Stripped 90hp water pump impeller hole - longer bolt or larger bolt dia?

[jim_s]

R&R'd the impeller, and upon putting the impeller housing cover back on, discovered that one of the cover bolts no longer grips. :-/ Took it back off, and indeed, w/o the impeller cover on (otherwise, the bolt doesn't catch any threads at all), that bolt sinks deeper into that one hole before it hits threads, and only has about 3-4mm of space beyond that point where it grabs threads before it bottoms out in the hole. So, I could find a longer bolt, and use that until the remaining few mm of threads also strip (probably another season or two if I'm cautious about it...), or I can try to tap it out to a larger diameter (the current bolt is M6, not sure if there is enough material in there to tap M7 or not - prob not room for anything much larger, though, I suspect. I guess I could try to deepen the hole a bit, and tap it further down, but I could see that mainly just managing to destroy the few threads I have left in there, in the process of drilling the hole deeper past them. There is of course the option of trying to put a thread insert in there, but that'd chew up even more space, and I'm not sure how much larger that hole could be made before Bad Things happen.

Suggestions? (I briefly considered just running it w/ just 3 of the 4 bolts in there, but that's more of a desperation move that I might reserve for the case where there were no other options.

 

[Mybiggles]

How about you JB weld a stud in the hole?

 

[jim_s]

Here is my concern about the helicoil - not much space around that hole, unfortunately... (the one right in the middle of the photo, with only about 2-3mm of material between the hole and the shoulder it sits on)

Update: On further examination, that hole is already oversize - about 8mm dia, so the external diameter of a 6mm thread insert would probably easily fit down in the existing hole, w/o the need to get closer to that shoulder than it already is.

 

[Faztbullet]

Drill it a tad deeper and rethread...those are metric threads. Usually need to heat a tap to soften Loctite before removal

 

[gm280]

Fatzbullet idea sounds like a good idea. But you can also install a helicoil there as well. There is plenty room to install one. And then it will always be a good solid thread hole. JMHO

 

[jim_s]

Ok, turns out the angst was all for nothing. The 6mm helicoil fit in the existing hole perfectly - did not require any enlargement over its current size (aside from drilling out the old threads down inside the hole then tapping for the helicoil, but the overall size of the outer hole did not change). The insert went right in, and it's now stronger than it probably ever was. It was tempting to do the other 3 while I was at it, but I figured there was no point in inviting trouble. Great ideas were provided by all, and I'll definitely keep those in my mental list for other future such problems that don't lend themselves well to a helicoil. Thanks Much for everyone's help and suggestions!

 

[Faztbullet]

Helicoils and Loctite do not work well together. The Loctite will bond the insert to the bolt and make it b**ch to remove next time ...been there on a customer unit when screws was helicoiled.