DF520 DHW coil studs

While performing annual maintenance/gasket change on my DF520 I broke off two (2) studs on the DHW inlet port to the boiler. I prepped the metal (it's in good shape with very minor pitting) and I was able to install the gasket and return the DHW coil to service with the help of some high temperature RTV. I have not experienced any leaks but realize this is a short-term fix. I have the old style 5 gpm coil (p/n 9551800), straight bolt pattern with the early style coil gasket (p/n 34833). The coil is in great shape due to water treatment that has reduced boiler inlet make-up hardness. I would like to purchase 2 studs (p/n 7037017) and restore the original 8 bolt pattern. I don't think it is necessary to go for the extended flange weld approach at this point. I am trying to find out if the studs need to be welded in OR can you use a drill/tap set and then screw the studs in the new holes? It looks as if they were welded in during the manufacture (~1989) but I could be observing corrosion of the stud in and around the boiler penetration. I would like to do this after the heating season before I lay-up the boiler. How good would the repair be if the studs were used in newly tapped holes? Is that stud p/n 7037017 correct for this repair? Any thoughts on this common problem?

I don’t think you want to drill
And tap as you will be drilling into the pressure boundary of the boiler and won’t have enough thread depth with out reducing the boiler wall thickness which I call the pressure boundary . Welding the studs would be the way to go , but the extended flange in my view is the proper thing to do as it brings the gasket out we’re you can see where it leaks. And not hidden under the insulated jacket

What he said!

Not all boilers are created equal, but i drilled and tapped 2-3 studs on my keystoker. Ran the bolts from the inside out. Has been working since i put into service.

example of the dilemma i was reassembly the unit so I had the luxury of turning to weld it on the flat but position welding is possible just a little more sweat and cussing and spark burns. My 520 is is not a dual fuel either

My keystoker had a broken stud. It had been drilled and tapped by the guy I bought it off, at only 1/4 thick I drilled it out and tapped it with a 1/8-27 NPT (finer threads then the larger sizes) and then turned a 1/2-13 set screw down and turned male 1/8 npt threads on the front, use the hex head of the hex screw to drive it in to seal. Almost 7 threads (1/27=.037. .250/.027 = 6.8) engaged and didn't leak during pressure test or in operation.

Go ahead and drill and tap. You can get the studs at Napa. Use thread sealant from Napa also. Get the studs that have a little spacer in between the two sets of threads. That metal is 5/16 six thick and will hold just fine. Remember,You’re not torquing the crap out of it. Use two nuts tightened against each other to tighten into the boiler. I would do it right thru the plate and gasket without removing them. Drain a bit of water out first. Good luck. It’s a two dollar fix per stud. It’s a 1/2” x 18 thread size if irc.

Really appreciate the insights. I will plan for this summer project.