How severely can a spark plug hole in an aluminum engine head, which previously had a repair insert and has since blown out, making the hole bigger and showing signs of erosion, be damaged and still be fixable? Basically, there needs to be enough material to ream or drill out so that complete threads can be tapped to accept an oversize spark plug insert. The image above shows a spark plug hole after a Napa Fix-A-Thread insert failed, leaving the hole in worse condition. Erosion on the side of the hole occurs when compression leaks along the side of the insert. This may sound like an exhaust manifold leak that gets louder as the hole size gets bigger. Over time, this erodes the aluminum, sometimes making the hole unrepairable. Notice that the eroded area is worse on the compression side. The image below is of the same spark plug hole after a Calvan thread tap was used.
Since there is still an eroded area showing, it's our recommendation to go up a size, to the Time Sert 5600 kit. This vehicle owner decided to proceed with the use of the Calvan kit along with High-Temp JB Weld as filler for the void. He reports that everything seems to have gone well and has driven the vehicle hard for a few days, including some off-roading on a hunting trip. We hope that it holds well for him. However, it's possible that over time there could be issues. We have known of a few cases when a Calvan insert has blown out and the Time Sert 5600 was big enough to do a "last chance" repair. So, to answer the question about how bad the erosion can be: The Calvan kit uses an 11/16 reamer, so ideally, if using that kit, the reamer should be able to make the hole nice and round with no void. If using the Time Sert 5600 kit, the manufacturer says the 5600 will do repairs up to .73". The OD of the largest insert is .77" prior to the threaded install driver being used, which expands the bottom of the insert, locking it in place. The reamer from the 5600 kit is pictured below.
The reamer honestly won't do much if a Calvan pulls out because .73" is about the OD diameter on the Calvan insert. With that being said, the 5600 has been used successfully and I would still run the reamer in to help clean the hole up for accepting threads. The OD of the largest 5600 insert measures .77", and then the installation driver expands it a little more when installed. This has proven to be large enough to successfully repair a spark plug hole after a Calvan style insert has failed.
Question: After researching online, it seems like the Calvan is a very good repair. Why would one fail to start with?
Answer: The Calvan kit is a very good repair. It's our guess that copies of the Calvan-style inserts found online from Amazon and eBay may not live up to the same standards as the original ones. Also, it's possible that no thread locker was used or an incorrect type was used that failed to withstand the heat. The installer may not have cleaned the threads and dried them prior to using thread locker, which would be a reason for it not to adhere properly.
Question: Can't I just purchase an M18x1.5 tap and buy just the 5600 inserts?
Answer: The outside of the largest insert in the Time-Sert 5600 kit is an oversize M18x1.5, not a regular M18x1.5. Also, the installation drivers are needed to finish the installation by cold-rolling the inside threads of each insert and expanding them so they will hold.
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