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OSSA top end suggestion


pmk
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Recently, there was a need to remove the cylinder from one of my friends 72 MAR 250s.

With the head removed, we expected the cylinder to slide easily off the studs.

P, time and water had corroded the studs and cylinder along the cylinders entire height.  We soaked the 4 studs for a couple weeks hoping to have the cylinder release.

When the cylinder refused to release and be removed, next I attempted to unwind the long studs.  While i could see several of the studs twiting as I tried to remove them, each would untwist back to where they originally were positioned. Seeing this I feared the studs would break causing greater problems.

Heat was even applied hoping the studs would release and unwind, still the studs and cylinder remained as one.

Ultimately, being extremely careful not to damage the cylinder, I resorted to an air hammer on the ends of the studs.  Yes this would ruin the studs but drastic measures were needed.

With two people, one holding the cylinder and the other operating the air hammer, we spent about 30 minutes with the first 15 minutes just barely getting the cylinder to move away from the base gasket.  Once we saw movement, we tried to unscrew the studs.  One removed easily, while 3 refused.  

Continuing, the cylinder moved away further and allowed a second stud to be removed.  The last two studs refused to be removed.  With care the cylinder was raised until the studs sat just proud of flush.

With the cylinder about 1 1/2" above the base gasket surface, the remaining two studs were cut while the cylinder liner was protected from damage by the saw.  With the studs cut, the cylinder removed easily.

The studs were able ro be removed with no cylinder damage, but did require a well supported cylinder and smart blows from a hammer and punch.

With short stubs of the remaining studs, a small bit of heat was applied and the reaning cut studs removed easily.

Some may not realize that by design, the 250 OSSA cylinder and head do not have a gasket sealing the studs to prevent water from filling the gap between the stud and cylinder.  The water can collect and remain causing corrosion.

When shoping for studs Alex Snoop advised using a high quality paint to minimze corrosion by painting the studs.  This will be done during reassembly.  Currently the entire motor is apart awaiting a crank rebuild, bearing and seals.

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I usually hang OSSAs upside down so no rust particles fall into the crankcase as I take the cylinder off.

That design is also a good reason to run a motor until it is warm after a wet ride or a wash

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2 minutes ago, feetupfun said:

I usually hang OSSAs upside down so no rust particles fall into the crankcase as I take the cylinder off.

That design is also a good reason to run a motor until it is warm after a wet ride or a wash

Good tips.

This motor was coming apart for a full rebuild, so rust in bearings was no concern.  For engines just seeing top end work, upside down to disassemble is good.

This engine came out of a recently purchased used bike, no way to know the past history, but it was certainly rusted studs holding the cylinder on.

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