Very tight JIS screws

I've got to remove some screws to pop out the external shift mechanism, before I can split the cases on my knackered KZ440 engine. 
I've run into a spot of trouble though. The screws that hold them on are so tight it's crazy.

I can pick a screwdriver bit which seems to fit, but turning them would damage the screw heads, I think. Do you think a JIS screwdriver would help? I've also got one of those impact drivers where you hammer on the end of it and it loosens screws. However, I'd rather not use that in case of damage to the screw heads or to the engine case. From the look of the screws, this engine has probably never been opened up before, although I can't be sure. Do you think these screws might just be obscenely tight? And is there a specific, ideal tool to get them out or will I end up ruining them with heat or an impact driver?

Do not try to turn a JIS screw with a Phillips head.  Use a JIS screwdriver, preferably a Vessel Impact Screwdriver.  Here is an example.  Be sure to get the right size head.


www.amazon.ca/Vessel-Megadora-Impacta-P2...a-422610599465&psc=1


 

VESSEL MEGADORA IMPACTA (2PC. SET)
Brand: VESSEL

$35 shipped off Amazon.  These are the right tool for the job.

If you have a hand impact driver I think the bit size is 5/16”. These should work I’m sure there are other available. 


rhinotools.com.au/product/jis-impact-screwdriver-bits/
 

Very useful guys, thank you.

Impact rotation + probably Vessel then, since that's 3 recommendations.

The other thing to have on hand is a set of left handed drill bits. If a screw head strips use a bit about the same size as the shaft of the screw and don't drill too fast. Most of the time the screw start to come out just before the head pops off. It does come off the shaft of the screw unscrews with your fingers.

Be sure to get a 3/8" drive impact. I use an extension long enough to reach from outside the frame using the largest bit that comes with it. Not JIS but fits well and a few hits the screws come right out. 
Steve

I have learned to move to an impact tool at the first sign a screw will not turn with a reasonable amount of force. You are less likely to bugger up the screw head with an impact tool than a standard screw driver. I also have and use my #2 and #3 Vessel impact drivers, but they don't work as good as my 60 year old, solid steel, Craftman impact tool.

I've bought this Vessel in the mid 70s. Made in Japan it came with JIS bits long before I knew what JIS was about. It has a half inch drive under the front end so I can attach (with a reducer) a 3/8 inch socket. An impact driver is a must have when it comes to an older bike.



 

wow - my impact driver from 1979 has the same Vessel case!  25 years later I found out why the bits worked so well on my Z1 - when I learned about JIS screws.  

Scotty

All good points, thanks. 

I've had a handheld impact driver for years. It will move anything, but I've only used it a couple of times. I just don't like the idea of transmitting direct lateral impacts into the engine case. I may have to end up doing that, but for now I'm doing nothing until I get my JIS screwdrivers and bits through the post.
I like powered impact wrenches more, because the hammering force they produce is only rotational. I think I should be able to use the detachable adaptor from the end of my impact wrench on my powered impact wrench, to fit a JIS bit on the end. 

There may be other issues when taking apart this engine. I'm not expecting it to go 100% smoothly. The bolts holding together the crankcase halves have brown heads... 
Worst case scenario: it will need replacement upper and lower crankcase halves. But that's thinking way ahead... First resort tool will be the powered impact wrench from here on in, I think.

The lateral impact is converted to rotational by the internals of the impact driver, I would seriously recommend not using an impact gun as it will rip the screw heads.(the rotational force of the impact wrench will push the "bit" out of the screw) In many cases a sharp lateral knock on the screw head will release the tension in the threads allowing them to undo without damage