Make a slow bike fast - KZ750 Twin

Seems like a neat project.  My brother had his XS650 rephased, I understand it is supposed to run smoother that way and make some more power.  I can't vouch for that since hes hardly riden it since it was finished and likes to jump from one project to the next...
 

I really wanted to rephase this engine, but because its a forged single piece crank the cost of a custom crank ruled that out. The glut of cool performance parts for the XS650 makes it very attractive as a platform.

The engine build is what started this project, I was initially building this for my other 750 twin, but decided an engine such as this need a chassis that was built to the same standard. 

Prior to this project I had never gone that deep into 4 stroke engine theory. This project had me reading several books on tuning 4 stroke engines, lots of late nights reading on the interwebs going into calculations and theory and then trying to apply that to my fairly conservative engine build. Looking back knowing what I know now there are some things I would have done differently but you gotta start somewhere. Below is what I've settled on, its not the end all be all, but its the culmination of my research and how much money I was willing to sink into this project. 

Transmission - 1983 twin, under cut by APE. The 83 twin has a lower 5th gear ratio than the 76-82 models resulting in a higher top speed.

Clutch - Basket from 83 twin - this clutch basket is aluminum saving over a pound in rotational mass, it also actuates from the right side of the engine allowing a much larger output shaft sprocket. Stock clutch disks with 20% stiffer springs from a Ninja 650. 

Balancer - additional rings pressed on sprockets 

Case - Bored for big sleeves, oil gallery tapped with 1/4" NPT for oil cooler, APE HD cylinder studs, grade 12 main studs.

Crank - Balanced and mirco-polished by Revco Precision 

Connecting Rods - Custom rods from Carillo. uses King bearings for a ZX -14. the ZX14 uses bearings with the exact same dimensions as the KZ750 the only difference is the location of the bearing tang. these things are pure art. its a shame they are hidden in the engine. 

Pistons - Custom JE pistons with custom iron liners - 86 mm. 10.25/1 in this application 

Cylinder head - custom valve train and porting from Schumann Motorworks. 1mm oversize titanium intake valve - stock size titanium exhaust valve. Custom bronze valve guides to neck the valves down from stock 8mm stem to 7mm stem. beehive springs and titanium keepers. 

Cams - Web Cam custom grind.  IN: .406" lift @230 degrees of duration. EX: .418" lift @ 261 degrees duration 

Exhaust - 1.5" 18GA 13"L primary, step up to 1.625" 18GA 13"L primary, 2" 18GA collector 16-20"L. I've always wanted a set of high pipes despite being impractical on a race bike. Ive ordered the bends from Burns. I want to build something crazy. 

Carbs - TBD but likely a set of FCR 39 carbs. back up will be RS-38 neither of these carbs would be technically legal in WERA but i love pumper carbs. easy enough to switch out later if i need to. 

Ignition/coils - Optical ignition and coils from Power Arc.

Bottom End assembly is complete.

Cases were very corroded. They were too big to fit in my crappy HF blaster, so 2 layers of duct tape and i got them blasted for $60. The blasting revealed how pitted the cases were so I chickened out and went black on the case paint. cleaned manaicaly like 4 times before painting with KG- Gun Kote satin black and baked them in the oven.

Whoever took off the cylinders before me did a number on the cylinder mating surface. There were some deep gouges from them prying off the cylinder. I wanted to make sure there were no high spots so I very lightly lapped this surface with some 220 grit on a surface plate. this revealed a low spot and a little more lapping removed it.

I removed the sludge plugs on the crank and after a thorough cleaning I reinstalled the plugs and tacked them in with some silicon bronze.

The Carillo rod bolts are tightened to the amount of stretch with these CARR bolts the stretch is .0065"-.0075". this was about 45 ft lbs. I clamped a drill vice down to hold the crank during the process so i could get accurate measurements with a micrometer.

I found some grade 12 socket bolts to replace the main crank case bolts. fitted with a thick hardened washer.

Sadly this project is being put on ice for the moment. and this engine is getting boxed up and stored for probably at least a year until i can get a shop setup again. I am moving out of state and will be dealing with all of that.




 

Very cool. I hope your move goes well. In the mean time, a bunch of us here are going to be suffering from withdrawal until the project gets back on track.

Hi Vic,
One thing caught my eye.  Not sure if it will be an issue. Your socket bolt diameter doesn't neck down when it reaches the threads like the oem bolt to the left. That may be a site of stress concentration on the socket bolt.
I've included a link from Kevin Cameron on the issue.

 

The new bolts are thicker so they can't neck down.  The old bolts will stretch more than the new ones at the same torque level.  Only minor question is whether or not that matters?  

wrote:

Hi Vic,
One thing caught my eye.  Not sure if it will be an issue. Your socket bolt diameter doesn't neck down when it reaches the threads like the oem bolt to the left. That may be a site of stress concentration on the socket bolt.
I've included a link from Kevin Cameron on the issue.

 

 

Its things like this that make me love this website. Never would have occured to me that this could be an issue. thanks for sharing the video. The CARR bolts on these custom rods also had this necked down feature- clearly there is credence to this. What I find interesting is the APE  heavy duty studs fly flagrantly in this face of this logic. Perhaps they are snake oil and a marketing ploy? either way i would be surprised if these grade 12 bolts were at risk of failure if the APE studs were OK. I mostly subbed these bolts out for cosmetic purposes truth be told. The OEM bolts were generally very low quality, but the main/cylinder studs might be an exception.

I noticed this today... same tapered feature on these driver bits. I may revisit this on the crank bolts 

I think the guy in the video on cylinder bolts has no clue about how threads are made. The reason that the bolt is "necked down" is that the thread has been cold rolled into the blank. This forms deeper grooves (the thread roots) in the blank, and the material from those is pushed up into the thread crests. This is a really good way of making threads:
     - It preserves the grain structure of the blank (cutting a thread with a die or single pointing it interrupts the structure)
     - It puts the surface in compression, which improves stress resistance - similar to shot peening
     - It's fast, gives an excellent surface finish and there's no waste material

Doing what he did - machining the shank on an already turned thread so that it "looks" like a rolled thread will not do anything to decrease stress at the start of the thread. It will however result in the bolt stretching more for a given torque, which may be how he "cured" the problem.

Here's how threads are rolled:  www.productionmachining.com/articles/5-f...bout-thread-rolling-

For a lot more useful information on threads and bolts in general, see www.boltscience.com/ - lots of good information on antivibration washers and bolt torqueing.

Ian

 

"the guy" ?

Did you watch the video? he never talks about thread forming, its all about stress concentration.

from boltscience - "High performance bolts are often designed so that the plain shank is smaller than the stress diameter of the thread. This is done so that the stretch that occurs under the preload induced from the tightening process is maximised. "

would this not spread the stress at the threads over a larger area by allowing the bolt to stretch instead of fracture, thus achieving his goal as stated in the video?