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Changing Spark Plugs

September 23, 2010 | Troubleshooting & Repair

  By Dan Metz


Over the years of owning my 356, I've followed with great interests the many posts on 356Talk. To say that this font of information has proven to be invaluable is too small a compliment. Anyone who follows the threads there for an ex-tended period of time realizes that certain topics reappear periodically. One of those topics is changing spark plugs on a 356. Because plugs are a maintenance items that simply cannot be avoided, it is a topic worthy of discussion. Many of the posts on plug changing come down to one or two issues: "How do I managed to get my *@#$ hands in there?" and "How do I get the spark plug started when I can't even see the opening in the cylinder head?" Hopefully, this short note will help answer both ques-tions. By way of background, my own car is a 356B-T6 equipped with a Super 90 engine, Weber carburetors and adaptor plates on the intake manifolds. That combination is probably as difficult as things get, working-space-wise. Therefore, most of not all of what I write here will be if anything somewhat easier with cars that are less constricted.

Tools Required

First, organize your toolset. You'll need the following items pictured in Photo #1:


  • 1/4" or 3/8" drive ratchet wrench (3/8" shown in Photo No. 1)
  • spark plug wrench with gripping insert (mine has foam)
  • 3", 6" and 12" extensions
  • universal joint
  • spark plug gapping tool
At some point all of these tools will be required.

Recommended Plugs I recommend Bosch Platinum Plugs - WR7BP. They're usually not available from your local auto parts store, but they are available from most vendors of 356 parts.


Editor's Note:
Other spark plugs recommended by 356 Registry members:
  • Bosch W6BC
Further Note:
The lettering and numbering system on Bosch spark plugs is as follows:
  • W - 14mm thread
  • R - Resistor type
  • 7 - Heat range
  • B - Thread reach .460 extended tip
  • C - Copper center core


Remove the spark plug wires at the spark plug end by gripping them at the connector. DON'T PULL ON THE WIRE ITSELF, or you will be in danger of separating the wire from the connector. You might want to measure the resistance of the plug wires while you have them off. All should be in the range of 1,000O or thereabouts. If you find any that have much more resistance, or a resistance very different from the other three, that plug wire should be replaced. Reconnection in proper firing order is easy as long as you don't take the plug wires completely out. The coil wire resistance will be somewhat larger.

You'll need a different tool set-up for the rear-most plugs (cylinders 2 and 4) than for the front ones (cylinders 1 and 3) and, of course, the front ones will be the most difficult to manage (front and rear mean front and rear of the car, respectively). To get started off on the right foot, let's work on rear-most cylinders 2 and 4 first.

Put your spark plug socket on the end of either the 3" or 6" extension (Photo #2 shows a 6" extension) together with the ratchet. No u-joint will be needed here. When properly seated on the spark plug, the extension will extend outward at about a 10-15 degree angle from horizontal, as shown in the photo. When the spark plug socket is fully on the plug, it will be far enough into the engine that you won't be able to see the socket any more, as Photo #2 shows. Be SURE the socket is seated completely; you don't want to round over the hex on the spark plug. Once fully seated, you can readily remove the rear two plugs easily.

Before you install the new plugs, you should check their gaps. The Porsche Shop Manual recommends a gap of 0.020-0.024" for my engine, so I usually gap at 0.022". Using a somewhat tighter or wider gap changes the effective heat range of the plug, but only very slightly. I use only Bosch WR7BP platinum plugs. I was put onto these plugs from our Group, and got them from Zim's; obviously there are other sources, but you certainly won't be able to find them at Auto Zone, Pep Boys, etc.

Once gapped, put a plug into the foam insert of your spark plug socket with the ex-tension attached to it, and HAND-THREAD the plug into the cylinder head. Be VERY CAREFUL here! The only serious mistake you can make during the whole process is to cross-thread the plug. Take your time and f-e-e-l the threads engaging. You can put a little bit of anti-seizing compound on the threads, as this not only helps lubricate the threads and ease the plug into the plug opening, but also aids in removal at a later date. Finish installing the plugs on cylinders 2 and 4 by tightening. When you tighten them up, go finger tight, then just enough more to compress the gaskets, usually 1/4 to 1/2 turn more on the ratchet. Don't overtighten.

With the rear plugs changed, we now move onto the more difficult ones toward the front of the car. Cylinders 1 and 3 are much more difficult to access than 2 and 4. To begin, put together the plug socket and a 3" or 6" extension and a u-joint, with the u-joint on the end of the extension that has the plug socket. Now you have to be both creative and attentive to your own body. How you reach behind the carb will depend on your own flexibility. I work on both plugs from the right-hand side of the car, probably because I am right-handed. For cylinder 1, I find I can essentially reach straight down-ward and get the plug socket onto the spark plug. For cylinder 3, I find it best to lean across the engine and reach down, using my wrist as a u-joint during the initial locating of the plug. The key issue at this point is to just get the spark plug socket firmly seated on the plug. If you're having difficulty, a small hand-held mirror will put you into the correct area. No matter how you position yourself, though, you are not going to be able to visually see what you are doing; you must work by feel.


Once the socket is seated, attach the 12" extension to the u-joint, and then the ratchet to the 12" extension. Your u-joint will be making a pretty severe angle at this point, and the 12" extension will work best when it is almost vertical. Photo #3 shows the set-up attached to the plug for Cylinder 3. Depending on the quality of your u-joint, you might have to undo the set-up, move the plug wrench one hex (or 60 degrees) on the plug, reattach the 12" ratchet and try again in order to be able to exert torque on the plug. Once you can exert torque on the plug through the ratchet and angled u-joint, you can break it loose. Then, disconnect the 12" extension and ratchet and hand-thread the plug out of the cylinder head. To install the new plug, again place it into the plug socket, hand-thread it until you can't turn it any more, and then tighten after re-attaching the 12" extension and ratchet. It usually takes some fiddling to attach the 12" extension to the u-joint when you can't see the connection, but not much.

A couple more points: if your spark plug socket foam insert has lost its grippiness and won't hold a plug, get a new one. Nothing is more frustrating than having a new spark plug drop out of the socket. If that happens, it will usually (but not always) just fall onto the shop floor. If it doesn't, forget about it, replace it with another plug, and just drive. It will fall out somewhere! Also, it helps to have the newer style extensions (Snap-On's are nice) that have knurling on their shafts. They make turning the plugs in and out by hand lots easier.

The first time you do this procedure, you will probably take an hour or two just getting the hang of what you are doing, how to position yourself, and the like. After you've done it a time or two, though, you will be able to change plugs in less than a half hour.

Good luck! You are now a 356 plug-changing wizard (unless you are fortunate enough to have a Carrera engine - but that's another story altogether!).


Editor's Note:

This is part of our module on how to tune up your 356. Components include:


Profile missing thumb
Nicolai Richter Friis
April 14, 2017 at 6:34 AM
Bravo thx best Nicolai
Aa493c9f9299cdb0d51a6d201e2b655c 2547
Lee Callaghan
December 28, 2017 at 9:06 PM
Great article. The one thing I would add is use a rubber plug attachment so that when you hand turn in each plug, you can make sure its not cross thredding. Plus its easier to hand tighten each plug until it just needs a small nick to tighten. On a C, undo the fuel line retaining screw at the front of the engine to allow the line to be pushed out of the way of number 3- the worst plug to get at. If you do pull a lead put of the socket, it should screw back onto the top of the socket by rethreading it but its much easier if you pull on the socket plug directly and not the leads to remove it. When all 4 plugs are off and leads are out of the way, use the extra soace to remove oil and debris from the tin plate as its much easier to clean. Also, check the fuel line joins are all in good shape as well as using a mirror you can see a lot more of them. When you restart for the first time, its amazing how much less lumpy the engine will be using new plugs. And make sure you run it up to temperature on the first restart as otherwise you risk oiling the plugs. Good luck with changing them. The second time will be much easier!