www.alpha-centuri.info
Last update 10 January 2021

Alpha Disc Valve Engines

The RD-Alpha Centuri 250cc twin

RDAlpha - Alpha Centuri on outside, Yamaha RD250 crank, Greeves scrambles chassis, to be raced

Work on this project started after the completion of the original Alpha#2 about late 2018. Based on the time it took me to get the Alpha#2 complete, this project will take ages too. I was so enthused by working on this Alpha#2 engine that I wanted one of my own, one that could be raced in BHR, one that could be risked. New crankcases were made in modern alloy with additional meat inside to give a close fit for the smaller RD_Yamaha crankshaft. A Greeves 1959 scrambles chassis was available, and used in the day by Alpha Bearings, so all good to go!
What could possibly go wrong?

2018 Here are the new crankcases, lightly machined just to get some datum surfaces, in the background are the alloy discs which will become the induction and the disc-valve covers. The original Alpha crankshaft I found quite fiddly so after due measurement I decided to build an Alpha with a Yamaha RD250 crankshaft.
Now the crankcases are somewhat machined inside, enough to accept the centre discs, and the through bolt holes. The three centre discs are outline machined so that the crankcases can be bolted up and machined as a unit. This work requires the barrel surface to be created and the barrel liner holes inserted. Then the crankcases are turned over to machine the inlet manifold flange, and bore down to create the induction tract. Finally, the gearbox mounting surface machined and through bolt holes created. Yes, the factory drawing is wrong! Holes machines out of position by 1/16". Does not match the Albion gearbox. Doh!
Disc valves are cut by CNC, four out of an A4 sheet of whatever.
The inboard disc valves are held captive in carriers which are keyed to the crankshaft. Between the two inner crank webs are two carriers with their discs and oil seals plus a centre bearing. So the carriers and the bearing sit inside the main centre induction machining, and two covers encapsulate the disc-valves. These covers have crowded roller bearings and support the carriers to create a five bearing crank. These components are the final test pieces to verify assembly and fit.
See here the disc-valve and a carrier sitting against the cover plate with the oil seal in place, against this go the centre bearing and the other carrier. The carriers are marked out with pilot holes by the CNC so that I can control the valve cutouts and vernier location holes with the keyway slot.
Feb 2020
The crankcases were machined to bearing size and a ground mandrel made to hold them tight, then the barrel surface was cut the best we could judge and this became the datum. Barrel spigot holes were machined using the CNC functions. Carb face and gearbox mounting face cut flat based on degree rotation from barrel face. With fingers crossed, we drilled a 19mm hole down into the carb inlet and down into the disc valve carrier. I had drawn it up with CAD, had angles and measurements that I was happy with, but still I had my fingers crossed. In the end you just have to do it.
March 2020 and the start of the covid lockdown - self isolate in the garage!
This is the point at which some kind of hours recording started, previous investment in the RDAlpha was probably 100 hours

The results were as good as I could have wished for, no major mistakes. The barrel spigot cut impinged into the top of the disc valve covers and this will give me a datum TDC location. The marker lines are the transfer port cutouts.
Now we will find out how good my CAD work was. Using a rotary table on my lathe/miller combination I cut a trapezoidal shape which is the port for the disc valves. What you see is unfinished, correct size to within a few thou, but now needs a finishing cut. The inlet tract is spot-on(ish).

After seemingly spending a week making jigs, I am ready to move forwards.
Here are the crankcases ready to be machined some more.
Time for a mockup of what it will become.
Crank centre section is nearly done, just a few more thou running clearance required.
Also being considered are mods to give this engine a 5 bearing crankshaft by putting bearings into the disc-valve covers.

Here is a home-made bearing on the left, loose crowded rollers in good Villiers tradition, on the right is a purchased 62x40 bearing slimmed until the tool did not quite touch in inner roller cage.
Yamaha and Alpha cranks, the Alpha was a five bearing crank, and I think the Yamaha beefy enough to be fine with three.
May 2020
Another check to see if all is as it should be, thicker base plate to match longer con rods. It was immediately obvious that the crank will not turn over as the con rod fouls the crankcase mouth, not unexpected.
Disc-valve pack assembled to dummy crank, seems ok and I have the choice of a 2mm carbon fibre disc (black) or a 1mm nylon disc (white) with steel as a reserve, and of course I can make some more from any material I like
Crankcases slimmed again on drive side, and test if the drive pulleys line up.
Sparks will be from the PVL that I previously used on the Alpha rebuild
Same twin spark PVL for Yamaha twin, new casing
On the timing side the fit is tight.
Yamaha outer flywheels with new shafts in EN24T with NJ205 bearings
These bearings provide for easy assembly, hold the crank well, I plan to hold crank with 2 thou of compression.
Left:Drilling the main bearing oil feeds
Centre: Boring the tapered inlet tract
Right: makeshift tapered reamer using hand reamer with blades reversed
June 2020 - Inlet with (now) tapered inlet tract
Carburation will be by DelOrto or Amal Mk2 or Mukini (all with 1400 main jets), or possibly 10TT9 with Gardner style flat needle connected direct to fuel tank
Maybe everything lines up now?
June 2020
Centre assembly nearly complete, does not turn like oiled-silk yet, still a bit tight.
Inlet timing IO=154 BTDC IC=26 ATDC total 180 degrees, max allowed for two cylinders from one inlet tract
Compare this with 190 degrees for Suzuki RG, 200 for Kawasaki KR250, and 229 for modern racing Aprilia, and Walter Kaden (Mr Original 2Stroke) 215 degrees in 1965 - surely an inspiration for the Cutler brothers if they knew that then, but I doubt that they did as that information would surely have been a secret.
Piston of choice at the moment is this forged racing Weisco piston from a YZ125
Pin is 16mm, height is 20, pin offset (to avoid rear port), exhaust lubrication holes
I liked the KMX125 pistons but they are for chrome bores only. Shame.
Suzuki RM125 pistons are nice, pin is 14mm so 3mm rollers required for 20 rod eye required, hence experiments with rollers below. These are the pistons fitted to the restored Alpha Centuri above, using crowded roller small ends in original Alpha rods which were honed out to suit.
With piston choice comes gudgeon pin size, and I wanted to see that I could use 14mm 15mm and 16mm pins at will. Rods normally have a 2mm caged needle bearing, but crowded rollers are good too, and 2.0 2.5 and 3.0 rollers with rounded ends are available.
Left: 20mm rod eye with 15mm pin using 2.5mm rollers
Centre: 22mm rod eye with 16mm pin using 3.0mm rollers. Hardened steel washers to protect pistons from needle ends
Right: two rods already shown together with standard caged 2mm rollers (20mm rod eye with 16mm pin) for comparison.
Holding the barrels is a problem, correct mandrel is required
I tried to make one, not very successful, so here is the purchased one that works well!
Barrel liner top lip (by design) too thick, now it needs to be trimmed to get exhaust port height finalised
Ignition drive is taking shape
Traditional tang driven shaft with PVL rotor on taper
Sparks from PVL twin spark unit (which used to power the Centuri rebuild)
Prototype drive shaft has hole to allow cranks to be split without removing TS crank web. Threaded to permit plugging. This feature may not be in first "production" crank build, but if I have to pull crank apart more than once I will change my mind.
July 2020
Looking ahead to engine plates, ground clearance and chain line need to correct.
 
Left: More an artist's impression of what will be, it does fit, and I can get the heads off without taking engine out of frame,
Centre: Exhausts will be either side of frame down spar, conveniently just less wide than the exhaust separation.
Right: With frame clamped in same attitude as on track, engine/gearbox top surface pretty much level, no obvious problems yet, can't last.
Engine held by straps, engine plates (too thin 1.5mm steel) by nails
Managed to buy a pair of exhausts for Yamaha LC250, I used to race an LC250 and believed them to be good candidate for close fitment.
Well, my guess was better than I hoped, they fit really well, not much cut'n'tuck to do here
Front engine plate may need to be stepped to give exhaust more room
Delay with engine plates while I wait for ordered 4mm electric steel plate cutters
... and my TIG welder is broken!
Stepped 3mm thick engine plates now completed
Inserts into barrel now bridge the angled gap between cylinder and pipe
Top gearbox mount and injury protection from broken chain to complete when TIG returns
Footrest hanger templates to finish
Frame will remain as tatty as you please until after brackets finalised and rolling road tested
Lean angle at 40 degrees and clearance is over 1" without rider, rubber mountings may pull pipes in further
How much it scrapes in action remains to be seen
August 2020
Last chassis work before returning to detailed engine build
Can the pipes be pulled in to maximum clearance, yes, with cutting and welding, but not until after rolling road tests
Front forks will have tapered roller bearings, see front stand bobbins and linkages for twin sided front 6" brakes
Individual cranks are now pressed and trued with 118mm conrods
Here is trial assembly with dummy bearings to get side-to-side shimming right
Next step is to complete assembly with disk valves keyed to crankshaft, and set valve timing
Crank now ready for assembly with disk valves when centre section gas flowed
Sept 2020
Crankcase transfer ducts are mostly complete, tedious dremmel and hand finishing to come, hours of fun!
Finish needs to be as 200 grit for best gas flow
Effort invested since March lockdown about 750 hours over 24 weeks. Would never have happened without lockdown!
Oct 2020
Left: a little difficult to visualise, view from piston down to duct controlled by disc-valve
Centre: Same duct from the side, as seen through barrel plate
Right: Exhaust stub, both ID and OD need adjustment, hence OD shim, new stubs to be made from thicker tube or solid
Final work ongoing
Left: Underside of barrels with base plate, final hand blending about to start
Centre: Crankcases and baseplate from top, almost complete
Right: And finally, a milestone, engine complete and under powered test with a little WD40.
Friction levels originally gauged by turning with cordless power drill with friction control.
Without valves driven, friction levels are negligable, several days ago the friction with valves was 15+
Hours with shims at 0.25 and 0.50mm later, and friction is 10+, more measurement and more hours ...
When I got to be able to turn/spin it with friction set at 7, I thought "good enough" for now
November 2020
Left: Dual spark PVL ignition driven by tang into slot in crankshaft
Centre: Initial start will be standard fitment DelOrto carb, 29mm choke. 1400 main jet
Right: and hoping to use Amal 10TT9 modified to work like a Gardner, to be connected direct to fuel system
Left: 10TT9 carb refitted but needs dogleg adaptor to reduce downdraft
Centre: Home-made head gaskets
Right: Engine about to be sealed, TDC volume is 12.5cc
2021 Here Already
Left: Engine loosely in 1959 Greeves scrambles frame (as used for RAS Silverstones)
Centre: Standard Yamaha LC250 pipes fit remarkably well, note 40mm square tube to act as lower frame cradle
Right: Frame has double sided 6" brake, engine awaits front engine plates, and tapered roller head bearings.