Applicable to: '90 - '97 '99-'05 '06+ 1.6 liter 1.8 liter
Tools to repair stripped or cross-threaded spark plug socket.
When I bought my car, I found that the PO had managed to strip the #4 spark plug threads to the point that I only had two or three threads remaining (out of ten) - enough to hold the plug in place but definitely a problem when I added the SC. A series of compression and leakdown tests I recently performed resulted in totally stripping the remaining threads to the point of unusability. Rather than replace the head, I decided to have a mechanic repair the stripped threads using a Helicoil, Threadsert, or other means. I wanted the work done by a mechanic I trust because I didn't want to mess around with drilling and tapping the new threads, cleaning out the resulting shavings, and then installing the thread repair insert or coil. After a bit of research, I decided that I could repair the threads myself using the Spark Plug Thread Repair Kit from Time-Sert. In fact, a mechanic on a motorcycle forum said this tool was as "perfect as it gets" and I agree. I'm of the opinion that these inserts are stronger than the head material itself, likened to cylinder sleeve linings used in an aluminum block. When I get around to it, I'm going to install inserts for the remaining spark plugs. I will also make this tool available to anyone who is local (e.g. SoCalM Tech Day) and needs it - you will, however, have to provide your own inserts. CHEERS!!!
This kit is AWESOME, the repair took about 20 minutes and is one of the easiest procedures I have undertaken. The earlier issues I noted are non-issues with this kit; specifically, The repair can be done on the car, just make sure that the cylinder to be repaired is not at TDC so that there is room for the tools. No drilling is necessary. The kit comes with a "step-tap" where the first threads are the same 14mm size as a spark plug; thus, aligning and starting the tap is a no-brainer. All that needs to be done is to thoroughly grease the lans and grooves of the tap before threading it into the head (the grease will catch the shavings and keep them from falling into the cylinder). The larger section of the step tap will cut the threads for the insert. Once the larger threads are cut, you leave the tap in place and overlay a seat-cutter on it and prepare the seat around the new hole. Once this is done, withdrawing the step tap removes the shavings as well. At this point you can vacuum the cylinder out with a shop-vac but I found this unnecessary since I didn't see any shavings on top of the piston. After cleaning the prepared hole, you oil the insertion tool, lightly thread a repair insert on the tool, and then screw the insert into the prepared hole. It is impossible to install the insert too far because it has a flange/flare that limits how far it will go. There comes a point where a lot of force is required to turn the insertion tool. This is the point where the unfinished threads at the deepest end of the insert are cold-rolled to expand and seat the insert, these cold-rolled threads keep the insert from backing out as a coil would. When the insertion tool spins freely, the installation is complete. You simply remove the insertion tool, inspect the seat, and install the spark plugs per normal procedures. If you ever need to remove the insert, it can be backed-out using a counter-bore screw extractor or drilled out with an appropriate drill/tap. Basically, this repair seems to be permanent. The recessed spark plugs on the Miata require that the kit be the "extended reach" kind (p/n 4412E); otherwise the tools are too short and won't work. The thread inserts themselves are of a specific length and cannot be shortened. In my case, I used the 14mm diameter X 15mm length (p/n 44129) for my spark plugs which are NGK BKR5E. The tool is expensive but the inserts are fairly cheap.
Under 30 minutes to remove completely
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19 May, 2007