late by time we got it apart last night - will have a real good look at the pitons and rings later

Thanks guthrie - these are foreged pistons - no obvious marks on piston but will have a real good inspection later or tomorrow

 

I think a tear down is in your guys’ future, letting the engine sit so long and manually spinning it over on old assembly lube, that gets scraped off with every turn, is a recipe for metal to metal contact.

 

It’s tore down as far as head is off and nos 5 and 7 rod and piston assemblies are out ..

 

"Orange" who?

 

if the rust is causing the piston to "grab" at the bottom of the bore, then it sure could cause a knock.

might want to tear it all down and get in there with a bore gauge, and a hone, and then clean it all up and see how everything fits together.

 

i think your line of thinking could be right squirrel - i may put the piston and rod back in and turn over by hand see what happens when the piston gets to bottom of the bore - then like you say clean all up and see what got -

thanks for your thoughts lads

 

If you have it down far enough to inspect 2 pistons, and there is rust on the bottom of the block, I believe I would pull it the rest of the way apart, and at the very least, really inspect it, run a ball hone through all the cylinders, clean everything up before everything gets lubed back up, and put it back together. You are already investing in the gasket set.

 

I’m of no help but I’m curious what it is so I’ll bump it back to the top of the page for more eyes to see it

 

Shouldn’t a piston skirt knock get quieter as it gets hot? I assume they are hearing it at idle as well. Also not where you would hear a piston skirt. I would suspect valve train. Just my two cents.

 

We called it piston slap. I had a SBC 400 I built using all used parts and it had it for a minute or 2..At normal operating temp it was quiet.

 

If the piston skirt is getting "stuck" at the bottom, it could make a noise in the rod bearing or pin.

btdt with a block that was not honed big enough all the way to the bottom of the bores.

 

Was the engine stored with anything in the coolant passages? Normally it's dry. If there was anything in the coolant passages, I'd also consider a spray crack detector for that bore. Might be a good idea in any case.
https://goodson.com/products/goodson-glow

As said, tap each piston to see if the sound is different, too.
I assume the rod bearings looked happy, since that was not mentioned.

 

Going out on a limb just a bit, but while you're checking make sure the piston to cyl head clearance at TDC is sufficient. Usually .040" is a good number, and most will run fine at .035" but tighter than that or any extra clearance and the piston may touch the head in the quench area. Measure the thickness of the used head gasket and how far above the deck the piston is at tdc and subtract.

 

Is the block square, did you check all the cylinders?

 

Thank you for all the thoughts .. block was machined and then all put together couple year ago and had sat around stores with no coolant in the block until recently. Taken on board all thoughts above .. I didn’t get chance to get to it yesterday .. back to it later today .. will report back any findings .. thanks again ..