Loud tapping noise, ideas?

[Gregs672000]

Greetings all. So as some may remember, I run larger valves and a high lift cam in my U20. I've had the block relieved to allow room for the larger valves and cam (.537" valve lift) during the previous rebuild. Afterwards, the engine had a slight tick sound that I attributed to valve to block contact. I tried a cam with less lift (.478") and honestly do not recall if the sound went away, but the power did and I swapped the high lift cam back in and ran it for a few months/several drives. Most recently it has become louder to the point I thought I might be hearing a loose valve seat. I listened closely with a stethoscope and it sounded loudest at the valves at 2 and 3 cylinders, but it was still diffuser and not totally clear as to location. Whats more, the ticking sound became much louder at 2000 rpm, and I could not hear it below 1800rpm or so... hmmm). So, off came the head, and after much evaluation (head/seats fine, no damaged/bent valves, lapped the seats just fine) I decided I still must have valve to block contact (I had one slight shiny spot and what could have been some marks on the block) so I clearanced it some more, filled the reliefs with clay, assembled the head with the inner springs only, kept the same tight valve lash (8/10 hot), used an old head gasket and bolted the head down (2 head studs used, tight but not torqued) and turned the cam by hand several times. I found NO metal to metal contact and could measure the depth of the indentations in the clay, finding what looks like plenty of clearance. Should be good, right? Not. Same tap. So yesterday I installed the lower lift cam, and it's just as loud!

I believe I have eliminated external sources (water pump, alt, distributor mount, other vibrations), it's not sounding like a timing chain issue or a loose lower chain guide, and the bottom end sounds good (doesn't sound like a rod bearing, noise is clearly louder up top, it has new jackshaft bearings at last rebuild, oil pump fine and checked a few weeks ago when I removed the crank scraper, showing at least 40lbs pressure using a new electric sender).

This is my current theory: When I built the engine last, I noted that one of the wrist pins was binding on the rod, enough that if I was holding the pin between my fingers and swinging the rod back and forth it would bind and stop at times. Pretty sure this was #3 rod. So, I carefully worked the end of the rod with emery cloth until it no longer bound up. It did take some time... but here's what I think is happening now: I think the wrist pin is binding causing the piston (forged Arias) to slap against the cylinder wall. At lower rpms, it's able to compensate, but as rpms increase it can't keep up and starts slapping. Im also wondering if it may have resulted in broken ring(s). I have not done a leak down or recent compression check, but neither of those had revealed the broken upper piston ring in #4 (my fault, improper gap, Total Seal rings) that lead in part to the previous teardown. What is interesting is #3 spark plug is slightly darker than all the others, which makes no sense with EFI and suggests there's an issue there (saw the same thing in #4 when the ring was broken).

So, what I'm asking is whether anyone has experience with something like this? Thoughts about my logic? Anything else I might consider? It's pretty clear I will have to disassemble the engine, but I really really really don't want to miss something and still be scratching my head with a disassembled engine... Thanks!

[Gregs672000]

Greetings all. So as some may remember, I run larger valves and a high lift cam in my U20. I've had the block relieved to allow room for the larger valves and cam (.537" valve lift) during the previous rebuild. Afterwards, the engine had a slight tick sound that I attributed to valve to block contact. I tried a cam with less lift (.478") and honestly do not recall if the sound went away, but the power did and I swapped the high lift cam back in and ran it for a few months/several drives. Most recently it has become louder to the point I thought I might be hearing a loose valve seat. I listened closely with a stethoscope and it sounded loudest at the valves at 2 and 3 cylinders, but it was still diffuse and not totally clear as to location. What's more, the ticking sound became much louder at 2000 rpm, and I could not hear it below 1800rpm or so... hmmm... how can a valve NOT hit at lower rpm?). So, off came the head, and after much evaluation (head/seats fine, no damaged/bent valves, lapped the seats just fine) I decided I still must have valve to block contact (I had one slight shiny spot and what could have been some marks on the block) so I clearanced it some more, filled the reliefs with clay, assembled the head with the inner springs only, kept the same tight valve lash (8/10 hot), used an old head gasket and bolted the head down (2 head studs used, tight but not torqued) and turned the cam by hand several times. I found NO metal to metal contact and could measure the depth of the indentations in the clay, finding what looks like plenty of clearance. Should be good, right? Not. Same tap (Son of a... !.) So yesterday I installed the lower lift cam, and guess what, it's just as loud! Now, it could be that the width of the valve remains an issue, but it sure did not show up in the clay.

As an aside, I believe I have eliminated external sources (water pump, alt, distributor mount, other vibrations), it's not sounding like a timing chain issue or a loose lower chain guide, and the bottom end sounds good (doesn't sound like a rod bearing, noise is clearly louder up top, it has new jackshaft bearings at last rebuild, oil pump fine and checked a few weeks ago when I removed the crank scraper, showing at least 40lbs pressure using a new electric sender).

This is my current theory: When I built the engine last, I noted that one of the wrist pins was binding on the rod, enough that if I was holding the pin between my fingers and swinging the rod back and forth it would bind and stop at times. Pretty sure this was #3 rod. So, I carefully worked the end of the rod with emery cloth until it no longer bound up. It did take some time... but here's what I think is happening now: I think the wrist pin is binding causing the piston (forged Arias) to slap against the cylinder wall. At lower rpms, it's able to compensate, but as rpms increase it can't keep up and starts slapping. Im also wondering if it may have resulted in broken ring(s). I have not done a leak down or recent compression check, but neither of those had revealed the broken upper piston ring in #4 (my fault, improper gap, Total Seal rings) that lead in part to the previous teardown. What is interesting is #3 spark plug is slightly darker than all the others (that's new), which makes no sense with EFI and suggests there's an issue there (saw the same thing in #4 when the ring was broken).

So, what I'm asking is whether anyone has experience with something like this? Thoughts about my logic? Anything else I might consider? It's pretty clear I will have to disassemble the engine, but I really really really don't want to miss something and still be scratching my head with a disassembled engine... Thanks!

[Gregs672000]

Somehow quoted myself while editing... sorry. Second post has a small bit more clarity and fixed typos...

[redroadster]

A good boroscope could help ,something imbedded in the piston ,or like you said a loose valve seat
Let me guess, its got 20w50 oil
When it knocks pull #2 or / then #3 plug wires see what changes , if so theres a play problem in piston or rod to crank ,could be a piston skirt slap
#3 is the hottest cyl , most often the problem cyl
Do a completely cold compression test , for the V seat. id check the valves adjustment with a go nogo Guage
Have the oil analysis done
Sure it's top of cyl ? Not the oil slicer pan Thang?

[Gregs672000]

There's nothing embedded in the pistons... I've had the head off and inspected everything before reinstalling the head and not solving the tap. Before installing the head, I pushed on the pistons and tried to make them move in the bores checking for a rod bearing or piston pin fracture... no movement. No loose valve seats, no sign of seating issues, valves lapped perfectly, all valves checked for bend and have none. I have pictures of all of it.

[JT68]

Seriously doubt it is a valve seat. That would be an atypical failure mode for a valve seat. (if a seat comes loose, it will slip out of the AL and you would normally have a major failure by wedging the valve open and causing piston contact)-or in the very best case you would find a valve that is not returning fully-causing a massive valve lash clearance and often spitting out a lash pad and rocker. Check all your rocker clearances at this point.

Otherwise, seats don't "tick" since they are immobile.

20/50 weight oil is absolutely, certainly not the issue.

With Arias pistons, the "normal" old school 2618 alloy race piston knock will usually occur cold and at slow speeds. Increasing the speed and temp makes the slap go away.

My first suspicion is the head work. Something is probably amiss that got overlooked or it has other issues.

If the valves were/are contacting the block, it would have had a very brief (minutes?) life before a valve was bent and not sealing properly. That would be serious valve abuse, possibly catastrophic and probably fairly obvious. (a witness mark somewhere and a bent valve) I'm guessing not that, but remotely possible especially if the compression is gone on that cylinder.

We recently rebuilt an engine that had a pin failure from the previous rebuild. The pin had a mile of clearance in the piston and the pin bushing failed completely. That created a pronounced and annoying click at 2k. Not a hollow rod knock sound. Not super loud either, just very unhealthy sounding. The sound was most like light piston/head contact. After 3k , you could not hear it because of all the other engine noise. It was not present at idle and the whole thing would have blown up if the use had continued. This is uncommon, but potentially disastrous. Have you been running the engine hard? if so that is a possibility.

The jackshaft would not typically make noises like you describe-that would likely cause a chain noise.

If it isn't the piston pin or a rod bearing, you are back to the cylinder head or it is something else very unusual for a u20.

If it was me(here at the shop), I would put the head on a separate bottom end, run it on a test stand and see if the noise stayed with the head or was cured with a fresh bottom end. Otherwise, you need to take the head back off and decide if you want to pull out the pistons///

[Gregs672000]

Thanks for the analysis JT. I've done a bit more research and am pretty convinced it's a wrist pins especially considering I had noted some issues before that I thought I'd addressed. The typical wrist pin sound starts at 1500-2000 rpm per google and Youtube discussions just like mine, and is harder to hear as rpms go up and the motor gets louder. It would get a little less loud as the engine warmed up and the pistons expanded. I'm hopeful that I've not done damage beyond maybe a broken piston ring... won't know until I open it up, probably next week. I'm also hopeful that I don't need to replace pistons given that they are Arias and custom machined... grrrr. I have a couple spare rods that I can likely use, thought they would probably need a little balancing work to match the others. Pretty sure it's #3 but I'll be checking every one. Agreed on the head and valves... they showed no issues at all and were arrow straight, no funky sealing issues on the seats, had 210lbs of compression and no leak down I could detect a few hundred miles ago. Ah well, one of these days I'll get this all sorted out and stop having to tear my engine apart every few months...

Sounds very much like you described. And of course I run the engine hard sometimes!

[Habitat.pat]

I tried posting this several time & it didn't show up so I am trying again!

Since I am a push rod guy, I can't address this problem from experience but will post the following in case it jogs someones memory & actually helps!

Is the noise at the rate of the cam or crank?

Might it be coming from the area between 3 & 4 on the block in the area of the oil pump?

Are the cam towers tight?

Could it be a cracked or loose cam gear?

Good luck.
Peace, Pat

[Gregs672000]

Thank you for the thoughts! I'm think it's loudest in the center of the engine (2 and 3), but I will check the oil pump. I did have it out a few weeks ago when I removed the crank scraper, and it seemed fine, but I'm going to leave no stone unturned! If it don't find any wrist pin issues I will check into a timing gear, but the sound is not there.

[Gregs672000]

I woke up early and headed into Datsun surgery. After some time and disassembly, I found the problem in #3 as I had suspected. What I didn't expect was to see this... THAT ladies and gentlemen is a fractured wrist pin. I've NEVER seen this before. I had a little binding problem with this pin on the rod that I carefully clearanced when I last built the engine... it swung free and clear and showed no more binding, but clearly I WAS WRONG. And it's cost me... how much is yet to be determined. The piston is damaged in the pin bore. These are older Arias forged pistons, custom machined... not gonna find a replacement for that one. I will be taking it to my machine shop tomorrow to see if it can be bored for an oversize pin... no idea. One side is fine but the other is loose and has some scoring. All the other pistons/pins/bushings are fine, but I will likely have the bushings replaced on all the rods and honed to fit. Just hoping against hope I can save #3 and have it be 100%, otherwise it's new pistons, at least a hone (or a possible bore of the block/engine removal from the car), more gaskets, more teardown etc etc. A single Arias piston can be several hundred, and then it would need machining... so that's not likely even if they could find one, which I really doubt.

So, knock knock, who's there? Broken wrist pin. Broken wrist pin who? Broken wrist pin that's really pissing me off!
Datsun down for a few weeks... wish me luck tomorrow...

[Habitat.pat]

Sorry to see this, Greg. I haven't been in that many engines & have not seen or heard of this failure before. Good luck finding a replacement.

Peace, Pat

[theunz]

Well it’s certainly not a good day although it could have been much worse if you had ran the engine any longer. If you put in a bigger wrist pin then your balance will be off and you will have to do a complete tear down to get it back in balance. It might be possible to install oversized pins in all four pistons and not upset the balance. Of course that all depends on wether or not it can even be done, and of course at what cost. You will already be in it for a full gasket set so it would only be time that you would save. At least you wouldn’t have to take a chance of disturbing the finicky rear main seal. Sorry to see you put in this position.

[redroadster]

20w50 is not the problem , oh yeah
I was the warranty clerk and tech line operator for a UHaul RPS engine remanufactor plant , we had a rash of new piston tops separating from the rest of the piston ,on engines we put out ,15 different trucks mostly out west, the plant head and the piston company man argued back and forth over if there product was to blame .
I was to find out all the voyages of these trucks after they got the engines , some blew coming on the down hill side of the rocky mtn.s , but others were only in Texas , I looked up the bills on the engine installations finding out the tech put in 20w50 oil , that was deemed the culprit by the piston manuf. as he had seen it many times before , the thicker oil ,likely more cold and on decelleration works on
Pulling the top off on the intake stroke
Might be just fine in hot hot Texas summers ,but those mountains get cold Uhaul stopped service centers from using any other oil but 10w40 and with friction modifiers added if you want it 20 w50 has no improvement in operation

[Gregs672000]

I rather doubt a science based analysis would conclude the use of 20/50 weight oil would result in a piston pin failure. I think the piston manufacturer found a convenient scape goat. I've had most of this engine see over 250-300k miles, have owned it for 37 years and always run 20/50 from day 1. I had identified an issue with this bushing and pin at the previous build, and it's my ignorant failure to fix it properly that likely led it to fail, not the weight of the oil, which did a fine job with all the other pins etc. The engine never sees air temps below 60-65 degrees, as it's a summer only car, so flow was not an issue.

With all due respect, the scientific method says that correlation does not prove causation, nor does a sample size of one prove anything. Our country and education system would benefit a lot from understanding and applying the scientific method so they could better analyze the dribble that comes out of many politicians that would never stand up to scrutiny or a court of law... or 60 courts of law...

[Gregs672000]

Mike thanks for the thoughts. I had considered balance earlier when thinking I may need to replace the rod itself and knowing mine had been lightened a little and the replacement would need some attention... had not considered the pin. There are many different pins out there with varying wall thickness and weight etc, so I will take that into consideration as either a single pin or all 4. I'll be seeing the guys at the shop later today and will see what they say.