MTFCA Forum

While cleaning out a few things this afternoon I came across this tap. It is marked STEVENS NEW YORK and is before the change/addition of the Walden Worcester to the logo. It is marked 1/2 14 and it measures out as such, 1/2-14. The only thing that I could think of or come across is the tap for the T-201 Oversize Tap and Bolt Set for Ford Cylinder Heads.


The only problem is the description from the Stevens catalog. The description says that the tap is the same pitch as the original head bolts (14tpi) but it is 1/16" oversize, which would make it 1/2-14 (Ford Head Bolt, 7/16-14). The description in the catalog continues on to label this as a 9/16-14 tap and special oversize bolts. Is this just a typo in the Stevens catalog? It states the same 9/16 in multiple examples of the catalog. So, is this part of the Stevens T-201 set or for something else?

Interesting

since the head is drilled 29/64 for clearance on the original bolt (.453), then I don't know how the ad could be correct, and that's a real whopper of an error.

I have studied that page in the Stevens catalog for several years. I have never really understood how it all works and have thought there may be a typo or something mis-stated. The 9/16-14 probably should read 1/2-14. If ya'll figure it out I would like to know what you decided.

Well, I'm going to stick my neck out here. I think that tap IS for the kit. I also think the 9/16-24 is an ad man's typo.

Here's why:

the head is clearance drilled at 29/64 from the factory
a 1/2-14 thread only takes a 27/64 clearance drill (that thread is still available, though the body is normal length, normal diameter, and too short to do this repair)
the replacement bolt has a significant increase in diameter over the original bolt and is likely just under 29/64 in diameter across the unthreaded portion
the tap in hand likely has a body diameter just under 29/64 diameter (you can check that)

therefore: I believe that the tap actually makes shallow threads in the head, and once down to the block, the large OD upper length of the tap maintains alignment to the head and the threads now more deeply cut into the block. The threads are continuous from top of head to bottom of hole in the block, with the only difference being the threads in the block are shallower and serve no purpose other than to pre-guide the tap into the block.

With the Stevens bolt keeping the original sized head, once installed, it would be invisible to the observer, only showing itself when removed as it will thread and thread and thread out while being unscrewed. It would sure surprise me if I ever saw one!

Scott_Conger wrote: ↑

Mon Jan 23, 2023 10:47 am

Well, I'm going to stick my neck out here. I think that tap IS for the kit. I also think the 9/16-24 is an ad man's typo.

Scott,
I agree with you, I think that this is the tap for that set and that it is just a BIG typo that went overlooked by a lot of people. I also believe that how you describe it as how it "should" be used is correct. According to the description from the catalog, that is all that should have to be done. It seems to me to be a very cheap/poor/bad way to fix the problem though. The major diameter of the thread body is 0.502" and the shank of the tap is dead-on at 0.500" and a 1/2-14 UNS would use an 11mm tap drill (don't quote me on that, working from memory), which is just just slightly larger than the 27/64" tap drill for a 1/2-13 (and less than the tap dril for a 1/2-20). So the bolt hole in the cylinder head, in theory should work.

The next problem would be the special bolts needed for this application, not even a standard thread pattern when this would have been made originally, hence the Unified Special thread form. This tap is nothing more than a novelty/curiosity now. Even in the Stevens catalog (on the very next page) when describing the Cylinder Bushing Set, they are quoted as saying that "the Oversize Cylinder Head Bolt is widely used for this job today.", has anyone ever seen one out in the wild?

The description for the bushing set goes on to say that it saves having to drill the motor head to take the oversize bolt. So the description for the Oversize Cylinder Head Bolt says no need to drill, but the description for the Cylinder Bushing Set says that it saves having to drill

I would have sprung for the Cylinder Bushing Set myself, not too different from a HeliCoil.

remarkably, the 1/2-14 is a not-so-expensive off the shelf item right now
why anyone would need one is inexplicable, but they are being made and they do not cost $100+ each (in fact they are very reasonable)
BTW, I agree with you, it isn't a very satisfactory sounding fix but perhaps mechanics knew full well the trouble you can get into pulling the head and if they didn't tighten too much could get this fix out the door for $5 or so and blame the resulting blown head gasket to something else.

Thanks Scott for taking the time to walk thru the process. I had came to about the same idea. I just had a hard time convincing myself that such a bad typo was possible. Pep, I know you are also a collector of of Stevens tools. But if you want to sell the tap, I am definatly interested in it. I do not have the tap and would love to add it to my Stevens collection.

Here is the copy of T 235 I made years ago.

Pep, sent you a message