The Stovebolt Site Forums The Shop Area Engine Shop 235 engine knock

My 235 has developed and engine knock...The truck sat for approx. 6 months outside and inside then when started it sounds like a rod is coming through the side. Good oil pressure...no change when I pull plug wires...I have removed the water pump belt and no change. Checked for oil to the rocker shafts. I can not feel and exhaust leak. Help...what else can I look at prior to pulling the pan.

bent push rod

Thanks...I have not checked the push rods or lifters...would valve trouble show in a compression test?

A bent push rod could still be doing a fair job of opening the valve while rapping the side cover. A mechanic's stethoscope is handy for finding knocks, as does a long screw driver with the handle against the ear. Broom handle works too. (Take the broom part off, as the bristles will tickle your ears.)
Carl

thanks Carl...will inspect push rods and lifters and keep all informed..

Still looking...push rods all appear straight...only removed 3 of the 12 and related lifters. My opinion this is not my problem. I am really not looking forward to pulling the pan so any other possibilities (places to look) will make me more comfortable if removing the pan the only solution. The engine is very clean inside and it has good oil pressure...1956 engine..51 truck.

'56 235 in a '51 truck. Sounds like mine!

A great old truck. I love the 5 window cab. I am working hard to stop this engine knock....all signs point at a rod...so Iam going to drop the pan and take a look this weekend. Provided the crank is ok I hope to replace the bearing or bearings and get it back on the road...

By the time a knock louder than a rattle develops, the crankshaft is usually trashed. Be sure and measure all the rod journals- - - -more than a thousandth or so out of round or score marks you can hang a fingernail into means the crank needs to be reground or replaced. Try rolling the engine over by hand with all the spark plugs removed- - - -you might have a piece of debris on top of a piston that's hammering against the cylinder head. One knocking engine I worked on had swallowed a screw from the carburetor air horn and it was imbedded in the top of a piston.
Jerry

Another thing to consider, if you find a bad rod bearing, there's a very good chance the oil hole to that bearing is plugged. If it were oiling it probably wouldn't be knocking. And it doesn't take long to ruin a journal on a crank when no oil is there.

If you find a bad bearing, pull the engine and tear it down.

The crank will need repaired and cleaned. And the metal from the rod failure is likely pumped through out the system. So be prepared to clean the block oil passages as well.

Thanks Roy & Hotrod....good advice. I will keep all informed after I look at the crank.....I did drain and filter the oil today and found nothing in the removed oil.

good oil pressure "generally" means good rod and main bearings. I would pull head and look for foreign objects and or carbon on top of piston/s. if the piston is hammering an object it will really sound out.

It can still show good oil pressure if a oiling hole to the one rod is stopped up.

But finding nothing in the pan may be a good sign that the knock is something on a piston.

Engine knock update....rod bearing show wear and are all using standard bearings. None are damaged enough to cause a knock. Crank mics 2.311-2.312 and in great shape. the 3rd main back from the engine front is shimmed ...should I be concerned? I have not looked at the mains because my oil pressure is good. I have not removed the head to check the piston tops and valves. I thought I would put the bottom end back together...start it and see what it sounds like ..if it still makes knocking sounds I will remove the head and go from that point.

A while back someone had a knock that turned out to be a dent in the oil pan that the crank would hit.
Joe

Get a hold of a small bore camera and pull the spark plugs. Insert and look at the tops of the pistons and cylinder walls. Had a nut inbed itself in the top of a piston in a 305 V6 and it sounded like a rod knock.

Engine knock update...removed the head and it looks really good so I removed the pistons and found two cylinders with heavy pitting. The knock seemed to be near this area. Will this cause a knock? I also found one different piston with different skirt. The crank on first pass mics within range. The engine ran good and did not burn oil...only a recent knock...

Yes. If there is enough clearance between the cylinder wall and the piston it can make a knocking sound.

If it has one piston different to the other it sounds like some one tried to patch it together.

Pull all the pistons and check for cracked skirts, worn out piston pins, etc. Pitted cylinders will cause oil consumption and compression blow-by, not a knock. If you choose to hone the cylinders and re-ring without boring the cylinders for new pistons, be sure to scrub the cylinder walls thoroughly with hot soapy water and a stiff-bristle brush. I wore out a brand new set of rings in 2500 miles once, because of an inadequate cleanup after honing.
Jerry

All pistons look good with the exception to one being different Definitely replaced for some reason. Several piston walls very pitted and will require the boring bar. How much over can I bore safely and what can I expect to have to pay for engine cleaning, boring, new cam bearings and crank evaluation. I also looking for recommendations on pistons ,rings and new cam shaft with lifters.

A proper engine rebuild (done by you, but machine work done at the machine shop) runs about 2k in this part of the country. I have heard prices much higher. You CAN go to 80 thousands over, but 60 is really as far as I would take it. For the 2k, the machine shop will take care of surfacing the head, changing out the valve guides, valves, etc, clean, bore the block, install the cam bearings, and provide you with a gasket set, cam, lifters, push rods if needed, etc.

I am doing a writeup on this now at:
http://devestechnet.com/Home/Project1959235 in detail. You don't have to be this thorough of course, but you can decide what shortcuts you want to do. I am still waiting on the machine shop, so its not finished yet.

Find yourself a good machine shop locally. Talk to the owner, get to know his ability just by talking to him. You will know if you have come to the right place.

.060"-.080" is considered to be the maximum safe overbore, but these old engines have a lot of iron in them. I've seen race engines go 1/8" (.125") over and last for quite some time, but I wouldn't recommend that for street use. When installing thinwall cylinder sleeves we bore 3/16" oversize (0.1875") without penetrating through to the water passages, and a 1/8" wall sleeve needs a 1/4" overbore. It would be foolish to bore one that big without installing a sleeve to bring it back to standard bore, but the metal is there to allow holes that big to be cut.

If the other cylinders aren't badly tapered, it might be possible to sleeve the pitted cylinders, hone the rest, and re-use the pistons you have now. There would need to be less than .005" of taper wear, however. If you choose to do the full rebore, have the machinist bore the rust-pitted cylinders first, and whatever oversize it takes to clean them up will dictate the oversize the rest of the cylinders need to go. Be sure to replace the cam bearings, and clean out ALL the oil passages during the block cleanup. I use rifle brushes and long gun cleaning rods, after removing all the oil gallery plugs, followed by a thorough pressure wash of the oil galleries.
Jerry