I've got a '94 Disco with the 3.9 that I picked up a few months ago. I've being going through, doing the usual tune-up items; as it's a pre-96, it's timing is set by the distributor vs an ECM. The distributor needed some attention, such as a new vacuum advance and new mechanical advance springs. Even after that, if the timing was set at the correct 6* BTDC, it ran like a dog; it ran quite well at around 12*-14* BTDC, which seemed a bit odd.

Today, I picked up a piston stop to see if the marks on the balancer were just off, as they can slip over time. In the process of finding TDC, I made a big ooops. This piston stop has a brass 1/4" threaded rod; I accidentally set it too long, an since the plug hole is angled and the rod was soft, I didn't notice contact and bent the rod...now I've got a fairly loud knock in the top end. :-(

I can't imagine it's a rod knocking, since there was not excessive tension as I turned the engine...my best guess would be that the piston stop made contact with and bent one of the valves and that's what's ticking/knocking. Any thoughts as I ponder my sad, sad day?

 

Man, that sucks.
What did it do to your piston stop? Did that get bent? Is there maybe a piece of thepiston stop rod in the cylider?
I think your timing problem was due to excessive carbon build up on the valves and maybe a chunk of carbon is in there knocking around.
I hate to say it, but the only way to find out for sure is to pull the head.

 

Heres a thought, remove the valve cover and see if any of the valves have a different movement to them, know what I am trying to say?
If one valve moves further or shorter than the others, or the spring is longer or shorter.
You get what I'm trying to say?

 

The piston stop did bend; it was in a 'L' ish bend, not a 'bowing' bend due to the plug angle and nothing broke off. The leg of the 'L' in the bend was probably 1" or so, so it I'm guessing it reached a valve.

Maybe I'll pull the valve cover off tomorrow since it's going to be rainy. I don't think there's too much in the way to get it off. I'm hoping any issue can be resolved by taking off the heads and it's not a full-rebuild issue...I'd really kick myself if it comes to that. If anything, it'll be an excuse for fresh head gaskets, cleaning out the carbon, and some new valve cover gaskets.

*sigh*

 

I think at worst it is a bent valve.
Look at the positive side, you didnt poke a hole in the piston.

 

How much noise do you think a chunk of carbon (or anything else) would make? This is a fairly loud tick/knock (more of a knock)---

 

I suppose it would depend on the size of the chunk.

 

Just to note and to possibly help in the diagnosis, compression is on par (120 psi warm) with the other cylinders, and I don't notice the noise while cranking.

 

If compression was low then I would guess a bent valve, but it is normal so I would have to say that something is in there.
Maybe do a induction cleaning and see if it breaks it up.

 

Well, after dinner and a short nap, I thought I'd go back out and re-check the compression on the #1 cylinder...125psi and no clack when rolling it over. I thought I'd see if the noise was compression related, so I pulled the compression tester and left the #1 plug out and fired it up; I couldn't hear a clack, but there was also a lot of wind noise coming from the #1 plug hole. Next, I thought I'd see if it was ignition related, so I put the plug back in but didn't connect it...a little rough, but no noise. I hooked it back up, started it up, and it ran a little rough from the excess fuel, but it cleared up and no noise.

As you suggested, there must have been a bit of carbon or the like in the cylinder and running it at idle with the plug out for those few seconds must've expelled the matter. Thanks for all the insight...now I'll be able to sleep good tonight!