Semi OT - drilling 304SS broken exhaust bolt (Yanmar marine)

A friend was changing the exhaust mixing elbow on a Yanmar 2GM. One bolt broke off on removal, but had severe corrosion (crack?) causing that? Cast iron riser, broken piece rusted in.

Bolt is m8x1.25. A2-70 (304 stainless).

Other than the normal issues of 304, any additional issues from the corrosion or some thermal cycling? Water cooled exhaust, so it hasn't gotten that hot.

Will try one of the other bolts for drilling first, but looking for any other suggestion before drilling in to the broken one. Good access to drill.

Broken flush,prefer not to try to weld a head onto it in place. Enough material on the riser to tap oversize if needed.

Thanks in advance.

mfisher said:

A friend was changing the exhaust mixing elbow on a Yanmar 2GM. One bolt broke off on removal, but had severe corrosion (crack?) causing that? Cast iron riser, broken piece rusted in.

Bolt is m8x1.25. A2-70 (304 stainless).

Other than the normal issues of 304, any additional issues from the corrosion or some thermal cycling? Water cooled exhaust, so it hasn't gotten that hot.

Will try one of the other bolts for drilling first, but looking for any other suggestion before drilling in to the broken one. Good access to drill.

Broken flush,prefer not to try to weld a head onto it in place. Enough material on the riser to tap oversize if needed.

Thanks in advance.

Click to expand...

Make a drill guide from 1/2 X 1/2 X by how ever long. One end bolts to a good bolt and hole. The other end is counter bored for the broken bolt with a smaller hole as a pilot for the drill. Use mustard as a lubricate. Making a drill jig will save you time in the long run. Left hand drill bits some times help. 1/2 is just an example of size.

Try a left handed drill bit it can sometimes spin it out --- try heating it first and spreading paraffin wax on it -

If it was so tight that it wrung off, you can forget the left hand drills, easy outs and such. I spent several years at a pump shop having to get stainless bolts out of corroded parts. about 99% of the time, the only option was to drill it completely out and tap the next size up. Hardly ever got one to back out, even with heat, monkey pee, begging, and third world religious ceremonies.

Biggest problem is keeping the drill from running off the stainless into the softer surrounding material. Best bet is to drill a small straight pilot hole in the center (drill guide suggestion above is good, but start with like 3/16" or so), then step drill it up until you can shell it out with a diamond point chisel and punch. Run your drill slowly, use lots of pressure and coolant of some type. If the bolt work hardens, you are screwed.

To close out the story -
I took one of the other bolts (also 304) and was able to drill that pretty will with a 3/16" bit in the drill press. So there is some hope.

Back on the boat - bolt was broken off just below surface. I dried to back it out with a punch, but not luck.
Center punched it, put a bolt in one of the other holes as a reference axis, and started drilling. I wasn't able to put enough pressure with a 3/16 to keep it cutting (and wanting to avoid heating and work hardening it) so I went down to 1/8".

1" blind hole, with about 3/4" of bolt in it.

1/8" drill was working well, in about 1/2", when it chattered and snapped. Not what I wanted. Drilled 3/16" down to the broken drill, but couldn't clear the broken drill out.
Quit for the day, and then came back today with a couple carbide options.

Switched to a 3/16" carbide center cutting 4fl endmill. That went right through the broken drill, no problem. Took that endmill down through the rest of the bolt, and hit the space at the end. So far so good.

Went to 1/4" and drilled the first 1/2" or so of the bolt.
So now I have 1/2" deep of 1/4" hole, then 1/4" deep of 3/16" hole.

Before I went further, I figured i would try a Ace (old, fairly high quality) tapered square extractor.
Tapped it in, but since I had the larger hole up front, it was grabbing down at the bottom of the bolt. This is what I was hoping for, since I was guessing that there might be some more clearance / less rust down there (rather that having it grab up top, and put more pressure on the threads and rust).

Wouldn't you know it, after some working back and forth, the broken bolt started to come free. No damage to the threads, no tapping to a larger size. Just clean the threads with a tap and done.

Sometimes you get lucky.

What I think made the difference was that the extractor was grabbing at the bottom, rather than the top. I seem to have better luck doing it that way, for whatever reason.

Glad that worked out--busted SS bolts in a softer hole are a bugger.

Another trick for future reference is the good old hand-powered impact wrench: McMaster-Carr

I take a dead sharp metal chisel and first make a slot in the broken bolt, fairly deep. Then use the straight-slot bit on the impact wrench. The force of the blows sometimes helps loosen the thing, along with the torque that the wrench supplies.

But the task just sucks, doesn't it?

One issue was that the manifold was still on the engine, and rather crusted up with carbon/rust buildup (as is typical with the Yanmar GM exhaust manifolds). I was concerned that any significant impacts would start to dislodge junk, and possibly send it down into the valves. No way to easily stuff a cloth in there to protect the valves without pushing crap down. Not knowing the internal geometry, I didn't want to take the risks.
Pulling the manifold off would be much more work (with other risks), but at that point the broken bolt would become a non-issue, since the manifold would need to be replaced soon anyway (but hopefully it can wait until after the boating season, rather than right at the start).

Oh yeah, you did say it was in place and I missed that bit--good thinking on not knocking crud down there.

I wish they'd make all marine fasteners from 316. 304 doesn't last any longer than a good hotdip galvanize steel for me in salt water.

Curious.
I don't recall the 3GM (or any other small Yanmar) using stainless bolts for it's exhaust manifold. I'd venture someone changed out the hardened steel bolts at some point in the past. It's not as if these particular bolts are even supposed to get wet... if water is back there, you've got problems!

The bolts are holding the mixing elbow (stainless steel exhasut elbow with the water inlet pipe coming into it) onto the manifold.
Manual calls for stainless bolts. Normaly these are dry, but in this case there was a slow water leak over years past the gasket and into the lower bolts (this broken one in particular).

Stranger still... a stainless steel exhaust elbow? Not on any of the multitude of 3GM's I've encountered during 10 years working for a Yanmar dealership.

The JH series has a stainless manifold as an option, but I've never seen it used on the GM series.

The most common manifold setup on a GM is nothing more than a cast iron flange-to-pipe 90 mated to a cast iron upside down U. They last about 7-10yrs before they either rot out our clog up internally.

I don't have a photo, but if you do a Google image search got 'yanmar 2gm20f mixing elbow' you get the normal cast iron U shaped ones as well as the simple stainless ones (for above water line installs I believe is the difference).