machinists advice with a broken bolt on my adapter plate? p1994 Force 50 hp

[djsforce]

Just to keep it short, I replaced the adapter plate gasket last winter, only to have it fail near the end of my boating season this year. It was easier to take apart the second time, and now I'm in new territory again, dealing with a broken bolt where the leak occurred. Sorry, I have pics, but can't seem to upload them. The broken bolt was a 1/4"-20NC and 1" in length, right where the warm water from cooling the engine dumps into the lower leg. Keep in mind I've never done this type of work before. With the help of videos, I drilled out the broken piece and re-tapped the 13/64" hole to the original thread size. Unfortunately when I went to put it back together the threads were not strong enough to hold. Do I go the next size up to 5/16"-18NC? If I do, I can make the top part of the bolt fit in the adapter plate, but a socket will no longer fit. I was thinking I could grind a slot in it to use a screw driver to tighten? The 5/16"-18NC calls for a 17/64" hole, since I am having issues with strong threads should I drill to that size or re-tap with my original 13/64" hole? Any experience/advice is appreciated!

 

[Jkjones2709]

If you have room to drill it out and install a thread insert in it. Put thread lock on the insert prior to installation. You can buy them from Grainger

 

[josh prstn]

If it is accesable u can install a helicoil they are usually stainless so it wont corode that would be the best choice so that all of the bolts will remain the same size. The most cost effective way it to tap the hole to the next size up or if u like to mix things up check the minor/major diameter on metric and chase the hole with a tap. Like In previous reply definatly use thread lock and make sure the hole is clean lock tight dosnt bond well with oil and metal chips. Also after installing the helicoil thread new bolt in to set the coils and remove it for threadlocker to cure

 

[Jiggz]

Using helicoil or inserts requires you to drill to a larger hole (unless you can still use the existing 1/4-20 hole with permanent threadlock and use a smaller bolt) also which is not much different when going to the next size bolt of 5/16-18. The problem with the 5/16-18 with a larger hole it could weakened the walls due to thinner sides. The use of metric at this point is not recommended as crossing the existing threads with different pitch will only exarcerbate the situation, by stripping most of the existing threads. Re-threaded the existing hole 13/64 with a larger tap can crack the walls (due to higher pressure) of the hole making the whole situation worse. Of course, without a picture I am only assuming the location of the bolt hole is right next to a water jacket. However, if thinning walls is not an issue, go with the helicoil or inserts (preferred) option first, just make sure you use permanent red threadlock/sealant (hi-temp) and ensure it is properly cured (usually 24 hrs) before torquing it. Note 1/4-20 bolts on aluminum torque should be no more than 70 in-lb or a little less than 6 ft- lbs. The reason this is preferred is for uniformity of the bolts. And usually, this requires the use of a 5/16-18 hole, which would be your next best option. Note, personally, I go a little smaller with the drill bit hole than recommended. For example, instead of a 17/64 for a 5/16, I try using a 15/64 or even 16/64. And finally, don't forget to use anti-seize on all the bolts, properly torqued and all with lock washers.