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Urban Myth Lore


billyt
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Like many of you on this site I have been around the trials scene for over thirty years now.  

Heard many urban myths, street lore, snake oil, and some basic nonsense about mechanics of trials bikes, their engines, carbs and suspension.

In your opinion what are some of these myths and lore that are off the wall?

Love to hear your experiences on this topic.

 

 

 

 

 

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I had this lad Tom Cholmondeley (Google him) at a wee club trial in the middle of nowhere.

Rumour has it his 4rt had a rifle mount on it.

Sadly he's passed away and no way will I take responsibility for him twisting his hip on section 4.

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5 hours ago, breagh said:

I had this lad Tom Cholmondeley (Google him) at a wee club trial in the middle of nowhere.

Rumour has it his 4rt had a rifle mount on it.

Sadly he's passed away and no way will I take responsibility for him twisting his hip on section 4.

His trials story is the greatest never told :D but don't go on his land whatever you do :blink:

 

If you google him and trials, you'll not get what you expect :lol:

Edited by cabby
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2 hours ago, oni nou said:

If you use a water-cooled piston in an aircooled engine it may seize up because of the tighter tolerances used on an water-cooled engine.

No, this is true. I read it on here so it must be ....

Edited by b40rt
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  1. SuperMoto

     

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    I always had to run our YZ490 dirt track engines (same basic engine as the air cooled wr500) at about .006" piston to wall clearance when using forged pistons like Wiseco. We found the air cooled barrels distort so much when you run them hard, that anything less would cause the piston to tighten up on long straights. The bores just don't stay very round when they get hot so it takes extra clearance to keep everything working together. Big air-cooled engines aren't well behaved like the new water cooled stuff. It sounds like marbles in a coffee can when it's loose and cold, but it always worked well for us.

    It sure is a lot easier to go fast with the newer stuff .
     
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I suspect freddy is pretty close. For example Porsche specs their 90mm (3.5433") piston to bore clearance at approx 0.003". This is on a precision engine that can survive on a racetrack off the showroom floor, it is also an air cooled cast aluminum cylinder with a coating and a forged aluminum piston both of high quality alloy content. Running a cast iron cylinder with an aluminum piston will definately need more clearance and it also depends on whether or not it's air or watercooled as the aircooled run a little hotter and tend to warmup faster. I remember my old stroker Ford Windsor, with street mufflers on you could hear the piston slap for a few moments til the pistons expanded.

Lose is like a worn engine an might smoke a little, too tight and the piston seizes or badly damages the bore, you decide. http://bbs.homeshopmachinist.net//wink.gif

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This has alway been an issue. Motorcycle piston clearance vs American V8.

Typical Honda water cooled piston clearance is .0004 to .0015" new, service limit .002"

Have a cyclinder bored by a machine shop not use to those numbers it come back at .003 or .004". Already well past the service limit or more than twice the spec.

Typical air cooled Honda runs. .0004 to .0017" with service limit of .004"
Example 1986-1995 XR250 R .0006 to .002" service limit .004"


Wonder why some engines have piston slap cold ...........start with .005 or more for clearance.:eek1

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