How to.... win the battle on Philips head screws

[jensen]

Hi Brewsky,

Thanks for the reply, it's becoming an long thread this way, and only about loosening screws or should I say : how to .... not screw-up screws.

But now serious, which screws are we talking about ? Because all the screws in the engine, except the screws of the ignition cover seem to JIS, the rest seems NOT to be JIS.

I mean, look at the pictures and compare with the drawings, no way I can detect any JIS screws in the pictures in this thread.

To show you I made an image of an oil pump screw, this came from a '61 C77 engine. I'm sure no one was in there before. Now look at the screw closely, and compare the picture with the drawings.

Rectangular edges ? where ?

Compare those with the screws from the ignition cover (those are true JIS and are the same as the drawings are showing).,

btw, the pictures are from my next thread "how to disassemble and inspect an oil pump",

Jensen

[brewsky]

I agree that especially the 6mm screws don't look like the perfect 90 degree cross pattern for whatever reason. Even the McMasterCarr JIS drawing doesn't look like it, but it is clearly labelled JIS on their web page and different from their non-JIS drawing.

The obvious difference seems to be the head sizes. (10.5mm for JIS and 12mm for current standard metric).

I think there has to be some additional distinctions such as differences in angles and dimensions within the relief cuts, but without the detailed manufacturing drawings who knows for sure? Surely the "designed to cam out" distinction is more than a simple radius difference?

Some additional food for thought in the pics attached. Note the 5mm screw with the dimple is definitely JIS, and it does not even have the perfect rectangular cross. The screwdriver tips do show some subtle differences.

[jensen]

Hi Brewsky,

Thanks for the pictures and the reply,

Like I said, the smaller (ignition cover screws) are more like someone could expect compared to the drawings. But the 6 mm, like you also show in the pictures, doesn't have any signs of rectangular edges, which are so significant for the M5 and smaller screws.

How is it possible that, when both are designed and manufactured from a JIS drawing, the 5 mm and 6 mm screws are completely different ?

After all, we all know that JIS means Japanese Industrial Standard, the word standard is controversial when looking at the 5 and 6 mm screws.

This is only about the shape and size of the head, I'm not discussing the threads differences,

Well, I found that this screwdriver is a "perfect" fit for the 6 mm JIS srew heads, and that is not surprisingly, because the 6 mm JIS screws are almost the same as Philips heads, infact, I can't feel any differences between a 6mm JIS and a 6 m Philips head when I use the mentioned screwdriver,

Jensen

[brewsky]

How is it possible that, when both are designed and manufactured from a JIS drawing, the 5 mm and 6 mm screws are completely different ?


Jensen

The only reason I could imagine for this would be the different depth of relief, and possibly that the cross section changes with depth. Don't know for sure though.

What is significant for me is that the phillips are "designed to cam out" at a given torque, and the JIS are not. How the stated goal is achieved is the question, and without design drawings to show differences, who knows for sure.

One thing for sure is if you are trying to remove a stuck screw, you certainly have to defeat any "cam out" tendency. And you are correct that the inward force applied along with the torque and introduction of additional friction of diamond coating or other compound helps.

I have also used air impact with a light touch on the trigger with success, but you lose the feeling of impending failure that way, much like an impact driver.

[conbs]

Brewsky and Jensen,

This is a great thread. Thank you both for providing so much information.

In the pictures you have posted I think I do see important differences. Start with the screw drver tips. The circumference of the screwdriver shaft seems to come down farther on the JIS vs Phillips. The very end of the screw driver tip also seems to have a bigger flat area on the JIS vs the Phillips. Either of those difference alone would/could change the angle of the outside surface that results, but perhaps there is no difference in the angle with both of them being different. But it does appear to me that the screwdriver tip goes farther into a JIS head than a phillips.

I agree with you that the well defined cross shown in dspictions of JIS head screws doesn't seem to match the reality of the screws. BUT, I think focusing on the look of the center of the cross misses the likely difference. I think the screwdriver tip going farther into the screw head and the resulting larger radius of the angled surface from center line of the screw could be what helps avoid "cam out" with JIS screws. Being farther out from center line of the screw head would give more leverage and allow more torque to tighten and loosen the screws. It would also explain why a more pointed Phillips head screw driver doesn't fit deep enough and ends up screwing up JIS head screws. The angled surface of the Phillip screw driver can't match up with the corresponding angled surface in the JIS screw head. While this would not explain why a #3 Phillips seems to fit JIS heads, maybe there is some other engineering similarity at that size that would.

Just a guess from a self-trained, home-garage "mechanic".

[cknight]

Yes, excellent thread. I have been under the impression (for a very long time), that the JIS fasteners had a shallower depth in the screw head, and that a Phillips driver (depending on its quality) would bottom out at the tip before the JIS flutes would be fully engaged. By taking about .020" off the tip of the Phillips driver, the engagement in JIS fasteners would then be much more substantial. Urban legend? The Reed and Prince fasteners have more of a true cross appearance (no radius between the flutes), but also have a different taper angle, and the drivers come to a true point. Regards, Chase

[Hondac71]

In my experience the t handle screwdriver from the common tool kit supplied by honda works best, these can still be bought new..(partnr 07776-99962 i believe)