Spark Plug Tap

KV Bruce

Member
Working on a 1947 Hercules IXB-3. Little flat head 4. It had RJ12YC Champion plugs in it.What tap would I use to clean the threads? Thanks in advance. Bruce
 
The RJ12YC is a 14mm 1.25 thread.

What kind of thread problem do you have?

I seriously doubt that is the correct plug for that engine! The thread is too long. There will be threads hanging down in the combustion chamber, which will carbon up, making the plug very difficult to remove.

I'm thinking a Champion J8 is the plug, but double check that.

Someone may have put the long plugs in because one of the holes was stripped, maybe the longer plug reached the last remaining thread...

If the threads are just rusty or carbon packed, the tap would clean them up. Hint: Pack grease in the flites of the tap, it will catch the grit it removes.

But, if the hole is stripped, it will need a thread insert to repair it.
 
14MMx1.25 Thanks. Bet your right, not the correct plug. The reach on the plugs is about right (maybe a little short) I do have the head off. Threads are ok, just want to clean them some. I'm chasing all the block holes too.
 
I run J8C (At least I think its J8C, its the longer of the J8 plugs) plugs in both my Hercules IXB-3 engines and they work fine.

If there is nothing really wrong with the threads, you could just run a thread chaser in there to clean the threads out. Most Auto parts stores sell one for spark plug threads.

(It sure helps if too short a plug has been installed: Sometimes there is so much carbon below the old plug threads the correct plug won't thread in and seat.
 
You don't need a tap at all . Find a plug with a thread that suits , the longer the better . Cut a fine diagonal groove across the threads from the top to the bottom with a Dremel . This will create lots of sharp edges that will scour the threads in the head when they are screwed into it . Cleans better than a tap as the steel is softer and less likely to cut thread material rather than accumulated carbon and muck . Screw in slowly , back out often , proceed with care .
 

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