When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Unfortunately when doing spark plugs on my 1988 Chevy Corvette, I had a hard time getting the spark plug to thread into #2, and after removing some tubing to get better access, it will not grab and go in no matter what I try. I tried the old spark plug and it will not take either. It’s just the beginning threads. The rest of the threads are ok. I believe I have to use a back tap for the best chances of fixing this, however I’m not sure of the size I need to buy. Does anyone know what size back tap I need to get? I thought it was 5/8” or 14mm but I’m not sure and I don’t want to mess this up worse! It has the 113 aluminum heads.
If you have a different suggestion let me know! I don’t want to take the head off the car right now, that’s more time than I can current put in. But I have many tools to clean up any loose shaving, and grease to catch them on the back tap.
14mm threads, the plug has a 5/8 nut.
I've done inserts before, but never saw a "back tap" till I looked it up. Looks like that may do the trick if the other threads are ok.
I would try a thread chaser. Worst comes to worse you could install a helicoil. I had a similar problem with an LS2 head. Though I took the head off and ran the chaser from the chamber out. It did a great job. If going from the buggered side in you betting get it right.
I would try a thread chaser. Worst comes to worse you could install a helicoil. I had a similar problem with an LS2 head. Though I took the head off and ran the chaser from the chamber out. It did a great job. If going from the buggered side in you betting get it right.
I used a back-tap when repairing the spark plug threads on a 1988 MB 560SEC. For cars that are built like bank vaults, MB used notoriously soft aluminum on the engines and heads. Get the proper size back tap and take your time. Put the piston to the top to keep any shavings from going down too far. I used tap oil and an adapter kit for my shop vac that is used to clean computers to suck out any shavings. The hard part about all of this is the lack of room to work. See if anyone you know has a enscopic camera you can borrow so you can visually inspect the threads before you do anything. If you do not own one, I would high recommend you invest in one as they are worth their weight in gold when you need one. You can buy them at Harbor Freight, Amazon, etc. Good luck and keep us posted.
OP, you have the right idea. The back tap is the way to go. Saved #4 I think on my ‘89. Same exact problem. Emissions pipe in the way. Slowly worked the back tap in to out. Took an hour just to be safe easing in and out. Use the good inside threads to correct the outside threads. Amazing tool.
I tried a back tap and it wouldn’t grab, it kept jumping sort of like when turning a cap the wrong way, but I was turning it counter clockwise as I should. I got stuck on something twice, I believe it may have been the valves, which is obviously not good. I don’t think this is something I want to go into further as I don’t want to screw this up more than I have.
If I had to go aftermarket heads that’s fine, but I’m still going to see if I can salvage this one. Not ideal to happen after the other successes I had.
The head is a long way from junk. If you don't want an expert to do it like I suggested then get a cheap LED camera and find a crank position where the valves and piston are out of the way. You never said what position the crank was in. You might have some carbon built up at the bottom of the hole. Might need a small brush like a copper pipe cleaning wire brush to clean the carbon from the bottom so the backtap can start correctly. Dan
The head is a long way from junk. If you don't want an expert to do it like I suggested then get a cheap LED camera and find a crank position where the valves and piston are out of the way. You never said what position the crank was in. You might have some carbon built up at the bottom of the hole. Might need a small brush like a copper pipe cleaning wire brush to clean the carbon from the bottom so the backtap can start correctly. Dan
No need for the expense(cheap or not) of a camera. Put #1 piston at TDC. #2 has fired 90° previous and is on its way down the bore out of the way with both valves closed.
No need for the expense(cheap or not) of a camera. Put #1 piston at TDC. #2 has fired 90° previous and is on its way down the bore out of the way with both valves closed.
I am sure you are correct. For myself, I would like a quick look at the threads with the camera. To see how bad the top is screwed up and if there is carbon at the bottom. Do you know if the spark plug threads all the way through or stops a few threads from the bottom? Dan
I am sure you are correct. For myself, I would like a quick look at the threads with the camera. To see how bad the top is screwed up and if there is carbon at the bottom. Do you know if the spark plug threads all the way through or stops a few threads from the bottom? Dan
I can't imagine a head with the spark plug hole not threaded all the way through. Exactly what would be the purpose of that? If such a thing existed, then whoever invented and markets the back tap sure went through a ton of expense and time to develop a tool that would be non usable.
Well I went to take the wheel off to get better access through the side, and I couldn’t get it off, this decided to strip on me too while I was trying to get the center cap off. The tool is bad, anyone know where I can find a replacement? I tried looking everywhere online. I might have to look at the for sale section on here for one.
I can't imagine a head with the spark plug hole not threaded all the way through. Exactly what would be the purpose of that? If such a thing existed, then whoever invented and markets the back tap sure went through a ton of expense and time to develop a tool that would be non usable.
HAha There are two ways to read my question. I was really asking if the spark plug reaches the end of the threaded hole NOT if the hole was threaded all the way. Thanks, Dan
Here is a picture of a spark plug not reaching the bottom of the threaded hole. The built up carbon would make it hard for the back tap to start correctly. Dan
Last edited by Whaleman; Jun 13, 2022 at 08:18 AM.
SUCCESS! I talked to a mechanic friend, he said the tap going from outside in will be fine, it’ll grab onto the old threads. And he was right. I got the spark plug in. Seems to be ok! I used this tool right here, he gave me it to use.
Stripped threads on #2 plug
