View Full Version : tapping tranny cover for overdrive switch

09-07-2006, 09:01 PM
okay i've never used a tap and die set before. trying to tap a new hole in my transmission cover for an overdrive isolator switch. does anyone know the appropriate size 9/16 etc? also procedure; do you drill the opening a size smaller than the actual tap used for cutting the threads?

09-07-2006, 09:12 PM
The tap & die set will list the appropriate size drill needed for the thread size. I don't know offhand what size thread you will need.

After drilling the hole, start the tap into the hole, then add a light machine oil around the tap for lubrication. Turn the tap in about 3/4 turn then back it out 1/4 turn, then in 3/4 and out 1/4; repeat until the hole is fully threaded. Backing out the tap pushes out the material from the new threads and prevents it from fouling the process. Make sure that you get the tap started straight and keep it nice and straight as you begin. After a while, it will folllow the hole but be careful as the top cover is aluminum and easy to tap but also easy to strip out the new threads because it is so soft. You shouldn't need to push the tap very much, use gentle pressure.

Dave Russell
09-07-2006, 11:37 PM
Practice on some scrap first. Lot better than ruining a transmission cover.

09-08-2006, 04:29 PM

The tap size is 16mm x 2.0 (per the factory service manual). But, it would be best to measure and double check the threads on the isolator switch you've got, to be absolutely certain before cutting. If you don't have the tools to measure it, take the switch with you to a good store that sells good quality taps and dies. A good tool supplier will have what's needed to accurately measure the threads and set you up with the right size tap.

The tap will be labelled right on its shank with the drill size it needs. All taps are supposed to be labelled this way. (Although it can wear off or might be very small and difficult to read, so there are separate references such as Peter mentioned. And there are special storage cases that to hold taps along with their respective drill bits.)

Try to get a high speed steel (HSS) tap, not the garden variety steel that many hardware stores and Home Despot might sell you. HSS taps are sharper and will cut much cleaner threads.

By all means, whenever cutting threads in aluminum you *must* use a tapping lubricant that's made especially for use with aluminum. Ask for it at the store that sells you the tap. If you use other types of tapping lubricant or none at all, you stand a good chance that the alu will gall (sieze) on the tap and the threads will be damaged right from the start. In aluminum, the tap will typically gall when you stop turning and try to reverse it to clear the cuttings. The galled threads will pull out and that material sticks in the tap's threaded areas and will damage other threads in the alu as the tap backs out. A tapping lubricant formulated specially for aluminum is the best way to help prevent this.

Dave's suggestion to practice on a scrap of alumium is a good idea, too. Hardware stores sell aluminum stock in various thickness, try to get something about 1/8-1/4" thick. It will probably be a softer alloy than the gearbox top cover, but that's okay as it's more likely to gall than the harder cast aluminum. So, if you can cut good threads in the soft, sample piece, the top cover will be easy by comparison.

Also, you'll need a proper T-handle or similar for the tap (don't try to use an adjustable wrench). Ask at the store.

By the way, the isolator switch is fitted with one or more fiber washers underneath it, possibly to help prevent leaks, but mostly to act as shims that adjust the depth of the switch's probe within the gearbox cover.


09-08-2006, 05:04 PM
If all this is overwhelming, it would probably be cheaper to take the top cover to your local machine shop and have them do it. I'd guess around $20 but ymmv.

09-09-2006, 02:12 PM
it would probably be cheaper to take the top cover to your local machine shop and have them do it. I'd guess around $20 but ymmv.

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I was thinking exactly the same thing!

It might also be a good idea to have the second hole done at the same time, so it's there if you ever want to set up the OD on all the forward gears other than 1st. The second hole can be plugged now, easily changed later, if you (or the next owner of the car) wish. Probably wouldn't cost much more.

The exact dimensions to locate one or both holes are listed in the factory service manual. Let us know if you need them.