We love when our readers reach out to us to show off their projects – and Josh Mills’ unique Yamaha engine project caught our eye immediately upon opening his email.
“I have always wanted to go all out on a motor project but could never afford to do this with an eight-cylinder. I scaled down my dream project to two cylinders and was able to make something happen,” he says.
He’s built this wild piece based on an engine from a 1979 Yamaha XS650, but he’s touched every piece inside to turn it into something truly unique.
Nearly the entire engine has been machined and built in his home garage – he only farmed out the items that were absolutely necessary.
The crankshaft in this engine comes apart in four separate pieces and is splined in the middle. In a two-cylinder like this, one piston is normally on the downstroke as the other is on the compression stroke, but Mills has done what’s called a “277-degree crankshaft phasing” to optimize performance.
“I offset the crank throws by 83 degrees – 90 degrees isn’t possible without a custom center spline or sourcing different crank flywheels from other models. This leaves one piston at max piston speed at any given rpm while the other starts and stops at TDC and BDC as opposed to both pistons starting and stopping together in the stock configuration,” says Mills.
When it came time to select connecting rods, Mills stepped out of the Yamaha book and into the Honda catalog to make a marriage of parts. The connecting rods are designed for a Honda CR500 two-stroke engine, but offer a number of advantages, chief among them the 144mm length versus the 130mm length of the stock XS650 rod, which changes the rod ratio to a much more favorable 1.9:1 from the factory 1.5:1 ratio.
“The rods also came with hollow big end rod pins for a nice weight savings. I also chose to retain the two stroke wrist pin needle bearing assembly despite the nay sayers. I feel that the wider little end will provide plenty of support over the stock unit,” he explains.
With a plan for street/strip use in mind, 4032 forged aluminum pistons were selected from Wossner; these use JE piston rings. The use of these pistons allowed Mills to select much narrower rings (1.0mm/1.2mm/2.0mm) than the factory design – and reap the rewards of lower-tension rings in the process.
“I got the ring tension down from 6.25 pounds top, 6.75 pounds second, and 6.9 pounds oil to 2 pounds top, 2.125 pounds second, and 4.25 pounds oil. The material and design I went with was steel non-direction for the top, cast iron Napier for the second, and traditional three piece oil,” says Mills.
The Wossner piston uses a shorter wristpin–for more weight savings–that has a stepped internal diameter, but uses the same outer diameter dimension and is still full-floating like the stock unit.
By choosing this piston, pin, and ring design, Mills saved 14 grams per over the stock cast piston assembly’s weight to improve RPM capability substantially. Compression was raised to 10.0:1 with this setup. A custom .040-inch-thick Clark copper head gasket is used.
The oiling system is mostly stock, save for a modification that helps improve the oil spray. A new stock pump was used along with an oil cooler that’s been added to the mix.
The rotating assembly was subsequently balanced as two separate one-cylinder engines, and Mills drilled, inserted, and welded in the heavy Mallory metal to finish off the balance.
What would a home-built engine be without a home-ported cylinder head? Mills removed 2.5 cc’s of port volume from the intake side, and 1.5 cc’s from the exhaust side, and picked up 21 cfm on the intake and 9 cfm exhaust on a local machine shop’s flowbench. The intake ports now flow 211 cfm at 28 inches of water and .500-inch lift – a number that rivals some factory small-block V8 cylinder head ports.
“I removed the guides for the port work, shortened them on both sides to work with the high lift cam and increase port flow on the other side. Finished the head up with a three angle valve job…or really two angles and a radius. The factory valve job has a radius on the top cut,” he explains.
A Megacycle camshaft with .440-inch lift and 271 degrees of duration at .040-inch lift was used, set up on a 103.5-degree lobe separation angle; it’s a single-pattern camshaft with a symmetrical lobe, but Megacycle performed some modifications before sending it out.
“The cam has been cut in half and turned 41.5 degrees (advanced) on the number two cylinder and welded back together to match the rephasing of the crankshaft. I pressed on an adjustable cam sprocket and dialed the cam in four degrees advanced,” says Mills.
The dual springs from R&D Racing are a non-progressive design that use aluminum retainers. The intake springs measure 135 pounds of pressure on the seat and 282 pounds wide open.
Even more custom touches about in the area of rocker arms. As there aren’t any available for this engine platform from the aftermarket, Mills did what any enterprising hot-rodder would do – he adapted pieces from another application; in this case, Porsche–where he found suitable rockers produced by Mahle.
“I had to remove the radiused rocker arm tip, grind the end of the rocker arm for clearance and install some swivel feet adjusters to keep the valve stem tips from pitting,” says Mills.
“Despite adding a heavier adjuster the grinding that was done for clearancing saved me 5.7 grams once the rocker was fully assembled and compared to the stock unit. I finished the rockers off by regrinding the slider follower ends on a valve grinding machine using a rocker arm attachment along with some pieces I made to make the job easier.”
He re-used the stock 38mm Mikuni carburetors to provide adequate fueling. However, just as he has with the rest of the engine, Mills took matters into his own hands to help improve it.
“I purchased some velocity stacks for them and using an old Chrysler engineering formula for computing runner length ram effect in the third harmonic decided to add 3/4-inch carb spacers to get some additional cylinder filling near my estimated peak torque of 6800 rpm. Total runner length is 12.250-inch from valve job to a space in dead air that is a quarter past the trumpet diameter,” he explains.
Ignition is handled by a Pamco dual Hall Effect sensor electronic ignition with high-output coils.
From the looks of it, Mills has built one heck of a motorcycle engine; using a bit of hot-rodding ingenuity and a bit of science, he’s managed to correct the factory’s deficiencies along with improving the performance of the engine. A taller fifth gear has been added to the transmission, with a plan for potential runs at the Texas Mile or Bonneville if the opportunity arises.
It’s science, people!
If you’re interested in submitting your garage-built engine to Homegrown Horsepower, send a few quality photos and the details to [email protected].