Thread Forming Taps

shoeboxpaul

shoeboxpaul

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Hello, wondering if anyone has experience in this area. I want to clean up the threads in an engine block, 7/16-14. I know about the thread chasers sold by Craftsman, Snap-on and others and have used them in the past but, do not own any. A thread forming tap makes threads without removing metal and I have no experience with them. Since the threads in the block already exist, my thought is that a thread froming tap would just push the threads back into a uniform shape and remove no metal, leaving a stronger thread.
Anyone ever try this?
 
You are write that is the way they work no metal is removed. I used to for Besly tap co. and are very familiar with forming taps. They were originally made for soft metal but as they were further developed you could use them on steel. The drill chart is different than a cutting tap, so keep that in mined if using in a new hole. I would think it would work great in a engine block. The besly co. was the original inventor of the x-press tap, and had the patent on it. Is the block cast iron or aluminum ? If it is cast iron i would not use a forming tap
 
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Since thread forming taps are generally not recommended for tapping new threads in cast iron, I wouldn't think that they would be your best alternative for thread chasing either.

Ted
 
A lot of the purpose of chasing threads in cast iron engine blocks is to clean out rust and crud, not to straighten existing threads. Cast iron does not bend much, it breaks. I fear that the forming tap would just be smashing all the grunge and rust into the iron, which might not work out well. An ordinary cutting tap will cut the rust and grime off the existing threads, and then you can blow it out of the hole. Yes, the downside to using a cutting tap is that it will remove a slight amount of metal from the threads. If the existing thread is rusty enough or loose enough for that to be an issue, then a thread insert would probably be a better repair.
 
Thanks for the info everyone. I am not trying to repair threads and maybe only 2 to 4 out of 44 holes might contain some rust. I guess slow and easy progress with a good tap or a restoration tap may be the way to go because of it being cast iron, something I was not thinking about (thanks Bob, for reminding me). I have a HSS tap and also a Heli-coil kit for 7/16-14 if I run into trouble. Just thought I may have stumbled onto something of better quality than the restoration taps, assuming they are made of carbon steel.
 
Some of the choice of your best options depends on what you are chasing out of the threads. Are the threads contaminated by greasy grime, paint, rust, cobwebs, dirt, or what? You do not really need a thread cutting oil if you will not be cutting threads, and a restoration tap might be the best choice. You can use some sort of solvent, WD40, or whatever for oil and grease, water for actual dirt, a penetrating oil for rust, or whatever will loosen up what is in the holes. Let it soak a while before tapping. If the holes are already pretty pristine, which looking at your avatar they might well be, it might be best to just leave them alone or chase them dry. If you use oily stuff you will need to carefully degrease before painting anything.
 
Seems to me that if you're not removing any material and some of that material is rusty it will be pressed into the surface of the threads. Iron oxide is an abrasive mineral so the threads might be better but I think screwing any bolt into them will feel horrible and crunchy. If you only need to screw bolts in one or two times during the remaining lifetime of the engine block that's fine, but I would go with something that removes/loosens material. If the threads are bent due to some abuse or accident but otherwise free of corrosion then a thread forming tap seems ideal.
 
If its a bare block and hasn't already been done, I would have it hot tanked first, and then use a thread chaser to clean out your bolt holes. That's what they are designed for. I would shy away from using a standard tap as they will remove block material, and cylinder head clamping force and uniformity is important, especially in any sort of performance build. Mike
 
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