This is a overview of cleaning and rebuilding this 1975 Harley-Davidson SX250 dirt bike engine, originally manufactured by Aermacchi company in Italy. My friend asked me to sell it for him and I looked at it and did some research on whats out there to get some kind of price and what not. I saw that most people just took these engines and put them up for sale however they found them. I decided that is bullshit and made the decision to take this one apart and put new crank seals etc…
To be on the up and up, I have called this a rebuild, it was more like a take it apart and clean it and replace seals and stuff. The engine was in really good shape and needed some basics. I did this so that someone who might want to buy it, isn’t buying something that’s in unknown condition and quickly becomes a money pit. It’s quite a few hours of work and since I’ve been building engines for almost 40 years, I’d figured I’d do it. It is for sale and can be found here at CosmicMoto.net
The engine was dirty, but had been stored inside for at least 20 years and had more than likely been sitting around before that. While nothing was really bad, with exception of the clutch and sprocket, it was very dirty inside and those crank seals were old and hard as I expected, and that is not good for a 2 stroke.
Everything got cleaned up and all the bearings checked and they were all in very good shape. The original clutch was toast, and I couldn’t find Harley SX250 plates. So I had a new Suzuki clutch from EBC and it was almost a perfect match. I had to trim the drive tabs down carefully and I also had to add another steel and friction plate to make the stack height of the clutch match the stock stack height. It worked out perfect and this way I didn’t have to put 40+ year old “new” clutch plates in. I also took the transmission completely apart and inspected everything… all of it was in good shape.
I checked out the crankshaft and it was all good, wristpin and big-end bearings are good, and i assembled everything into the right hand side case. I sourced some new SKF seals for the crank, transmission and shifter and also bought a gasket set.
Next was the top end. The piston, rings and cylinder are in very good shape and seem to be standard, stock size. Very little scuffing. It all went together as expected and there is a very good seal in the lower end, as can be observed by a piston port 2 stroke. As far as the carburetor goes, I cleaned it out pretty well, once again so the buyer can see what is really there without wasting time. I had no intentions of actually rebuilding it. What is there is in good shape.
Last was to check was of the electrical system. I had to repair a wire that runs between coils, but the coils look perfect. I checked the coils with a meter and also put the coils in and assembled the rotor and spun the engine with an oscilloscope attached. From looking at the scope I would expect the electrics to be working as intended. That includes both coils for charging and ignition and the small pick up that triggers the CDI box. To be clear, I do not have a CDI box.
Finished Engine
