stripped screw hole

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Old 02-03-11, 03:47 PM
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stripped screw hole

A bottom screw, one of four which secures the valve cover on a small gas engine (on a pressure washer) of mine won't tighten into the hole when the cover is in place, although it will thread in and tighten up into the hole without the cover. So it seems to me that the somehow the threads lower within the hole are stripped/damaged because the screw doesn't grab down there when the cover is in place. I'm figuring the repair for this will be that the hole will have to be re-tapped? I really haven't done much in the way of tapping holes before so am unsure of the proper approach here. Do I just need to tap the hole with the same threading as the original screw, or do I tap a different size/threaded hole for a different screw? Plus, any quick general tips/comments about the procedure would be appreciated.
 
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Old 02-03-11, 04:49 PM
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Sometimes it helps to "chase" the existing threads. They could be boogered a little, and when chased they will correct and hold. It seems to me the deeper threads in your case are holding, but raising the bolt the thickeness of the cover raises it too much for them to grab. May want to try a bolt 1/2" longer as long as it doesn't bottom out (after you chase the threads). All else fails, thread to the next size up, but paint the head of that bolt bright yellow so you will remember it is different.
 
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Old 02-03-11, 06:01 PM
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1st I would try a 1/4 or 1/2" longer bolt.
2nd I would see if you have room to drill it out tap for the next larger size bolt.
3rd and the most expensive option is to put in a thread insert (Heli-Coil).

Tapping is not difficult especially in aluminum. There are two types of tapes depending on how deep the hole is. A standard tap has a lot of taper at the tip so the end 1/4"+ does not produce proper threads. This is not a problem if your hole is 1" deep and you only need 1/2" of threads but if you need more threads or when working in a shallow hole you need a bottom tap which cuts proper threads much closer to the tip. The bottom tap is a bit harder to start but it's usually not a problem in aluminum. Make sure to keep the tap lubricated. I turn the tap in 1/2-3/4 turn then I back the tap out 1/4 turn to break up the chips and turn forward another 1/2 to 3/4 turn and keep repeating.
 
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Old 02-03-11, 07:11 PM
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Thanks for those helpful replies. If I do end up needing to tap, I don't happen to have any specific tapping or thread-cutting lubricant. For just the one bolt, just for this one time anyway, think it'd be okay just to use some other light instead oil like 3-in-1 or WD-40 or something like that?
 
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Old 02-04-11, 04:19 AM
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Yes, you should be fine using almost any oil. 3-in-1 or even motor oil will work.
 
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Old 02-04-11, 08:56 AM
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If you tap or chase be sure to blow out the hole before reassembly.
 
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Old 02-04-11, 09:49 AM
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Think I'd have any luck just trying using a longer bolt but without chasing the threads first with a tap? Because I don't have a tap.
 
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Old 02-04-11, 10:31 AM
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Oh, I would definitely try a longer bolt. If you find one that's just a bit too long you use a few washers under the head of the bolt.
 
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