Quick Overview
375cc Hans Hummel big bore kit to suit the TZ250 4DP and TZR250 3XV V twin. This is a rare piece of kit and offers over 100 bhp - so is not for the amateur. We bought this as part of a larger package of TZ250 bikes and spares, so don't know the complete history. The kit has been in storage for many years, so is unlikely to be a simple bolt together job.... you'll need mechanical experience to set this up. As a minimum, it requires packer plates to set the correct deck / port height and there is bound to be some fiddling and bits to get everything built up right.
The crankcases on the TZ250 4DP/4TW and the TZR250 3XV come from the same Yamaha castings, so the HH big bore will fit both bikes, but due to the performance it is most likely to find it's natural home on the race track. The cases have been modified to accept the HH cylinders, a longer stroke crank and run the TZ adjustable ignition stator/rotor.
In the 1990s, Germany still boasted a healthy 500cc two stroke race series and those who couldn't afford the V4 500 opted for the 360cc big bore V twin. In standard form, the normal 360cc kit ran a 66mm bore x 50mm stroke, bit this one has a special longer 54mm crank, so is 375cc. The crank is freshly rebuilt and uses the later 5KE crank wheels with 4DP rods, to give the 54mm stroke. The crankcases have been trenched to suit the longer stroke and the mouth opened to accept the HH bigger cylinders. The cylinders have 7 transfer ports and monster exhaust ports, chrome is good, though the pistons included do feel a bit loose, so either it needs a rechrome or they are slightly too small for the kit. The cylinders and heads have been welded to take the extra torque stresses - see photos.
Kit includes freshly rebuilt crank x1, trenched cases x1, HH cylinders x2, HH cylinder heads x2, used o rings x4, new Wiseco pistons x2, rings x2, gudgeon pin x2, new powervalve blades and collars x5, powervalve caps x2, . Everything in the pictures is included.
Additional information: With a vernier gauge, the cylinder bores are approximately 66.7mm, the pistons are 66.5mm. Crank is rebuilt, I've had a look and it seems to have new 4DP rods, big ends, thrust washers and outer main bearings, the centre main looks to be a good low mileage used bearing. Crankcases are fine, no welding I can see. The powervalves are the same set up as the standard 4DP TZ250, so the wheels etc are the same. Piston gudgeon pin is 18mm O/D.
This is a full on race kit, so sold as a 'project' and comes with no warranty. If in doubt, please view before purchase.
