SmedSpeed XS650

 


Engine Valves



One of the hardest parts to source for the XS650 was good quality valves. Yamaha stopped making valves years ago.

Kibblewhite (KPMI) in the USA make good quality valves and valve springs, but there are a few minor design issues which they have not addressed completely. Notwithstanding these somewhat minor issues, they are expensive at nearly £50 each by the time they are landed here in the UK. Recently KPMI stopped supplying the spring kits that allowed the use of stock top collars.

Smedspeed in conjunction with G&S valves have designed replacement valves for the XS650. G&S valves in Godalming are well known and supply only top quality valves to the motor industry including Cosworth and many other racing/motorsport firms. G&S used to make valves for Tony Hall of Halco going back over twenty years, so we knew we had found the right supplier. 

After many design iterations a valve set that will perform very well in the XS650 engine. Although primarily designed for 750 and larger engines, they will work perfectly in smaller displacement engines of 650/707cc.

Engine valves

The stock XS650 cylinder head was designed in the days of “big ports” and reflected the then current thinking of racing engines of the time (G50 Matchless/ Norton international etc.) The valve seat internal diameter (ID) to seat size ratio is too large at around 90%/91%. This makes cutting a five angle/radius seat very hard, if not impossible, as the metal required is just not there. The ideal solution would be to make the seat ID smaller but, as this is not possible, the only option is to make the valve larger - not massively larger, just big enough to enable the correct seat ID to seat ratio.

The valves + 1mm on the intake and +0.75mm on the exhaust. These slightly larger dimensions will enable a five angle seat to be cut on the intake, and a constant radius seat for the exhaust, and give the correct ratios too. 

 The Smedspeed valve set, made here in the UK features:
+1mm oversize inlet
+ 0.75mm oversize exhaust
The design of the valve will flow better than any stock or replacement valve
The valve-head back angle reflects modern high performance valve design
The valves are true one piece stainless steel forgings
The valve ends are stellite tipped giving very long life (as OEM)
The valves are plasma nitrided for long stem life; they can be run in cast iron and bronze guides with no problems (unlike some)
Valve inlet and exhaust stem diameter at 0.315’’
Longer overall length to ensure that the stem protrusion figure is correct without sinking the valve into head

The Smedspeed XS650 valves will only be available as a set (4) as they will not be compatible with stock or other replacement valves, due to the design features listed above.

To compliment the valves, Smedspeed designed beehive valve spring sets are available,  made here in the UK. The spring and top collar save considerable valve train weight, reducing the chance of valve bounce if you are using the top part of the rpm band a lot. They are suitable as stock replacement items also, as the seat and nose pressures are only a small amount higher than stock.

Valve springs
The Smedspeed spring and top collar are 59 grams, the stock OEM items are 100 grams. The distribution of the reciprocating mass is better still, with the Smedspeed top collar being 12 grams as opposed to the OEM 21 grams. The top collars are manufactured by the same supplier as the valves, and are steel for long life and reliability.

Contact me for more details

DISCLAIMER
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.