MB RT255 Reed Valve


The beauty of our Race-Tour cylinder kit - is its versatility. Why? There are many reasons. You can use any crankshaft you want and with no machining of the cylinder or welding un like all the other kits on the market. The RT cylinder will work with 58/60 x 107mm and 60, 61, 62, 63 and 64mm crankshafts with 110, 115 and 120mm con rods. You can use them as they are designed - as a piston port engine. As standard they are really ride-able, drive-able, reliable and easy to set up. Or if you want, you can do some extra road or race tuning work to suit you. With some modifications you can fit a reed valve to gain a little more power and increase the fuel economy. Not only that but there's other advantages - extra exhaust studs which can be used with our MBgm Clubmans or you can improve any exhaust by fitting a MB 4 stud flange. There is our MB head drop design which locates the head in the bore so its perfectly centralised and you can set the squish clearance correctly using no gasket or with our various thickness head gaskets. Also there is the extra 4 bolt holes for the head on top of the 4 original studs.

Here we are showing a Race-Tour 255 kit using a 71mm piston and 64mm crank. Originally this engine started life as a standard piston port cylinder, the customer had done some miles then the decided he wanted it reed valving. This happens quite regular and it's part of the beauty of our RT cylinders versitility. We didn't like the 1st small reed valve because it needed lots of work and welding to get the flow to work correctly. So at a later date it was modernised with a MB Shorty reed valve with some extra tuning and a rebore with ceramic plating.

There are various ways to convert a Lambretta cylinder to a Reed design we have tried many ways over the years with many reed blocks and manifolds since 1985. Below are photos of various stages of this RT cylinder. First fitted with a European small universal reed assembly, this required so much work to flow we stopped using them, shown in the photos. With most reed conversions it's best to use 360 degrees inlet port timing, this is done by modifying the piston to accept windows and cutouts. Beware some pistons are not designed for this and become weak and will break. Our MB UK made pistons and our MBgm forged pistons can be modified with no problems as they are so strong. Some aftermarket reed manifolds fit with no modification, but we are not keen to use any of these and prefer our flowed and shaped MB shorty reed assembly. For these to fit some fins need cutting/grinding away, this makes no difference to cooling as cooling is increased with the larger reed manifold. Once the fins have been ground to shape so the manifold fits easily the inlet port can be cut to be opened and flowed to it's maximum towards the base face of the cylinder. There's no need go wider, this just wears pistons faster. Some cylinders can be raised, the RT is always at the correct height. To get the advantage of the reed conversion a boost port is required, this is a port cut above the inlet port, set between the ring peg positions which can vary on different pistons. Ideally this port should be as close to equal in the middle of the transfer ports but ring peg positions control this positioning. The reed manifold needs bolting to the inlet port then grinding and flowing and matching to the inlet port. Transfer ports can be altered inside the bore as can the transfer to casing feeds as can the exhaust port. Keeping the exhaust port small helps low down town/2 up type riding, increasing the exhaust port lifts power higher in the rev range, just how big you go depends on how the engine pulls and works and how high the horse power figure you will get.  

The photos show stages of the work MRB does to the cylinder.  

































































We offer various reed assemblies and tunes depending on the cylinder used, please email for prices.

Mark Broadhurst 13.6.2013 mark@mbscooters.co.uk