K301 Threaded exhaust elbow
 

New threads on a repaired elbow. Can only get a couple turns into the block.
What would be a good sealant to use to keep from leaking?

Depending on the model, 2 turns in won't hold. It will just rip out the threads. I'd try a better repair, or replace the elbow.

I'd check the threads in the block. You can use a pipe that is the same size to see if it goes in more than 2 turns. If it does the problem is with your elbow. If not you may need to get or borrow a tap to clean up the threads in the block. If the problem is with the elbow you'll need to replace or repair the elbow. The threads on the elbow can be cleaned up with a die of the proper size. Any decent plumber should have the tap and die to fix you up.

What I did was cut the threaded portion plus a little meat from a double wall nipple. After screwing the elbow in a coupling to assure it being straight, I cut the damaged threads off square in a band saw. then welded the new end to the elbow making sure it was square and the same length as before, useing another elbow as a guide. It also has messed up threads, both having been welded to a block.

There is not much left for threads in the block, unfortunately, and the repaired elbow slips in a ways before the threads catch. Same for a new nipple also.

I'll have to see about a tap to clean up what's left in the block I guess, if possible.

You can cheat a little:
A plumber or anyone with a pipe thread machine can set the die loose and make a larger diameter thread on the nipple after you chase the thread in the block with a pipe tap, that should get you a good fit with plenty of threads. BTDT:ThumbsUp:

Capital idea George!

I have a 1" die but I doubt if there is enough straight length on the Elbow. Maybe I can reverse the teeth in it. I found a 1" tap on the bay last night for not too much.
We'll get it figured out.

With the repair, the threads on the elbow are new, and at first glance, one would think it an NOS elbow. My son is an outstanding welder.

Again, what would be a good sealer for the threads?

Any pipe dope is a good sealer but it will burn off @ exhaust temps.
you really don't need it.
a 1" pipe tap hopefully will get the threads cleaned up in the outlet.
and go from there.
As far as what I was talking about in making the thread on the nipple larger, it needs to be done on a pipe machine with an adjustable die,
not a small hand operated non adjustable homeowner die.
Larger hwd and places like Home Cheapo and Blows have pipe threading services/machines that can adjust the dies open or closed, from the std size. it is just a matter of loosing a bolt on the die.
You might have to purchase a short length of pipe or 6" nipple long enough to be chucked into the thread machine, and give your son additional practice welding another nipple, make a trial fit in your newly chased thread in the block to assure you like the fit.
Usually 1-1/2-2 turns hand turns tighten then the pipe wrench finishing it.
I have used antisize to get tight enough to stay, but you don't want to put a cheater bar on the pipe wrench or you might split the head spiggot.

had the same issue
 

I had a similar issue , the threads on the oem elbow were worn as someone had run it loose in the block. Luckily the block threads weren't too bad. It would take a new nipple well. I ended up cutting the elbow just behind the threads and cutting a new nipple the same length. I was concerned welding it to the elbow being it was cast, but it welded up nicely. Good luck. I had checked the price of a new elbow , but at a bill plus I figured nothing from nothing left nothing. Vey Pease with my result.

My old elbows were the same way.
Got a tap bought, it should be here first of next week.