Top Dead Center - Air Whoosh

So this is what I need - Piston down and valves closed.

I have a spark plug that blew out of the head and I currently have a remote starter switch hooked up and a fuel line “threaded” into the plug hole. When the large whoosh or rush of air escapes I am assuming that the piston is at the top of the compression stroke (both valves closed) and if I bump it a lil more the piston will be traveling back down with valves closed.


Sounds right.

let me post a video…

Here is the video - I bumped the starter a bit more to send the piston back down after the rush of air.

Am I correct - the rush of air was the piston at the top of compression stroke with valves closed a small bump after that is sending the piston back down with valves still closed?

Yes. The rush of air out of the spark plug hole means that cylinder is at top-dead-center so the valves are closed. So if you continue to rotate the engine after that rush of air the piston will move down the cylinder with the valves closed.


Good, I need to make absolutely certain as I blew a spark plug (threads included) out of the head on the 5.4 Triton. I bought the Time-Sert kit and I am concerned if I don’t have the piston and valves out of the way the reaming tool may contact one or both.

I intend to place a small dowel rod down the spark plug hole (which on the Triton is on the top of the engine pointing down) and slowly turning the engine by hand to watch the piston travel as to further ensure I have the appropriate clearance.

On the power stroke, the valves stay closed until the piston is about 2/3 of the way down, then the exhaust valve starts to open…

Ah? The Power Stroke is a diesel engine. Diesel engines don’t have spark plugs.


The OP sounds like he’s on track. Just remember that the crankhaft rotates twice as fast as the camshaft. So if the timing mark on the crank pulley indicates piston no 1 is at top dead center, it might be that the valves are closed (the end of the compression stroke), or that one or more of the valves are open (the end of the exhaust stroke). If you have an assitant manually turning the crank pulley and if air is rushing out of the hole on the upstroke on the first cycle but not the second, that’s a good indication the first cycle is the compression, and the second is the exhaust. Using the wooden dowel idea is a good way to visualize when the piston is moving up vs down, esp helpful when working on other than cylinder no. 1. Use appropriate caution if you place any fingers near open spark plug holes. The compression force can be powerful, and the intake vacuum can tend to suck your finger into the hole. I’m not sure which is worse, but it is best to keep your valuable fingers clear of that area.

@Tester I think Caddyman meant “power stroke” as in the stroke that comes after the compression stroke and before the exhaust stroke, and not a specific engine.

@SeattleScott, if you’re going to ream the spark plug threads with the head on the engine, load the flutes of the ream with grease so it will hold the shavings. You don’t want any of those to get into the cylinder to damage the rings.

Four-Stroke engines all have an Intake Stroke, Compression Stroke, Power Stroke, Exhaust Stroke…

Sounds like the OP is looking for the Power Stroke to insure the correct repair conditions, piston down, (but not BDC) valves closed.

Good thinking @B-K. Details like that can be the difference between 1 days work and 2 weekends.

Doing job now, updates to come

So the job is complete after about 2.5 hrs. I must say that the Time-Sert is well built and the instructions (along with the YouTube video from them and another mechanic who documented the process on a 4.6) are fairly thorough.

I DO have to add unfortunately that the Time-Sert Kit I purchased was slightly defective which added about an hour to the job and was a bit stressful at the beginning. The kit I purchased (new) had a burr in the side of the Counterbore. Once it was placed into the head it would not spin freely and eventually came loose from the supplied wrench. Let me tell you that 20 mins into the job I was not feeling that confident as I now had a steel cutting head inside an aluminum cylinder head that would not budge. Needless to say after ample amounts of lubricant, busted knuckles, and blood we finally got the cutting head out and instantly realized the problem. 60 seconds of filing fixed the issue and it was essentially seamless from that point forward.

If I would critique even further I would add that it is imperative to ensure you have piston and valve clearance and here is a good tip (which ironically started this thread).

  1. 5/16 fuel line that has been crudely tapered at the end with a utility knife.
  2. Twist fuel line into the damaged spark plug hole.
  3. Purchase a remote starter switch. Locate starter relay on pass side of firewall and connect to right side (small) terminal with ignition lead detached and the large lower post on the starter relay. (I will include pictures later)
  4. Additionally purchase a 7/16" wooden dowel rod from the hardware store (cut to approx 16" in length)
  5. Bump the starter until an unmistakably sudden rush of air from the cylinder. (out the fuel line you just threaded into the blown spark plug hole)
  6. Bump starter (literally half seconds) after that whoosh of air has ceased.
  7. Place dowel rod down the chamber via effected spark plug hole and note position of piston by visually inspecting how far dowel protrudes into cylinder.
  8. Remove dowel and slightly bump starter a few more times and reevaluate position of piston by previously described process of dowel insertion.
  9. Once you feel confident that dowel is nearing BDC (Bottom Dead Center) of this cylinder you are more than far enough away from Time-Sert tools involved in operation.
  10. I would guess you need about 3 inches (that is honestly a rough guess but I felt comfortable at about that point)
  11. At this point valves should be closed and piston should be down far enough to complete procedure.

I would additionally recommend 2 other things -

  1. Grease - We performed all processes (cutting, reaming, tapping) with plenty of grease. Not so much that you are packing the cylinder but enough you can be sure when the cutting, reaming, or tapping has completed its process. We would cut with the counter bore, clean off metal shaving filled grease, re-grease tool, and re-do the process until it was fairly clean.

Same with reaming process and I will add there was a definite point when the reamer “fell” through to the seat of the plug hole. I would bet the reaming tool dropped an inch all of a sudden once it hit the seat and we knew we were good.

Same with the tap - I tapped the hole until I felt little resistance and tool came out clean with grease.

  1. Borescope - I purchased one from Harbor Freight Cen-Tech Model 67979 and was very impressed. No - it will not go through the plug hole into cylinder but you can easily see your progress and evaluate your cuts with the camera.

At start-up the truck ran fairly rough for a few seconds and kicked the Check Engine Light (I am assuming this had to do with residual grease and fluids we could not get out with shop vac) but after running for a few minutes and disconnecting battery for 3 mins to reset Check Engine Light it ran like a champ.

I hope this helps!

PLEASE - contact me with questions (or critiques) if you choose to perform this fairly simple operation.


Thanks for taking the time to post the follow-up @SeattleScott. Glad to hear things worked out well for you. Sorry you had the negative experience of a defective part. There are numerous reports here of parts purchased at auto-parts retailers being defective right out of the box. So you are not alone. I got a defective starter motor last year, and had to take it out and return it, after enduring the time consuming process installing it first. And it isn’t just cheap off-name brands, it is even well known name-branded parts. A lot of this stuff, no matter what brand name is attached to the box, is made off-shore now and the quality control appears that it isn’t what it used to be. Good idea to use the Borescope it sounds like. Congrats on getting her back on the road!

Great news from Seattle. Paying attention to details is often the difference in success and failure and your success speaks volumes, @Scott. And it might seem a fairly simple operation now but that is the reward for a job well done.