You may be installing new spark plugs into your DB9 as part of a scheduled service or as part of some other service issue. I am swapping mine out as part of solving a Lumpy Idle issue (check out my post on this). Regardless of the reason, getting it done right is important. In this post I will cover preparing the plugs plus how to install them properly.To get to the point where you’re ready to install your new spark plugs you’ll need to have removed your old ones first. This takes a fair bit of effort on a DB9 (understatement), and you can check out my post and video on Removing the Spark Plugs.
I’ve already discussed this thoroughly in my other Post on Parts your need to Change your Coil Packs and Spark Plugs. I updated that article in June 2020 with a second and better option than the original plug, so be sure to check it out.
You won’t need too many tools for this:
- 5/8” Spark Plug Socket (3/8” drive)
- 6” long extension (3/8” drive)
- Two 3” long extensions (3/8” drive)
- Note: You can’t just do it with a 6” extension, you’ll need these two in order to do the work on the rearmost cylinders
- Torque Wrench (3/8” drive) with a range that includes 18 Nm
- Oiling Can
Preparation of the Spark Plugs
I’ve read in a few online posts about people claiming there is no need to know or check the spark plug gap. Poppy Cock! This is ignorance. I believe it’s true that NGK has pre-gapped the Aston Martin branded and boxed plugs to Aston’s specifications, but lots can happen to tweak the gap and you’d never know it (a simple tap of the electrode on the bench will change the gap). If you don’t use official Aston supplied plugs and buy a ‘compatible’ model, you’ll have no idea what the gap is. You need to check the gap!
The Official Aston Martin Workshop Manual section 3.07 lists the ideal gap as 1.2 – 1.3 mm (0.047” to 0.051”). My spark plug gapping tool didn’t have a tooth in this range, so instead I used my feeler gauges and combined an 0.008″, a 0.016″ and finally a 0.025″ to add up to 0.049” (center of the desired range). I then carefully checked the gap on each plug. You want a ‘snug’ feeling easing the tool in and out of the gap. If it won’t fit, open the gap ever so slightly in baby steps. If it’s sloppy loose, a gentle tap on the tip will close it down slightly (again use baby steps). Fortunately all 12 of my plugs were gapped right out of the box.
Before installing the plugs I also put some clean engine oil on the threads so that they will install smoothly. I used my oil can and put a drop on the first few threads and then spread it around (see my video below for the technique on how to do this cleanly on two at once).
With the plugs all gapped and oiled it’s time to get them installed!
The Official Aston Martin Workshop manual section on this describes the process in one step “Install the Spark Plugs”. Succinct. My description below will be just a little more verbose and shares a few tips.
- Insert the Spark Plug into your 5/8” spark plug socket. I didn’t ram it all the way into the rubber portion of the socket, I actually insert as little as possible, just enough so it barely holds on (this is so I can extract the socket later). I attached the 6” extension to act as a handle as well.
- Carefully lower the Spark Plug and socket down into the spark plug well in the valve cover and cylinder head.
- NOTE: Be super careful to NOT bang the tip of the Spark Plug while doing this. If you accidentally to, lift the plug back out and recheck the gap. Better safe than sorry (and it will be a lot of work to get back to the plug later if you find the car isn’t running right after).
- Using just your hand start threading the Spark Plug into the cylinder head. Don’t use a ratchet until you have at least done several full threads by hand. I was able to use hand power to get them threaded most of the way down.
- NOTE: You need to be really-really careful here. The cylinder head is aluminum and the plugs are steel. If you cross thread the plugs as you install them and wreck the threads you will be facing a nightmare of replacing your cylinder head.
Once you have the plug gently bottomed out (but not tightened) it’s time to properly torque the plugs. You could just wing it and make them snug, but honestly that’s not the right way for something this important. The official torque spec is 16 – 20 Nm (12 – 15 ft lbs). I noticed that the box the spark plugs came in had a different torque method, but I ignored it and went with the Official Aston Martin Workshop spec.
- I added a 3” extension to mix as well to get some additional clearance while torqueing them – check out my video below. Set your torque wrench to 18 Nm (14 ft lbs) and do the final tighten on the plug using one smooth pull until it ‘clicks’ to let you know its done. Don’t overtighten the plugs!
- NOTE: Be careful to not ‘cock’ the socket and extension over while tightening. It’s possible to crack the porcelain insulator on the plug and wrecking it. If this happens, remove and replace the plug. If you slip while tightening and aren’t sure if you cracked it I’d suggest removing and inspecting it (better safe than sorry).
- Remove your extensions and plug socket. I’ve had occasions where the extension comes out of the plug socket since the socket has a rubber insert that grips the plug. Usually with a little finagling I can connect the extension again and wiggle the socket off. Worst case remove the plug and start over, this time don’t press the plug so far into the socket (so it’s gripped less tightly).
- Now do the other 11 Spark Plugs!
- Tip: When you get to the two rearmost plugs on each bank you’ll find you don’t have the overhead clearance to lower the spark plug, socket and 6” extension since they are under the dash cowl. This is where the two 3” extensions come in. Insert the plug and socket partway first, then add a 3” extension and lower that part way, then add the second 3” extension. There should just be enough clearance under the cowl to do this and use your torque wrench.
Check out this video to see this process in action: