TOPIC:

What a bizarre problem:

Way back in 1982 I bought crash bars to go on my 750 B4 twin (in front of the engine). They bolt to the frame down tubes just below the steering stem and have brackets that extend back and bolt on the front (lowest) engine mount bolt.

But, the stupid MAC exhaust extend about 1" too far forward (lazy dipsticks couldn't bend the pipes right) to allow me to connect the bracket to the engine mount bolt.

I want to mount the crash bars back on for the simple reason that the only two times I have been in an accident in 30 years was during the brief periods when I had them off because I was working on something.

I think I could mount them if I run a long 1/4" bolt on each side through the frame tube a little in front of the front bottom engine mount.

1) Would it significantly affect the frame if I drill a hole in it?

2) How hard is that steel to drill?


The frame tube does have other holes drilled in it a bit above the lower mount which is where the front upper mounts connect to the frame, so I wouldn't think another 1/4" hole would be a problem.

IMO a hole drilled in the frame is not a good idea. It will weaken the frame to some degree. (It certainly can't stregthen it) Whether or not a crack or failure occurs later only time will tell. It will also give rust an opportunity to enter inside where there is no protective coating like paint. It might take a while but it will happen. Lastly, If you want crash bars to offer anything other than a visual detail, I don't think a 1/4" bolt will offer the structural integrity in case of an actual lay over. Just my opinions, but it comes down to it, it's your bike, your ass, and you need to do what you feel comfortable with.

bountyhunter wrote:

What a bizarre problem:

Way back in 1982 I bought crash bars to go on my 750 B4 twin (in front of the engine). They bolt to the frame down tubes just below the steering stem and have brackets that extend back and bolt on the front (lowest) engine mount bolt.

But, the stupid MAC exhaust extend about 1" too far forward (lazy dipsticks couldn't bend the pipes right) to allow me to connect the bracket to the engine mount bolt.

I want to mount the crash bars back on for the simple reason that the only two times I have been in an accident in 30 years was during the brief periods when I had them off because I was working on something.

I think I could mount them if I run a long 1/4" bolt on each side through the frame tube a little in front of the front bottom engine mount.

1) Would it significantly affect the frame if I drill a hole in it?

2) How hard is that steel to drill?


The frame tube does have other holes drilled in it a bit above the lower mount which is where the front upper mounts connect to the frame, so I wouldn't think another 1/4" hole would be a problem.

Just a gut reaction, but I wouldn't bore a hole of any size into one of my own motorcycle frames for any reason. Being afraid it might weaken the tubing, foster stress cracks, or result in some other weakness sooner or later, due to vibration, etc.

Crash bars imo don't protect much if anything in a crash situation, but are mostly intended for mounting cruise pegs or extra lights. Whereas case protectors, especially those with three mounting points on each side (instead of only two) actually do afford some protection to the engine cases. Additional foot pegs (and maybe extra lights?) are often added to the case protectors.

Am unaware of a factory application where the design is for a hole through a frame tube without substantial reinforcement on both sides. Or any aftermarket item which requires a hole to be bored through a frame tube. Thinking most anything joining to the frame is connected by a tab welded to the frame (or perhaps a U-bolt arrangement for aftermarket parts).

Looked at the KZ750-B4 twin frame diagram on kawasaki.com and don't see any oem holes directly through the frame (only through holes in tabs welded to the frame).

Sorry, but I would consider the existing holes as already being a problem, and wouldn't consider drilling any more. Surely there's a better way.

Just my 2 cents -- and maybe worth even less.

Good Luck!

The frame is just mild steel, and the only difficulty drilling is keeping the drill centered.

If I was going to drill the frame, I'd make a boss (a thick walled tube that extends through the frame) and braze it to both sides. That would provide strength and support to the frame and keep the hole from cracking. Like the right front engine mount plate bolt holes.

Brazing a mount to the frame would be better, if you can figure out how to move the crash bars mount a little.

Tig welding would probably be better than brazing, but the equipment is more expensive and it requires more skill.

Patton wrote:

Looked at the KZ750-B4 twin frame diagram on kawasaki.com and don't see any oem holes directly through the frame (only through holes in tabs welded to the frame).

Sorry, but I would consider the existing holes as already being a problem, and wouldn't consider drilling any more. Surely there's a better way.

The holes already there are definitely OEM and are used to bolt the upper front engine mount brackets to the frame. And there are two holes on each side. They are designated "front upper mounting bolts" in the manual and they go through the mount bracket and through holes in the frame.

Patton wrote:

Crash bars imo don't protect much if anything in a crash situation, but are mostly intended for mounting cruise pegs or extra lights. Whereas case protectors, especially those with three mounting points on each side (instead of only two) actually do afford some protection to the engine cases. Additional foot pegs (and maybe extra lights?) are often added to the case protectors.

These crash bars are really heavy duty and mount in four locations including the lower engine mount bolts on both front lowers. They are really solid. The main reason I want them is not only for a possible crash but having the bike flipped on it's side in an earthquake.

Are there side mount case guards available for a B4 750 twin? I have never seen them.

Can you use some U shaped bolts and self locking nuts to hold the bars to the frame? 1/4" or 5/16" bolts can be found at most hardware stores.

The way these old frames noodle around I would never remove material. At the very least I would surely boss them. I would have to guess you are missing some mounting hardware since no bar manufacturer would recommend drilling the frame. I would find another way for sure. Velcro pillows to the sides of the bike for earthquakes. Maybe large springs on the sides. Airbags on mercury switches! Anything but cutting, drilling or blasting!!

Locozuna wrote:

The way these old frames noodle around I would never remove material. At the very least I would surely boss them. I would have to guess you are missing some mounting hardware since no bar manufacturer would recommend drilling the frame.

The stock mounting method for the crash bars is that the brackets attach to the lower engine mount bolt that goes all the way through the engine.

Because the exhaust pipes extend too far forward, I can't mount it that way. The brackets have to be about 3/4" farther forward for the bars to clear the exhaust pipes.

MFolks wrote:

Can you use some U shaped bolts and self locking nuts to hold the bars to the frame? 1/4" or 5/16" bolts can be found at most hardware stores.

I don't think so, the flat portion of the bracket isn't long enough for a U bolt.

Locozuna wrote:

The way these old frames noodle around I would never remove material.

I know what you mean, but since they already put four holes in the tubes to run bolts into the triangle bracket the engine mounts to, I didn't think it was a big deal.


Since the diameter of the tube is about an inch, the linear circumference is over 3". The 1/4" hole (both sides) would be taking away about 0.5/3.14 = 16% of the total material length. I didn't think that would be a big effect.

I guess when the frame tubes start cracking from the new holes out.....you might then see the big effect. Then again....Its your bike, your butt.... go ahead and drill away if you so choose. Would never consider such a thing on any of my machines. just my opinion. jonnybiker