SmedSpeed XS650


Engine Valves

One of the hardest parts to source for the XS650 is good quality valves. Yamaha stopped making valves years ago, and all the NOS (new old stock) have dried up too. It was entirely feasible to fit Yamaha XV 750 valves which are +2mm oversize, but they are also obsolete as well, good old Yamaha!

Kibblewhite (KPMI) in the USA make valves which are good, 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.

With this in mind Howard designed replacement valves for the XS650. He contacted Andy Grenside at G&S valves in Godalming who supply top quality valves to Cosworth and many other racing/motorsport firms. G&S used to make valves for Tony Hall of Halco going back twenty odd years now, so he knew he was in good hands.

After many design iterations he established 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 Howard commissioned are + 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. With regard to valve seat cutting, he liaised with a local engineering firm who have a Newan Contour BB valve seat cutting machine. These machines cut valve seats to complete perfection; they cost in excess of £75,000 plus tooling, and are usually the preserve of top end racing firms.

Together they  developed valve seat /valve shape profiles that allow for the best possible airflow that the XS650 cylinder head can provide. This technology is available soon for your rebuild, whether you want a strong running 650, or a high torque 750, or a killer 840/880, the best valves and seat cutting work will help you attain the goal you aspire to.

The Smedspeed valve set, made here in the UK, will be available in late February 2019.
They feature:
+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.

The cost is £128 per set of four

Although Howard is no longer running Smedspeed, he will be supplying those valve sets already ordered.

Made in the UK


Contact me for more details

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.