RCU Forums - Stripped Glow plug threads

I posted this several months ago. Repeated here FYI:

Quote:

You asked for it. Here it is:

The intent is to install an insert with an interference thread, then drill and tap 1/4-32 thread.

1) Preparing the head.

Secure the head in the vise in a manner that the drill chuck is perfectly aligned to the plug hole. This can be eased by putting a short length of tubing or bar stock in the chuck that fits closely in the plug hole. Then using a new or near new 21/64" drill, drill through the head. (This drilling must be smooth and accurate, errors here can cause leakage around the insert.) De-bur the top of the hole. Chuck a 3/8-24 tap in the drill, hand feed and hand turn the tap, using a good cutting oil, and cut the 3/8-24 thread in the hole you just bored. The head is ready.

2) Preparing the insert.

Using a short length of 3/8" OD brass rod, drill a 3/16" hole dead centered in one end, depth at least 1" for later use (We'll come to that.)

The easiest way I've found for the needed accuracy is to chuck the bar, and hold the bit in the vise, then use the drill press hand feed to bring the work down on the drill bit. The surface finish inside bore is not important.

Using your 3/8 ADJUSTABLE die, set it to the maximum size. then with the die stock flat on the drill press table, hand feed and hand turn the brass stock into the die, cut the threads to a length of 1/2" from the end. Start the drill motor, cut a slight chamfer on the end using a fine flat file.

3) Test fit.

Leaving the brass stock in the chuck, attempt to thread the cylinder head on it. It should not go on., but if it feels like it will ALMOST go on, you are ready for the next step. If it doesn't grab at all, decrease the size of the die, make another cut, and recheck. This fit is the critical step, you want to have the brass OD about 1 to 1 1/2 thousandth inch larger than the thread in the cylinder head.

4) installing insert.

Clean the head and roughed insert with any solvent that will remove all residue, even hot water and dish detergent will work, but all oil must be removed. Put the head in an oven, raise it to 450-500 degrees. Chill the insert, dry ice is best, but your freezer will serve if nothing else is handy. Suggestion: If you can pop the chuck out of your drill press, leave the insert in it and chill the chuck AND insert together. (The chuck makes a good handle.) When you have the maximum temperature differential, using an oven mitt or similar, screw the head onto the insert QUICKLY as far as it will go. NOTE: Be sure, please, to screw it in from the cylinder side, have it come out on the finned (top) side of the head. Important.

4a) If you can't get it on, or it just starts and goes no further, take the head off, go back to step three and cut the insert thread a little deeper.

5) Finishing.

Take the assembly out of the chuck, reinstall the chuck in the press. Clamp the head in the vise, using the cylinder side of the insert, with the plug hole (the 3/16 drilling you made earlier) aligned with the chuck. Again, using a short length of 3/16 rod will make this easier. Using your 3/8" endmill, cut the face of the insert to the original level for the glow plug seat. See? That hole helped you center the head, and let you do a plunge cut with an end mill! Then, without removing the work from the vise, drill the 3/16" hole to 7/32" for tapping. This is slightly smaller than the correct drill for 32 thread, but it will work, with care. Drill it down to the same 1" depth you had before. Put the 1/4-32 tap in the chuck, again, hand feed and hand turn the tap. If you fill the hole with cutting oil it will tend to push the chips out instead of letting them fall to the bottom of the hole, makes tapping easier. When you think you've tapped it deeply enough, try a glow plug in the hole without a washer. If it will seat, the threads are deep enough. There's no need to tap it further, and the taps are easily broken. So stop when you think you're there and check.

Remove the head from the vise, cut the inside end off, finish it to match the original combustion chamber.

You have finished. How about that?

N.B. It's not a bad idea to use a sharp center punch and stake the insert on the plug side, even though you'll never have a problem if the fit is right.

Bill.

This is obviously for a detachable head. If repairing a one piece head and cylinder it has to be done a little differently because you can't get to the inside as easily, but the basic operation is the same.

Bill.