Two Stroke Racelab

First race for the mc28 project, mediocre results but keep working on it.

Samarin is a company specialized in the construction of connecting rods and crankshafts and they are based in Spain. They undertake the development of stock conrods as well as custom ones. Here is an example of a special conrod that allows the use of later NX4 pistons on Honda Rs125 NF4 bikes. This conrod has 105mm centers (stock is 104mm) and uses a 15mm wrist pin (stock is 14mm) hence later NX4 pistons can be used in pre'95 era Rs125 bikes without any cylinder modifications. I believe Samarin also constructs connecting rods for the Honda Nsr250 bike but Tuck(NZ) knows a lot more than me about this subject, so check out his post at  edetuckracing.blogspot.com .

After about 10-12 working hours and several attempts on pipe routing, it seems like I now have a layout to follow. Templates are already drawn on 0.7mm mild steel sheet and are ready to meet the tin snips... Estimated weight of the expansion chamber, including exhaust flange and springs, will be about 1.500Kgrs.

I was offered to construct an expansion chamber and it's been more than a year since I've built one, so I couldn't resist.  For a start, I only have a pile of numbers and dimensions that will eventually take shape and form an expansion chamber for a Yamaha Z125 bike.  Edit :  Big thanks to Vaggelis (GR) aka  ½   Racing, for posting a schematic of the expansion chamber designed  in EngMod2T simulation program. 

...modifying YPVS control boxes for Jan-Mikael(FI), Jason(UK), Russ(UK) and repairing a PgmIII ecu for Nick(GR).  Check the online store link at the left hand side of the site, for more modifications - services. 

Rs250 wheels can be used on a Nsr250 mc28 but there are a couple modifications that needs to be done.  The front wheel is quite straight forward and you will only have to fabricate a 14mm spacer. All other parts can remain stock Nsr250 parts.  Rear wheel installation is a little bit more complicated.  The rear wheel collar should be changed with a NX5 one and the part number is 42606-MW4-000. At a first glance, it looks exactly the same as the mc28 item but it has a coned shape on the inner side that centers the wheel. Refer to the pic for the spacers, the titanium rear wheel nut is costly and optional though looks tricky and it's the cherry on the pie nonetheless.  Also, the brake disk bolts should be changed and fitted in the opposite way or the wheel should be machined as bolts are protruding out of the rear hub and they will foul the wheel.  The brake caliper will not fit with new rear disk pads and the center part of it needs to be milled about 2mm.   No bud

A nice addition to your toolbox is a set of fillers, for measuring piston to cylinder wall clearance.  They come in a variety of thicknesses but you only need a set consisted of 0.002, 0.0025 and 0.003 inch fillers. In metric system, above fillers measures about 0.05, 0.06 and 0.07 millimeters respectively.  You will have to measure the cylinder wall clearance at a 90 degree angle to the piston pin. Fit the fillers starting from the smallest size and install the piston until it slips into the cylinder smooth and tight and this will be your clearance.  Consult your service manual for suggested clearances.  Use several pistons to get a feel of how the fillers react to different clearances and you will quickly realize that blindly fitting pistons, even when all parts are new is not always the way to go.  A very handy tool that can save you a seizure or other engine damage, especially when you are changing pistons in the pits.