Piston Comparison

[rablack21]

John, how did you find a Vertex piston for your 250r? I couldn't find where their website showed one. I like some of the technology they are using in their piston. The silicon content to decrease thermal expansion so the bore clearance can be tighter and decreasing piston skit wear. Then the use of the Molybdenum Disulphide coating to also reduce overall wear. Not sure if they are up to snuff or not, but they certainly have some intriguing tech they are using.

[C-Leigh Racing]

I dont know about that loose of a piston clearance guys, not on a Pro-x cylinder & Wiseco piston.
I set my daughters 330 up 0.0025, on an old junk resleeved Pro-x cylinder, I had got out of my shop trash can in 2004. I broke it in myself for several days. I did do, the heating process on the piston before I had the cylinder bore & fit. That engine ran from 04 to 07 TT racing & only thing touched on it was a head dome replace from me cross threading the plug.
That engine build, was during the time, there were so many saying you cant make those big bores stay alive. I put everything I knew into building it & it did live.
I have a paper somewhere, listing all the hours, gallons of race gas/oil mix, number of laps that engine endured at local & national TT race events through all those years & it was staggering what it totaled out to be.
During that time, she won an AMA National open class TT championship with it as well, so it was not a trailer queen, that old R was seriouslly beat on.
Neil

[rablack21]

I'm not exactly sure what the cutoff would be for using .003 clearance pertaining to the piston size though, Neil. 330cc may be just under that threshold. I do believe exactly what you said about you engine and respect you opinion, but I had a different experience.

I had my first 363 piston set at .003 (technically .00295 by the bore man).This was using a Wiseco pro lite piston. I ran through 10 cycles on the engine prior to even getting on it and riding it. Then I rode it gently for the first time varying the throttle no more than 50% throttle. The engine was running plenty rich and plenty of oil. During the first ride it seized at 50% throttle. Tore it down, and it had a picture perfect four corner seize. Bore was too tight. And I use an amazing bore man. Nobody babies their engines more than me during break in. The four corner seize pretty much tells the story. I then checked with Wiseco and other engine builders. The cumulative recommendation was .004 clearance for that size and type piston. So I had the piston honed and ordered a new piston. Bore man set it up for .004 exactly and it ran fine ever since. That doesn't make this gospel, just been my experience.

[udontknowme]

I'm not exactly sure what the cutoff would be for using .003 clearance pertaining to the piston size though, Neil. 330cc may be just under that threshold. I do believe exactly what you said about you engine and respect you opinion, but I had a different experience.

I had my first 363 piston set at .003 (technically .00295 by the bore man).This was using a Wiseco pro lite piston. I ran through 10 cycles on the engine prior to even getting on it and riding it. Then I rode it gently for the first time varying the throttle no more than 50% throttle. The engine was running plenty rich and plenty of oil. During the first ride it seized at 50% throttle. Tore it down, and it had a picture perfect four corner seize. Bore was too tight. And I use an amazing bore man. Nobody babies their engines more than me during break in. The four corner seize pretty much tells the story. I then checked with Wiseco and other engine builders. The cumulative recommendation was .004 clearance for that size and type piston. So I had the piston honed and ordered a new piston. Bore man set it up for .004 exactly and it ran fine ever since. That doesn't make this gospel, just been my experience.

i dont see much correlation between piston size and cold piston clearance. what it boils down to is the amount of expansion of the piston vs the amount of expansion of the cylinder. if you needed .004 to keep from seizing, that suggests to me that the amount of expansion was quit different between piston and cylinder. the old 500cc plated cylinders only called for .002 cold. piston/cylinder expansion must have been very similar

[udontknowme]

one other thing, i dont care who bores the cylinder, you can bet your bottom dollar im putting my bore gauge in there to double check the work

[Jerry Hall]

i dont see much correlation between piston size and cold piston clearance. what it boils down to is the amount of expansion of the piston vs the amount of expansion of the cylinder. if you needed .004 to keep from seizing, that suggests to me that the amount of expansion was quit different between piston and cylinder. the old 500cc plated cylinders only called for .002 cold. piston/cylinder expansion must have been very similar

A plated aluminum bore has a thermal expansion rate very close to the thermal expansion rate of a pure aluminum bore.   An iron sleeved bore in an aluminum casting has the thermal expansion rate of a solid iron cylinder but close to the weight of an aluminum cylinder.  


The thermal expansion rate of aluminum is almost 2 times that of iron.  Because of this fact, plated aluminum cylinders typically run tighter piston to cylinder wall clearance than when using the same piston in a iron sleeved aluminum cylinder to get the same operating clearance in a  running engine.  

I will go into more detail on this subject when I have more time.