Drilling and tapping Conrad heatsinks

I'm about to drill and tap to mount my F-5 IRF MOSFET's to my Conrad heatsinks. I had planned on drilling (and tapping) for 6-32 mounting hardware, but NOT all the way through the base of the heatsink. (Didn't want the mounting screws to be visible among the cooling fins of the heatsink). I'm having a hard time finding a "bottoming tap" that will allow me to tap the threads all the way to the bottom of the drilled hole. I'm to the point where I'm considering drilling "through holes" through the base and into the finned area, and using a conventional tap to run threads all the way to the fin area. (Not as esthetically pleasing, to be sure).

Your thoughts? How did you mount your MOSFET's? Partial threads in the base, or through-holes? I'd appreciate your thoughts, before I bugger up my $$$ heatsinks.

home made bottoming tap

Try grinding the end off a starting tap, and when grinding dip the tap in water every time you grind a bit off, don't grind until it changes colour of the tap or the hardness of the tap will be less. Make sure to leave about 3 threads on the end of the taper, tap the hole then grind off another thread off the end of the tap till there is one thread left. Use a tapping compound or a light oil. Make sure to clean out the hole every time the tap starts to get tight in the hole and start again till you get to the bottom. Full thread strength will be fine when at least 1 1/2 times the of the diameter of the screw is used(ie-1/2" bolt 3/4" deep). Tap all the holes then grind a bit off then tap all the holes again until done.
Hope this works out for you.
Peter C.
You Only need 6 threads for full torque ISA standard so look at your number of threads You need beyond the thickness of the transistor... Do you have a torque device that measures in inch/ounces or the equivalent newton metric???...32 tpi / 6 ~6 threads about 1/5 of an inch (.20 ")not very deep.
do the math... whats the max torque on a 6-32 machine screw???

OK I digress There a number Engineering types here that can give you more specifics the key thing here is , relax get it close use the largest screw head and the largest washer available to secure you device to the heat sink...
nuff said

sorry for the tyraid, Elwood
A self tapping screw would be a really good idea. You only need a few to several inch pounds of torque and most sinks are made of the lowest grade of extrudable alumium alloy and it usually has terrible machinability. Swallows taps by the handful. I suppose you'd have to have the Proper size hole for the screws to work well.
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Yes you do...self tappers are a godsend I just can't bring myself to that point being a mechanic by trade. I will limit myself to machine threads>>>as I said before torque specs are infinitismal so... what ever feels good as long as the holes are perpendicular and provide aducate heat transfer, use a hammer and a nail if you can modulate the force needed to gain propper heat transfer...

Regards, Elwood

Greg Erskine

Paid Member
2002-01-05 11:56 pm
Hi Ken,

I don't think there is enough meat on the Conrad heatsinks to drill a blind hole and tap it easily (by me :D that is). I would suggest you drill and tap through the base of the heatsink, but grind the ends the bolts so they are flat and flush, then paint them black.

If your holes are inboard from the edges you probably won't notice the tops of the bolts in the shadow of the fins.

The Conrad heatsink are cast which makes tapping a little more difficult I believe. I have always used Conrad heatsinks and find I have to clean the tap 2 or 3 times during the tapping process. Half way through tapping it starts to bind and I need to remove the tap and remove the swarf with a toothbrush and kero. Once I have tapped through the base I blow the swarf off before removing the tap.



Paid Member
2006-01-31 6:22 am
I'm wondering if the quality of the taps might play a part? I've used the Conrad sinks on 4 amps now and had no problems. I do have some good quality 3-tap sets with the bottoming tap. Using tapping oil, I just make a full rotation with the tap and then back it off half a turn to break the chip, repeating until I hit the bottom. There seems to be plenty of meat there to get more than enough torque without having to go through the side of the sink.

Thanks, all. One more simple question (and I think I already know the responses I'll get....). At one time I had planned on mounting my amp boards only with the tabs of the MOSFETs, into drilled/tapped holes in the heatsinks. (I thought this might be adequate, due to the light weight, and relatively small size, of the amp boards.) Having said that, my amp boards are drilled for four mounting points (one in each corner of each board). For reasons of "good construction", should I use all six mounting points for each board (2 MOSFETs + 4 on the board itself). I'm guessing you'all are going to tell me to drill/tap all six.

Probalby a dumb question, but I wasn't able to tell if there is a preferred approach by looking at the F5 pics posted to the forum..... Thanks, in advance....



Paid Member
2003-03-06 12:59 am

Thanks for the great advice. Probaby will go the McMaster-Carr route, and chucking it in my drill press to keep alignment true. Thanks, too--for the tip about "required depth" to give me a satifactory fastening. Re: "buy a couple because you'll probalby break one or two......" Oh, boy--something else to look forward to....!

For what its worth, I eye balled mine and drilled them with a hand held cordless drill. Worked perfect!

Tap magic is good cutting oil for these. I dilled device mounting holes deep and used a conventional tap in a 3/8" heatsink base. Removing the swarf is the trick, occasionally blowing out the hole with air does it for me but I've also heard that using wax or grease in the hoile moves the swarf out. Even if you through drilled the heatsinks, the holes should be unnoticeable to any but the most intrusive inspector. Of course, you'll always know.;) The through holes could also be backfilled and paint retouched, if you end up going that route.

BTW, don't break a tap, it is a PITA to the whole project. There is really no worry about threading aluminum with a quality tap, if done by hand.
Ok, this topic seems to have taken the usual detour :D

A few simple guidelines about threading aluminum:

1) Alcohol is by far the best cutting liquid for aluminum.
Anything else "made for aluminum cutting", is made due to the issue of the fire hazard of alcohol, as the industry does not like flammable liquids in free air.

2) Never use a tap or die, that has been used for cutting anything besides aluminum, as such tap will be wast inferior to one that is used for aluminum only.

3) Stay clear of el-cheapo taps. A good quality tap this small, can be had for like 15us$, and is well worth every cent. The cheap ones are almost always either brittle or too soft, and frequently both.

4) get a small straight angle, and measure that you have managed to tap straight. This will lower the stress on your tap significantly.

5) For short blind holes, use a spiral flute tap.

If you stick to these simple guidelines, tapping heatsinks is a walk in the park.

Magura :D