Rod knock/bent valves question - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

nisileighty · 04-25-2018, 07:38 PM

I’m not sure if this belongs in the small questions thread, but I’m at a dilemma.

I’m fixing a 2006 Altima QR25 which was ran with low oil and spun a bearing. Upon removing the cylinder head to see if it was salvageable, I did a pressure test to test the valves and it lost pressure/valves were leaking. There was no visible bent valves, but the air was definitely coming through.

Now my question, if a rod bearing is spun or he connecting rod it loose, is it possible that it could bend a valve? At first I thought yes, reason being that there’s now extra room for the rod/piston to move up and down while the crank is still in its normal position, but then I realized that the compression of the air would still be pushing the piston downwards away from the valves.

Feel free to say your opinion! I’m interested to see what’s up with this thing.

Kingtal0n · 04-25-2018, 07:48 PM

if you are sure it spun a bearing/has an oil related issue,

why are you looking under the head?

dude it sounds like a mess honestly if a next engine is like 50% of the cost of the car or less just go next

nisileighty · 04-25-2018, 08:00 PM

Quote:

Originally Posted by Kingtal0n

if you are sure it spun a bearing/has an oil related issue,

why are you looking under the head?

dude it sounds like a mess honestly if a next engine is like 50% of the cost of the car or less just go next

Would like to see if the head is still useable to sell or use for another motor

mr.nismo. · 04-26-2018, 03:02 AM

The only reason the valves and Pistons would hit would be if the motor went out of time (chain jump, break) , conrod/wrist pin break within the upper part of the cylinder, or a valve dropped/floated. Either way, you would very clearly see damage to the top of the Piston itself, and the bent valve. A spun bearing is just that, a spun bearing. Get the crank checked for clearance. If you're lucky, it will just need a micropolish. As for the head, the valve seat will need to be regrounded, and subsequent valve replaced. It won't be too bad if you get a decent machine shop priced.

Sent from my SM-G955U using Tapatalk

kyral · 04-26-2018, 10:50 PM

a little extra play in the rods wouldnt cause the pistons to contact the valves

04-25-2018, 07:38 PM	#1
nisileighty Zilvia FREAK! Join Date: Feb 2016 Location: Seattle, WA Posts: 1,218 Trader Rating: (4) Feedback Score: 4 reviews	Rod knock/bent valves question I’m not sure if this belongs in the small questions thread, but I’m at a dilemma. I’m fixing a 2006 Altima QR25 which was ran with low oil and spun a bearing. Upon removing the cylinder head to see if it was salvageable, I did a pressure test to test the valves and it lost pressure/valves were leaking. There was no visible bent valves, but the air was definitely coming through. Now my question, if a rod bearing is spun or he connecting rod it loose, is it possible that it could bend a valve? At first I thought yes, reason being that there’s now extra room for the rod/piston to move up and down while the crank is still in its normal position, but then I realized that the compression of the air would still be pushing the piston downwards away from the valves. Feel free to say your opinion! I’m interested to see what’s up with this thing.

04-26-2018, 10:50 PM	#5
kyral Zilvia Junkie Join Date: Nov 2012 Location: wisconsin Age: 32 Posts: 448 Trader Rating: (7) Rides Gallery: kyral Feedback Score: 7 reviews	a little extra play in the rods wouldnt cause the pistons to contact the valves __________________ The Game you lose haha

04-25-2018, 07:48 PM	#2
Kingtal0n Post Whore! Join Date: Jul 2005 Location: South Florida Age: 41 Posts: 4,828 Trader Rating: (17) Feedback Score: 17 reviews	if you are sure it spun a bearing/has an oil related issue, why are you looking under the head? dude it sounds like a mess honestly if a next engine is like 50% of the cost of the car or less just go next

04-26-2018, 03:02 AM	#4
mr.nismo. Nissanaholic! Join Date: Feb 2012 Location: Oxnard, CA Age: 30 Posts: 1,890 Trader Rating: (31) Feedback Score: 31 reviews	The only reason the valves and Pistons would hit would be if the motor went out of time (chain jump, break) , conrod/wrist pin break within the upper part of the cylinder, or a valve dropped/floated. Either way, you would very clearly see damage to the top of the Piston itself, and the bent valve. A spun bearing is just that, a spun bearing. Get the crank checked for clearance. If you're lucky, it will just need a micropolish. As for the head, the valve seat will need to be regrounded, and subsequent valve replaced. It won't be too bad if you get a decent machine shop priced. Sent from my SM-G955U using Tapatalk