Mystery Knock, Please Help!

[GTRjames30]

So I'll try to make this as short as possible. When my engine is running there is a knocking sounds coming from the engine (I think the back, 6th cylinder area). It's really noticeable when the engine is cold and seems to get a BIT better when its warm. Also, it's alot more defined at lower rpm and harder to hear at higher rpm and the knocking follows the rpm's. So basically I figured piston slap, rod knock, etc. So I started by taking the head off to find nothing wrong with the head except for the #6 spark plug was extremely crooked by previous owner (almost 45 degree angel!) and was barely showing on the inside of the head (figured it was either not getting a spark at all or just a faint spark). Also, the top of the bottem end was in perfect shape, no scuffs or detonation. So now I took the bottom end out to check underneath and to my surprise, the bottom end is perfectly fine. No shot bearings, loose crank, nothing. So I'm extememly confused as to what this knocking sound is and where it's coming from? Has anyone on here experienced the same or similar problem? ANY help would be great!

[Mohsen]

possibly a spun bearing.

rebuild time.

[GTRjames30]

Checked all bearings and they're all strong with no play or anything. Even my mechanic/buddy was confused on this one. Is there a way of checking for a spun bearing other then feeling/moving them around with your hand? Don't want to take apart the block if I don't have to.

[vorse]

_{gunky lifters maybe?? is said engine a 26 going by your user being a gtr}

[SiR_RB]

Did you take mains caps off to visually check the bearings??

[abr33]

And measure them... you wont see 10 thou but will hear it

[slippylotion]

If you have pulled down that far already, you should just strip it and get the crank linished and fit new bearings, get the bores honed and fit new set of rings. Factor in a gasket kit decking the block and get the head skimmed and checked over. Shouldn't cost you a heap if you and your mate can put it back together.

Atleast this way she has been checked out correctly and freshened up for many years more use.

[gotRICE?]

possibly a spun bearing.

rebuild time.

A possible spun being, so rebuild? LOL! Wouldnt you at least try other things first? hehe.

I thought mine had spun a bearing at one stage too, i got crazy yuck sounds when it was cold, turned out to be the idler pulley.

[GTRjames30]

Thanks for the comments/help! Its an rb26 and I've decided to take it somewhat apart and change somethings. I didn't really want to go through the wait and process of machine work, etc. Was just thinking of changing bearings and rod bolts as everything else seems fine. Saves my budget a bit too lol. I'll be checking bearings in the near future and let you know what I find...hopefully something or I will still be very confused lol. If anyone thinks of anything else that could be possibly doing this please let me know, its greatly appreciated.

[NoFigJamHere]

Mate i'd bet you've actually already found it!!! Your first paragraph says it all..... Noise is near cylinder 6? Spark plug cross threaded and more than likely not wound down to the seat on cylinder...... 6!!!!! You would have metal filings all through the engine if it spun a bearing. And if have actually looked at the bearings and they look fine they obviously are ok, when an RB knocks because of bearing trouble u would already have metal bits in ur oil....A gudgeon pin is another possibility, is there excessive or unusual wear on the cylinder bores on exhaust and or intake sides?

[NoFigJamHere]

Also excessive piston to bore clearance (which u are aware is a possibility) can have those symptoms. How many km's/miles have you personally done on the engine? And how do you drive it? Like you stole it? Are pistons STD or Forged?

[GTRjames30]

Mate i'd bet you've actually already found it!!! Your first paragraph says it all..... Noise is near cylinder 6? Spark plug cross threaded and more than likely not wound down to the seat on cylinder...... 6!!!!! You would have metal filings all through the engine if it spun a bearing. And if have actually looked at the bearings and they look fine they obviously are ok, when an RB knocks because of bearing trouble u would already have metal bits in ur oil....A gudgeon pin is another possibility, is there excessive or unusual wear on the cylinder bores on exhaust and or intake sides?

Yeah I thought there would be metal shavings in the oil if it was a spun bearing as well. But there was nothing, oil was clean. It smelt of fuel in the oil but that was probably because it had a leak in the intake gasket and was running rich a long time ago, also if the #6 spark plug wasn't firing then it would saturate the cylinder with fuel too. Well i'm still leaning towards just replacing bearings with new ones for my own sake. Shouldn't be too hard to do a job like that. How would I check the gudgeon pin?

[GTRjames30]

Also excessive piston to bore clearance (which u are aware is a possibility) can have those symptoms. How many km's/miles have you personally done on the engine? And how do you drive it? Like you stole it? Are pistons STD or Forged?

Yeah I was looking for that too but there aren't any signs of wear or problems anywhere. I've personally only put about 6-10K km's on the car since I purchased it (insurance reasons mostly for the lack of driving, canada is horrible for RHD insurance!) and I never really drove it hard, just the occasional gives. STD pistons

[NoFigJamHere]

To check pins you need to remove the piston and conrod assembly from the block. Unless the engine has over heated at some stage from what you've mentioned i'd be surprised if it was pins.. If your going to replace the bearings get ones that give a little bit more clearance, if using OEM Nissan get Bearing grade no. STD 0 (black marking) for mains and Bearing grade No. STD 0 (no color) for big ends (con rods). Thats the thinnest shells Nissan makes for RB26 ;-)

[GTRjames30]

To check pins you need to remove the piston and conrod assembly from the block. Unless the engine has over heated at some stage from what you've mentioned i'd be surprised if it was pins.. If your going to replace the bearings get ones that give a little bit more clearance, if using OEM Nissan get Bearing grade no. STD 0 (black marking) for mains and Bearing grade No. STD 0 (no color) for big ends (con rods). Thats the thinnest shells Nissan makes for RB26 ;-)

Yeah I don't think its pins as well. Checked everything tonight and I have a strong feeling its the con rod bearings. They aren't in horrible shape but theres some play in them. I'm not too sure if I can order the STD 0 bearings since my crank is used and could be a bit worn down so i'm doing a plastigauge test on them to see what size bearings to get. my question is could i accomplish a main bearing install myself or should I leave that to a machine shop?

[NoFigJamHere]

Your crank wont be worn down dude, steel in much stronger than bearing materials. You really need to acurately measure the parts with the best equipment available to you. Plasti-gauge isn't my recommendation.. If your doing this type of job for the first time i'd suggest a machine shop does all the technical things so all you have to do is bolt the ancilleries back on.

[GTRjames30]

Your crank wont be worn down dude, steel in much stronger than bearing materials. You really need to acurately measure the parts with the best equipment available to you. Plasti-gauge isn't my recommendation.. If your doing this type of job for the first time i'd suggest a machine shop does all the technical things so all you have to do is bolt the ancilleries back on.

Yeah thats a good call on the machine shop, I don't want to take any chances. Thanks! I'll be hopefully bringing it to a machine shop asap.