Howdy all. After reading thru 11 pages of a search, I am not finding my answer or am not getting the right search terms to get the right posts. I need to drill an accessory bolt hole in the pass. side head. It has one hole near the outer edge of the head but I also need another one that is closer to the manifold (driver side has it). A friend had told me that if the head has the machined flat area, I could drill and tap it but did not recommend doing it on heads that did not have this pad area. After checking the heads again, the heads on this engine do not have the machined pad area but checking another engine I have, this 2nd engine did have the machined area. My question(s) is, can I drill and tap the head that does not have the machined flat or should I swap heads with the one that does? I haven't pulled the valve covers off either one, so I don't know which head # they are yet. As for drilling the hole, my plan is to bolt the bracket on, which for the hole I need to drill, the bracket's hole is about 2-3" deep. I am going to get some tubing that will have the diameter of the bracket hole and the inner diameter of the drill size needed for the tap. This should allow me to keep it straight while drilling, using the bracket/sleeve as a guide. I will put a stop on the drill to only allow about 7/16" to be drilled into the head, for a 3/8" thread. Is this a good/correct way of doing this? Thanks for any help or pointing me to a thread already here.
sounds like you are trying to mount some billet "street rod" type alternator and/or AC compressor mounting brackets. i suggest you get brackets that you do not need to drill and only use the water pump and intake manifold bolts. Alan Grove sells them , so does Bill's ..i'm sure there are others http://www.alangrovecomponents.com/ http://www.billshotrodcompany.com/17043/17169.html
Are you sure you need that hole for your brackets? Standard chevy heads with accy holes have 3 on one end, and 1 on the other. Meaning the front of the engine always has one head with only one hole. And the passenger side head is the one with only one hole at the front. I'd be very suprised if any OEM or aftermarket bracket set required nonstock holes. Good luck
Actually I am trying to mount a GM serpentine belt setup for a non HAMB friendly build (my '68 C10 truck). Picked up the whole system from the JY for $75 and it solves a number of things in one package (upgrade alt, A/C)
Take a picture of your heads. You can drill and tap just about any motor casting. Your friend is right about having a pad or you can face the area so you will have one. Are these special heads to you, or can you get the right ones for the job.
I disagree....bolt diameter is all thats needed. this is from Bearings and Drives...they have classes on ths type stuff. I still think a little more is needed in aluminum....
I've seen it done but on one head, it pulled out the threaded metal because the casting was thin(hollow). The other example was goobered up,,leaked, that part of the head is at an angle in relation to the front. Pre '68 Heads I think.
The reason you need a milled flat is to check the thickness of the material with Ultrasonic before drilling.
What he said he was going to do would only give him 1x if he bottom tapped with a tap that was hand ground with no lead on the tap.My 30 plus years of machining and engineering would lead me to use 1 x the diameter as a min.2x is what we ussually use in situations for high strength or vibration aplications.Burl
Hey , I put the same set up on one 2 years ago left the hole that wasnt there and it still works fine today . Dont give your self a head problem .
What metal bender said; That big bracket that everything is mounted to is hell for strong. Absent one bolt, it will still work.
markzhotrodz is correct, the old style SBC heads (283-327) don't have holes in the ends and are very thin casting on the ends. The hole locations you are missing will go into the water jacket with not enough threads to seal, plus the end of the head is not square with the bracket you are installing. Using an existing bolt hole would be your best bet...
Okay, that's good to know. I was concerned since the pass. side bracket only had three bolts holding it on, yet held the A/C compressor and belt tensioner. The driver side with the alt and PS pump gets 5 bolts, all of which line up. As for my heads, nothing special about them. Vortec heads have the extra hole in the heads but if I change over the heads, I have to change the intake and this all starts to add up. $800 to put on a $75 setup. Doesn't make sense. Thanks for all the input. For some reason, I had thought drilling heads we more commonly done. Guess not.
I've drilled them for the alternator, what's the big deal? I like to spot face (make a flat spot) and braze on a nut (3/8-16) then drill through the nut a bit with tap size, then run a plug tap in, then run a bottom tap in, then grind off some of the nut (till it gets down to desired height), then if possible use a stud loc-tite-ed in the hole. Simple and strong!
i did a v-8 swap on a 1990 astro van. i used a 1986 305,tried drilling the holes for the alt bracket. the drill wandered and i hit a water jacket so i ended up buying a 87 or later heads and installing them on my short block.
Didn't realize vortecs had an extra hole for the accy plate. Wonder if the non vortecs used in the 90s are the same way? I learned something today......
Put a TPI Corvette motor in my wife's X J 6 a few years ago, and drilled and tapped a hole in the aluminum head to use a Camaro serpentine belt setup. Had it for years with no trouble.
Well, I finally got some time to get back to work on this project. After having a buddy come by and give me his opinion and ideas, I decided what the heck, worst case I have to swap on my other heads, best case it works and using sealant with a thread stud some place in the middle. About an hour worth of work and everything came out perfect. Bracket bolted up with no issues. Here a few pics. After bolting the bracket on and using a transfer punch to make a center punch, I measure the distance so I could build a jig. Used 3/4" think aluminum stock, center punched with my measurements. I used a drill press to make sure the holes were square and did one at 3/8" and one at 5/16". Bolted it to the head, centering the center punched hole in the 5/16" side on the jig. Finding that case iron is stronger then I thought I was able to get the hole 3/4" deep in the head and did not punch into a water jacket or anything else. (sorry for the blury pic, had camera setting wrong) Then I used a starting tap to get the threads going and switched to a bottoming tap. Tapped very easily to my surprise. Moment of truth came when I bolted the bracket on and it fit perfect, without having to wollow out any of the holes. I was stoked to say the least. I can add my name to the list of people who have done this. Just thought I would share my joy of not having to swap heads.
I am not sure what that reply means??? I don't need new heads, just an extra hole, which I took care of. All is good.
OK, nine years later: I have the same issue and will probably do the same (drill a hole ...). But isn´t there a third bolt hole on the very bottom of the bracket? In my case it´s flush with a whole that was sealed with a bolt. The bolthole on the engine that fits the lower bolt of the bracket leads into a oil passage .... deep enough to leave 0,4" space for the bolt without restricting the oil passege. Using this bolthole sounds pretty wild for me .... anyone done that before? My engine the peripherals including brackets come from is a ´92 2 bolt engine. These brackets are to go on a ´75 4 bolt (where the upper right bolthole is missing on the head). Any help is very much appreciated!
are you sure it goes into oil passage? if you mean the one on left bottom it does not go into oil passage it does go into fuel pump rod cavity if so that will take bolt 1/2'' deep.if no bolt it will leak.
No, I‘m not sure. I blew air into the valve cover‘s breathers and It came out in that passage (bolt removed). But of course you may be right! Anyway, the funny thing is that that’s the missing third bolt hole to mount the ‘92 a/c bracket on the ‘75 engine. I’ll put some loctite thread sealant and a gasket inbetween and cross my fingers .... I’ll keep in mind the 1/2 inch depth!!!!
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