You can tell a lot about a motor just from looking at the way it was reassembled, or at least that is what I have learned from this Pursang Mk4 motor. I got this bike last summer/fall after replying to a Craigslist post about a 1974 Bultaco Pursang/Astro. As soon as I saw the word Astro I new I had to have this bike project. Bultaco Astros are, in my opinion one of the coolest models. The paint job on the tank and tail section is outstanding, they have a long pedigree in flat track racing, they are just awesome to look at.
Unfortunately or fortunately enough, this was not the Bultaco Astro that I loved. The frame and motor were mismatched and the motor is an 250cc mk4 Pursang motor and the frame is a mk9 Pursang "Astro." I was so frustrated to hear that this wasn't the same as an Astro, but couldn't understand why the seller put Astro in the listing. It turns out that this year Bultaco was the transition year and includes some of the design choices that went on to make a true Astro what it is today. For instance the frame has steering stops and similar gas tank shape, the rest is all different.
But anyway back to the motor, I am kicking myself for not looking closer when I bought it, all the gasket surfaces are missing the gaskets and held together with a RTV gasket sealant. Yuck! The red/orange stuff is all over every gasket surface and needs to come off before applying the correct gaskets and getting this motor running again.
So... Much...Sealant.
The seller told me that the transmission would probably be ok if I just wanted to connect the piston, and button up the top end again, but it always pays to check when you get a basket case engine because clearly if the seller had the top end apart, something must have been wrong at some point, right?
And I am so glad that I did check because what I discovered really threw a wrench in my whole plan to get this bike running. I dropped my new crank bearings into the bore and they slid in a little too easy.
So easy that I was just able to pull the bearings right out with my fingers. That seemed a little too easy for me, every other motor that I have worked on the bearings are interference to prevent spun bearings. I talked to a few people on forums and they agreed that that was too loose and that I might have to search for some new case halves. Another issue that I discovered in the transmission is that one of the bearings was cracked on the outer races and the cracked piece broke off when I was examining it. I would have been pissed if I put everything back together and found a broken outer race the first time I changed the oil!
The new case halves came in and they should work, now I just need to change all the parts from my old engine to the new engine. I just about changed all the bearings in the motor but got hung up by one of the bearings in the transmission so I decided to take the whole motor to Moto-Scoot to help me get the last bearing in.
Stay tuned!
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