Of course, I broke bolts getting my water pump off... 4 of them to be precise. After soaking the studs over night I got 3 of them out with vice grips and one of them broke off flush. I drilled out the stud with a 7/32" drill bit and now I need to re-create some threads in there. Since I've never done this before (I will be supervised by someone who has... but couldn't answer this question) any tips would be handy as I will probably do it Wednesday... What size tap should I buy? Common sense would say use a 7/32" tap... but is that correct? Would using the next size up (8/32 so the hardware store says... which would be 1/4") be the correct answer? I really don't know. Thanks in advance... Eric
tap size The bolts you broke were probably a 1/4-20 thread. On my 1972 350 there are 5 of these bolts holding on the pump. To repair the threads for a 1/4-20 bolt, you need to use a #7 drill bit (.201) to make sure the holes are the proper size. If the holes are too big, the bolts will not hold properly when tightened. Buy a 1/4-20 tap with a pointed end, not a flat end. The point makes it easier to start threading. Emaill me if you have any questions
I have an entire chart that shows correct drills to use for what tap you are using. I used to know them all like the back of my hand but of course now that I need to know them I have forgotten. I think as a general rule of thumb though, you should use a drill bit 1/16" smaller than your tap diameter. For instance if you are going to tap for a 5/16-18 bolt, use a 1/4" drill bit. In fact, that's probably the quickie method, it's probably a bit smaller. When you go to fine threads like 5/16-24, you might need to use a drill bit about 1/32" bigger than what you would use for a course thread, because fine thread bolts I don't think need as much meat to bite into. If I'm not mistaken I think the water pump bolts are all 1/4". Like 1/4-28 or 1/4-20. a number 7 I believe might do it. So I guess in short all I've done is confuse the sh!t out of you. ou:
Well, the hole is already drilled... we just took the bolts with us and compared the bit to them... 7/32" just happened to be the match. I'll bring in the drill bit I used to the machine shop tomorrow or wednesday and see what they think... and I'll ask them for a 1/4-20 tap... thanks for the replies and I'll email if I have any more questions as time goes on :grin:
actually 7/32 is a little big. but should be ok (famous last words) the decimal size for the drill for a 1/4 - 20 is .2010 a number 7 drill bit. 7/32 is .21875 nice chart for tap drill sizes located here. http://www.pos.net/reference/tap1.htm just be careful not to strip out the thread when you tighten it as now you don't have quite as much meat to screw into. Nate
Leave it to me to drill it out too big Hopefully I'll be alright ... especially since I don't have the money for a new timing cover! That link is cool - definitely going in the favorites. Thanks a lot!
