gt40 knocking

[g33b0]

oil pressure is staying around 50..... and goes up to 60 when i increase rpm and comes back down to the 50 range....................it idles at around 600 rpm .................what happens to oil pressure if bearing is spun?

[93nautique0710]

oil pressure is staying around 50..... and goes up to 60 when i increase rpm and comes back down to the 50 range....................it idles at around 600 rpm .................what happens to oil pressure if bearing is spun?

the oil pressure will usually drop 50psi is good oil pressure but not saying its not a bad bearing have you tryed killing each cyl by pulling of a spark plug wire seeing if the knock changes im going to pm my number give me a call

[g33b0]

i dont know if this is relevant.....but not all of the plugs shocked me when i pulled em.......only a couple of them...........when i pulled the second wire on the driver side, starting from front going to back of boat....to me the knocking seemed to change and quiet but wasnt completely gone

[steve-o]

Re: gt40 knocking

Oil pressure can be good with a spun bearing as long as you haven't run the engine down the road like what usually happens in cars. In a car the rods will get so hot they fuse to the crank or they spray metal in the crankcase, or in my case (racing) let go of the piston and damage the top end too. Clogs up the oil system.

[Nautiquehunter]

The thing that confuses me is how can you spin a bearing simply by de winterizing . Its my understanding this boat hasn't seen water since last year. It was fine then I still think its a lifter I cant wait to see what the mechanic says.

[g33b0]

i am just hoping i didnt get jewed when i bought it two seasons ago, and something was done to the oil (additive of some sort} to disguise the knocking sound.........

[jjgag60]

Knocking

Another way to check to see what is going on inside your motor is to cut open the oil filter. An engine that is having problems with bearings is going to have small chips of metal inside the filter on the "paper". To cut open the filter use a sharp knife or filter cutter, don't use a hack saw or any other object that is going to put metal chips inside the filter. Start cutting around the outside near the screw on area and gasket. Once you have cut all around it you can seperate the paper filter material from the steel frame. Open up the paper and look for any copper colored flecks. If you see any you could have problems inside your engine with bad bearings. Good luck.

[AirTool]

My bet is a rod bearing or wrist pin...I garon-teee.

I've forgotton what kind of wrist pin bushings 350 chevy uses (if at all). But it sound to me like if they use a bushing...that it is GONE. I've heard the exact same sound in other engines. Rod bearing failures at the crank are usually a soft growl from down low. I do not think it is in the valve train because the engine is running so smoooooth. (At least it sounds like it is....is the engine missing or idling rough?)

The other thing you could do is to carefully grab an oil sample and send it in for analysis. That might tell you if there is either babbit or bronze in the oil. Babbit would be a bearing. Bronze would likely be the wrist pin busing (if that is what chevy uses...need to check if there is a bushing or steel to steel fit. Steel in the oil might indicate a rounded off cam lobe.

[NautiqueJeff]

He's got a GT-40, not a Chevy.

[AirTool]

He's got a GT-40, not a Chevy.

That's right. I didn't look.

I'll see if I can find a guts breakdown to see how the piston/wrist pin/ rod are attached.

I'll still bet it is something associated with a rod until he tells me the engine is rough/missing.

[g33b0]

from just listening to it............i dont think the engine is missing.......it sounds like a strong solid grumble at idle...........when i removed the manifold on the driver side....the gasket appeared to be worn.......first chance i get, i think i will get new riser and exhaust gaskets, even tho yous guys dont think that is my problem........i hope yall are all wrong...lol............going to mechanic first thing in the morning............my nautique guy that i get my advice from hasnt seen my engine personally, but vouches that in his 15 years of gt40 nautique experience that he has never heard of a gt40 with valve, pushrod trouble,,and thinks i have an exhaust leak.........my prob is..........i know how i run and maintenance my new boat, which is immaculate to the T..... ..but i dont know how the last owners maintenanced this glorious beast, therefore i might have a spun bearing..........this is a sad sad day that i have to throw in the towel and take it to someone i dont trust and pay him for advice that yall are giving me....." i have done and will continue to do any and all(if possible} service on my boat.......i love to learn ,........and there aint nothing to it but to do it.........

[steve-o]

Just be sure to follow up because if I were a betting man, I'd throw a hundred down on that noise being made by a loose rod on a crankshaft.

[2gofaster]

With good oil pressure, a knock like that is more than likely a piston pin. The GT40 uses a pressed in pin with no bushing.

[g33b0]

explain more on the piston pin please

[AirTool]

explain more on the piston pin please

The piston pin or wrist pin is the 1" or so diamter pin that connects the piston to the rod. There is a pin that presses through the piston/rod/piston. The rod is sandwiched in between the piston on either side. The pin is about 3" long. It allows the piston to go straight up and down in the cylinder while the rod is cycling back and forth / up and down with the crank.

Find an eploded view and you will see.

Absent of an exhaust leak. You now have two experts (2gofaster and airtool) voting on wrist pins.

If you can get a small 1/4" bar or rod in through each spark plug hole, I can tell you a way to check for extreme slop. I've diagnosed this before on other engines.

You might be able to locate which cylinder using a mechanic's stethoscope. I often use the radio antenna off my truck as a stethoscope.