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PERA’s Core Corner: Trying To Make Sense Out Of Detonation Sensors

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Here it is the New Year and I’m sure many of us have gone through the challenge of making resolutions to lose weight, quit smoking and so on and so on. We’ll look back 3 months from now and try to make sense of how our resolutions got lost in the shuffle.

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This month we are going to do that exact same thing: we’ll try to make sense – or in this case “sensor” – of it all. Our goal is to understand where and when different size detonation sensors apply. We’ll look at a few different engines and find out how in some cases we can cover ourselves so it doesn’t make any difference which sensor was used in which original application.


Let’s start with an old friend: the 4.6L Ford to see if we can find a relationship between the 12mm and 8mm detonation sensor usage. Beyond the obvious size differences, there are two unique differences between the two sensors. The 12mm sensor screws directly into the block, whereas the 8mm sensor mounts over a stud that screws into the block. (see Figure 1). The 8mm sensor has a hole through it that goes over the stud and a retaining nut that holds the sensor down onto the stud. No big thing; anyone can figure that out right? Well, true if there was a clean line as to when the change occurred. Unfortunately, that’s not the case. The best information that I have at this point is that 2000 and later models will use 8mm and that 1998 and earlier models used 12mm. Which leaves us with 1999 – that model year Ford engine seems to go both ways.

Great – now what do you do, ask the customer for each 1999 engine sale if he has an 8mm or 12mm detonation sensor? Not likely. Getting customers to even know if they have a 1999 engine can be a challenge. But what if we could solve that problem by providing a block that can be used universally, and potentially use 12mm blocks in later applications, where that is a profitable option?

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Here is the solution to that dilemma: the use of an available adapter that can be threaded into the 12mm hole with an 8mm thread on the ID (Figure 2).

There is one small glitch. Since the insert is designed as a thread repair that will lock into the parent thread the lower extremity of the 8mm diameter decreases slightly to expand and lock the adapter into the parent thread.

This is not what we want in this case, so to be successful you must first run a clean and sharp M8 x 1.25? tap through the adapter to eliminate that diameter taper, thereby allowing the stud to bottom into the parent bore of the block.

To this point I have only found this adapter available from Time-Sert, so for more information visit their Web site at www.timesert.com.


Next up is the 4.3L Chevrolet engine that went metric in 1999 and had an “M” added to the end of the casting number. There were three types of bolts that became metric: the motor mounts, the starter and the transmission mounting bolts. The part numbers for these bolts were provided in a previous “Core Corner” (Engine Builder June 2003, page 16), but once again availability from the dealer is an issue. I did find one source of these bolts in kit form: Jasper Engines.

Now back to our original sensor issue. The detonation sensor on this engine is located in the rear of the block at the transmission-mounting flange and in 1999 was 3/8? NPT. In 2001 and later the sensor boss was 8mm, leaving us with 2000. Once again, there does not seem to be a clear line as to when the change occurred, so we are back to the adapter mode.

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However, for this particular adapter, you’ll need to make your own, because we’ve been unable to find anyone making a finished piece for this problem. Here’s how to do it simply and easily: purchase a 3/8? NPT solid plug, then drill and tap for M8 X 1.25? (see Figure 4, above).

You can install it into the block and ship it that way – if it’s not required it may be removed. Or, you can just send it along with the block with instructions to install if an 8mm det sensor is being used. See Figure 3 to show that the 4.3L block can be used both ways with this adapter.

Because I have had indications that the 2.5L and 3.0L Duratech engines go through similar sensor transitions as well, as I get more details on that information I will provide that to you. As soon as I know you will know.

Have a great 2005 and hope that these items are ones that will help keep you profitable. Remember: profit is not a dirty word.

Special thanks to those who helped me locate this information: Chip Helderman from Jasper Engines, Shai Dhanani from Yamato Engines, and Jeff Gentz from Gopher Motors.

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For technical questions, contact the Production Engine Remanufacturers Association (PERA) at: [email protected]

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