Tapping in the engine

Discussion in 'The Bench' started by Paul A, Aug 15, 2019.

  1. Paul A

    Paul A Active Member

    Recently I posted a thread asking for help in installing an HEI in my 72 Buick GS 455 and received a lot of assistance from forum members that led to successfully accomplishing the mission. My car is an original 455 by SN but not a Stage 1. However the prior owner rebuild the engine to Stage 1 specs - so i was told. But all I can really see are the roller rockers installed in what appears to be the stock heads.

    After getting the HEI to work and the timing set. I decided to install new spark plugs and cut my spark plug wires to a better length since they were all over the engine compartment despite being what was called custom cut wires. When I selected the plugs, I read several posts on this forum and settled on an NKG plug whose PN I cannot recall because my car is in the shop and I don't have access to the plugs. There were 2-3 members that concurred with the use of these plugs or the similar plug that was one notch colder or hotter, I cannot recall which.

    So I bought the NKG plus, installed them and cut the SP wires to length and started the car. Engine started fine but soon began to make a tapping noise and I had no clue as to what was wrong. I checked the wire connections - all looked good and pulled a couple SPs and they looked fine. I finally gave up and took the car to the shop. Ends up the tapping noise was the pistons intermittently hitting four of the eight plugs - obviously the ones I did not look at.

    My shop did a compression check on all 8 cylinders to check for piston damage and all were good. The four damaged plugs were only damaged on the very tip of the plug. The ground electrode was either touching or almost touching the center electrode thus reducing the gap to nothing or almost nothing.

    What did I do wrong and what is the probability of the pistons being damaged even though the compression is good?
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    That makes no sense to me. I don't see how a piston can hit the spark plug unless you installed a plug with the wrong reach. It would have to be off by quite a bit. I also don't understand why only 4 were damaged, why not the other 4? What was the part number? Call the shop and ask.
     
    Last edited: Aug 15, 2019
    john.schaefer77 likes this.
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    The only thing I can think of is maybe you installed plugs meant for aluminum heads? They have .750" reach as opposed to the correct plugs for iron heads at .460".
     
  4. greatscat

    greatscat Well-Known Member

    Even the AC LTS plugs don't hit and they extend into the combustion chambers, something else is amiss.
     
    sailbrd likes this.
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Gary,

    The reach on the LTS plugs is .715" The NGK FR-5 reach is .750" I can't think of anything else. The FR-5 is a projected nose plug. Maybe between that and the extra reach? The OP said he got the part number from a post here. That's my best guess.
     
  6. greatscat

    greatscat Well-Known Member

    Hi Larry,Maybe that extra .035" causes the interference.I don't know if the LTS clearance was that tight.What is his deck?Doesn't sound like its 0 deck, and he probably used a .040 gasket. He didn't say he gapped the plugs, possibly they were too close to begin with and he just installed them out of the box.
     
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    All the UR and FR-5 plugs I have installed in my engines come out of the box gapped at .041, but I always check them anyway. This is very strange. We will need to hear back from Paul.
     
  8. Paul A

    Paul A Active Member

    To All,

    Thank you all for the assistance. I had never heard of pistons hitting the spark plugs. I did call the shop and the plugs that I installed were NGK UR5 plugs. The shop I use has a friend that has been building and racing Buick 455s forever and he gave them the correct plug to use for my engine and all the tests to perform to ensure there was no damage. They did all the tests and installed the correct plugs and it looks like I am good to go. I have no idea what the thickness may be on the head gasket but I am reasonably sure the plug was too long. When I installed the plugs, I gapped them at .045 adding a little to the gap to compensate for the HEI distributor's hotter fire.
     
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Still makes no sense to me whatsoever. The UR-5 does fit all 67-76 BBB and the 350. It is a valid spark plug part number for your engine. The only way for the piston to hit it would be if you had domed pistons like a Nailhead. I have 0 deck and use the FR-5 and never had a problem. The FR-5 and UR-5 are identical except for the reach and plug seat.
     
  10. buick64203

    buick64203 Just plum crazy Staff Member

    is it a possibility that the plugs were installed that way? In other words, maybe the plugs were dropped and smashed the gap to zero. Then taken out of the box and installed without checking the gap? Thats the only thing that would make sense. And the shop just assumed the plugs hit the pistons when they took them out? its the only plausible scenario

    They do come in a box of 4. Maybe the 4 came out of the box gapped at zero?
     
  11. buick64203

    buick64203 Just plum crazy Staff Member

    And if the same type plug was re-installed, wouldn't the problem happen again? The shop didn't change the pistons right?
     
  12. Paul A

    Paul A Active Member

    I purchased the plugs, set the gap at .045 on all 8 and installed them. They weren't dropped or anything. I saw the damaged plugs after the shop removed them and on 2 of the plugs the ground electrode was touching the center electrode and on the other 2 the ground electrodes were almost touching the center electrode. The other four looked normal. When the shop took the old plugs out they did not recall which cylinders had the damaged plugs. The shop spoke to their Buick 455 expert that has built 455 racing engines forever and he gave them the correct plug number to use. The shop did a compression test on each cylinder and said the compression was in the 170 range. The looked at the pistons with a borescope and found no damage. They did say something about the engine being bored .040 over and they said that info came from the pistons apparently inscribed on the top of the piston although I did not know that was done. If that's accurate I guess the engine is a 473 CID. I just picked up the car and its running fine. Thank goodness no damage to the pistons.
     
  13. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    so what were the correct plugs that the shop installed?
     
  14. buick64203

    buick64203 Just plum crazy Staff Member

    X2
     
  15. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Again, this makes no sense to me at all. There is no way for a piston to contact a spark plug unless it is a domed piston at 0 deck. There a lot of guys on V8 that have used the NGK UR-4 and 5. Never have I heard of a piston hitting the plug and closing the gap. They do extend into the combustion chamber deeper, but I just don’t see how contact is possible. I have heard of SC3800 users closing the gap when they up the boost without supporting mods, and detonation frags a piston, and pieces of it fly around inside the cylinder and hit the plug electrode. I’ve heard of incorrect reach plugs causing contact but the UR plugs are correct for this application.
     
  16. buick64203

    buick64203 Just plum crazy Staff Member

    Doesnt the UR5 cross over to an AC 42TS?
     
  17. greatscat

    greatscat Well-Known Member

    Been my position too.
     
  18. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Yes, 42-43, but it is more projected than the AC TS plugs. Even a flat top piston shouldn’t hit unless it is above deck?
     
  19. Paul A

    Paul A Active Member

    I just looked at the receipt and it shows 8 ASP24 plugs. One other note the shop also thought in the beginning there may have been domed pistons but they had a hard time believing anyone would install domed pistons in a recent rebuild. Apparently they are a thing of the past?? They also said when they bore scoped the cylinders the pistons were not domed. The car is running fine now but only time will tell if their was any damage that has yet to rear its ugly head. I think or I am hoping that it was a combination of circumstances. Plugs that were slightly too long, possibly a thin head gasket and maybe I did not torque the spark plugs consistently when I installed them. The whole thing was weird to me. Importantly the car is running fine, for now and I will keep my fingers crossed that the issue is indeed solved. I do appreciate all the comments I received and I was glad to hear that the situation was a mystery to others as well.
     
  20. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

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