The Stovebolt Site Forums The Shop Area Engine Shop Small knock at start up

So I continue to find issues with the " rebuilt " 235 I bought. Not only did my rear main seal install not fix the leak I'm having, it now has a quick knock right at start up. What's weird is that it goes away instantly and is not heard again till the next time I start it. The engine is super quiet, runs strong and besides the leak, it's great. I have heard of something called the " Chevy Knock ", is this what I have?????
'59-'62 235

Does it quit the second the oil pressure comes up? Or have you noticed?

What seal did you put in it? Rubber or Rope? How much oil pressure do you have when running?

235s do leak, and an odd knock here or there may just be..An odd knock.

Kurt

If the engine is sealed right there should be any leaks, and if it is built right there should not be any knocks either.

Thanks for the replies. The knock goes instantly away, the very second ( or right before ) the oil pressure comes up. If I shut the engine off, wait a while, then start her up, there is no knock. It has a rubber seal, and per my year engine, I should be ok with that, right? The oil pressure on start up is easily 35, at operating temp it is around 25. That's what is driving me NUTS, besides the oil leak and knock upon start up, it runs and sounds soooooo good........

I have a rebuilt 235 with only a couple of thousand miles or so on it. I occasionally have the odd tapping or light knocking sound. It always goes away and it's usually while the engine is cold. Like yours, mine runs beautifully.

How loud is this knock? Do you have hydraulic lifters, or solid lifters?

some area of the valve train taking a bit longer to get oil after sitting awhile, worry if it doesn't go away .... if a 235 valve train is totally quiet it's got hydraulics or it's too tight - and if a 235 doesn't have some oil drips it's outta oil

Bill

The engine has Hydraulic Lifters. I know of the sound of solid lifters, it definitely is not valve train noise. The knock is loud enough for my next door neighbor ( who's and old school hotrod guy ) raise an eyebrow. He thinks the oil pump is not pressurizing the system fast enough. I told him I replaced the oil pump with a new Melling pump, he saw that my oil pressure was great at both higher rpm's and at idle. He just shrugged and had no answer for that.

I guess the questions will be who assembled the engine and what was the clearance it had with all the bearings,is the crank std,or has it been cut? Was there new cam bearings put in? And then why did you replace the pump? Was there a problem before, did it make the knock before the change of the pump? What oil do you put in it? With a rear main seal leaking maybe rope would be better, I know rubber would be better if it stopped the leak, but rope if done right will last a long time with problems or leaks, not easy to install with the crank in there already but it can be done. I am just trying to understand to help if i can.

Hi. I bought the engine from a guy outside Vegas, he said it was rebuilt from a reputable, local machine shop. I asked for the papers on it, he said he lost them in when moving from one house to another. I bought it anyway, now I'm regretting my decision. So to answer your question, I do not know the history of any of the bearings, whether they were replaced or not, other than the guy I bought it from saying they were. The leak has been ongoing, what I first thought was a rear main seal leak, was most likely the oil pan seal. I can say that now because I see what a REAL rear main seal leak looks like. Because now the oil pan is not leaking and there is a definite leak from between the flywheel and engine. When I first ran the engine ( proper break in procedure ), I noticed that the oil pressure was pretty low, 10 at normal operating temp idle. I was advised to change it, and upon inspecting it after I pulled the pan, it did look old if not original. No, the knock was not there before I changed out the pump, which does give some validation to my neighbors claim of this oil pump not pressurizing fast enough. If that's the case then why such great results oil pressure wise? have bought both a new rubber and rope style seal in preparation of doing battle with the engine this weekend.

Sounds like a wee bit of piston slap......just a clearance issue that is exacerbated with cast pistons.....friction quickly eliminates it most of the time!
I would not worry about it if it does not get louder and last longer!

I personally wouldn't worry about a small knock that is only present until the oil presuure is up. If anything I would observe it to see if it changes. When I worked in a garage years ago, I saw some cars with low mileage that would do that. It was not unusual.

My humble opinion... if the knock is loud enough for your neighbor to hear it... it's too loud. And it won't take long to get louder.

The oil pressure on my old 235 and even my old 261 was higher than what yours is even when both engines had many miles on them.

How long does it take for the oil pressure to come up on the gauge?

It sounds to me like it is related to the oil pressure, so I would look at bearing clearance. Especially main bearings.

Did you put the rubber seal in? Did the main bearing cap get torqued correctly?

Roy Rodgers is correct! if your neighbor can hear it, it IS too loud. There is way too many unknowns,and it depends on what you want to do about them now or later, if you can live with it then do so and see what happens, if you cant live with it and your not ok with the knocking everyday then don't try and put a bandaid on it, piston slap does not go away in seconds,it takes longer until engine gets warmer,and if you want to dive under and change out the rear seal,i would first check all the bearings when you had the pan off, take off one at a time the rod caps and look at the bearings to see any ware on them, like copper showing though at places on the bearings,make sure that you put them back on the way they came off too, they can not be mixed up and have to go on the same way they came off, once you have checked the rods then check the mains out and see what they look like,any copper showing you have problems,if you don't see any copper then go and get some plastagage and check the clearance out,you should have .002-.003 max in my opinion of clearance on the mains and .0015-.002 on the rods.if you have more than this that could be the problem, you can also check the size of the crank when you take out the bearings and look at the back side of the bearings and see what size they are,over sized bearings are normaly stamped what size they are, i also think that you engine should have more oil pressure than what you saying,seems a bit low but will work if you had the right clearances.

+1 on Benny's comments. You're hearing a main bearing knock due to excessive clearance after enough time for the oil to drain away from the bearings- - - -usually overnight. Drop the oil pan and inspect some bearings, both rods and mains since you're in there already. I'd be surprised to find "new" bearings. You'll probably find some of them worn down to the point where the copper backing is showing. If the crankshaft is round and straight (not tapered) install new bearings and check the clearance with PlastiGauge. Low oil pressure with a stock pump usually means worn bearings. The high volume pump just tries to hide the problem by overcoming the leakage past a wide-clearance bearing a little better, that's why we use them on race engines that are built with extreme clearances for low rotating friction. "Lost" paperwork usually means a seller has something to hide!
Jerry