SmedSpeed XS650


rephased crank and camshaft

The Yamaha XS650 responds extremely well to being rephased. The benefits include vastly reduced vibration, a significant change in inertial torque production over the 720 degree 4 strokes cycle, and a slight gain in midrange power. All this plus an attractive offbeat exhaust note that sounds good. It's similar to 83 degree V twin, as this is the configuration it emulates in sound and power delivery. The most common rephase angle (and the one I use) is 277 degrees, with the right side leading the left by 83 degrees. This method allows the cam timing to be set up on stock marks, and the ignition timing to be set from stock marks as well. It is possible to rephase an XS650 to 270 degrees which does give better balance and ever so slightly less vibration, but be aware it requires extensive machining work, and the use of two matching cranks, and a custom splined pin. For a slight improvement over a 277 rephase it's really not worth the massive investment in £££.

To perform a rephase I dismantle the crankshaft using a Genuine Yamaha press tool, and proprietary press tools designed and manufactured here at SmedSpeed. I clean and inspect all parts, noting any parts worn beyond service limits, this entails measuring and inspecting the four main bearings, and the connecting rod small and big ends. If all is OK, I realign the crankshaft; press it  back together, and true to Yamaha minimum specs <0.0002". As a precaution against any movement I TIG weld the crankpins to the flywheels, in two short arcs.

rephased cam kit

In addition to disassembling the crankshaft and re-phasing it, the camshaft and ignition also need changing for the revised 277 firing angle. In the past the accepted method to address the rephased cam necessity, was to cut a cam in half and either weld it back together or cut it and pin it.  A new one piece cam is the ONLY way to complete this conversion properly, cutting stock cast cams in half and re-welding them is a cowboy bodge at very best.

Smedspeed supplies only one piece EN40B billet steel camshafts, utilising an adjustable sprocket as pictured. I have masters of nearly every grind made for the XS650 from the standard XS1 grind to all out race cams, and nearly everything in between.  The cams were designed at Smedspeed and are manufactured here in the UK by a long established and respected cam manufacturing firm. 

Cams are available in both 360 and 277 formats; the adjustable sprocket allows the builder to “dial in” the cam to exactly what he requires without the hassle of removing/moving/replacing the sprocket as per the OEM design. The adjustability feature is extremely useful for moving the torque up or down the rpm band, it allows to a limited extent a wild cam to made milder and vice versa.

The new cams require the cam follower/rocker arms to be in very good or new condition, running worn rocker arms against a new cam could seriously damage both items. I offer re-conditioned items on an exchange basis. The materials combination chosen will ensure long trouble free camshaft and cam follower/rocker arm life.

Rephased Cranks

Be aware rephased engines rev a lot faster than a standard 360 configuration, due mainly to the fact that at no time are both pistons stationary. It is not uncommon to 'accidentally' find the rev counter in the red zone.....

Cam and crank kit

Rephased lightened crankshaft

The benefits of rephasing the XS650 are well documented now. We have raced a rephased 750 sidecar outfit for three years and broken nothing in the engine or the chassis as a result of the vibration, despite regularly running the engine to 8400rpm. There are still some that say it does nothing, but they are obviously in some sort of denial, me-thinks the sort of people who still think acetylene lighting can’t be beaten!

When the XS650 is rephased, not only does the vibration reduce by around 40%, but the engine gains rpm faster (revs quicker) as a direct result of the rephasing.
In a standard XS650 engine, when one piston reaches TDC, so does the other; for a moment in time (a very short moment) the pistons are neither going up or down, and stop completely (momentarily). The only thing that “pulls” them down the bore is the inertia of the flywheels/crank. Therefore the mass of the crank and its associated moment of inertia is important in helping the engine run properly and smoothly.

However when the engine is rephased, only one piston is at TDC at a time, the other piston and con-rod are moving nearly flat out down the bore or up the bore depending on where in the cycle it is.  On a rephased engine the moving con-rod and piston become part of the inertial mass of the crank.

This unforeseen “increase” in the inertia of the crank gives a much smoother tickover and low rpm manners; a few of the bikes I build have been run with no alternator at all, just to see what effect it has on crank inertia. The reduction of this big lump on the end of the crank actually makes the engine run better and this led us to thinking about a reduction in the actual crankshaft mass (mainly for solo bikes and racers/street racers). This lightening, if not taken to extremes, has no detrimental effects at all, no loss of tickover, and low rpm smoothness.

Pictured below is a crank I recently lightened for a Flat-track bike. The actual reduction in mass is quite small (under 0.5Kg, this is less than a 5% change to the total mass of the crank and con rods) but as the metal was removed from the outer diameter, it has a much greater effect than the small amount would indicate. This is not a radical reduction of material at all, however it will make the engine rev quicker, whilst still maintaining Swiss watch tickover.

The reduction in diameter has another bonus; it keeps the crank just a few more millimetres away from the oil which drags the life out of any crank. Shell Thuett told Yamaha back in the 70s that the oil level was too high and was viscously dragging the crank acceleration down, he was right all along!

It’s ever so cheap to do, and when we checked the balance it has had almost no change from a stock crank. If you want to do it, I can give you the figures.

lightened crank1

lightened crank 2

lightened crank 3

These are experimental parts intended for racing and as such no warranty is given or implied. If you decide to buy it is on the understanding that you assume complete responsibility for any possible injury, loss or damage that may occur as a result of using these parts and that you accept these conditions of sale prior to purchase.