Pro-X Piston Clearance

[m in sc]

The prox pistons are not oem as they were in the early 70's. they are different. therefore.... how would oem specs apply? they really don't, thats just material and construction differences.  lc cyls temps are more stable and do run a bit cooler agreed.   Aircooled cyls also will contract quicker than the more temperate LC cyls. Less thermal mass = (ac) quicker to expand AND contract. 

I'll take the advice of these guys as well with modern pistons in mind, they are experts on this i would say....

https://www.lasleeve.com/downloads/piston-clearances.pdf


If they are Yamaha oem NOS pistons, different story. But not the case being discussed.

On all the bikes ive run, ive run these specs with prox and have never gotten a 4 corner seizure due to piston clearances. even when i ran the lc hot as hell a few weeks ago due to the timing it didn't hurt the skirts by expanding, but the domes were melted. Same with a few aircooleds over the years. Regardless,  people can run whatever they want. I'm pretty sure since prox sells these for banshees, they are covering their asses with the 'use oem specs' and not for 50 year old aircooled bikes in mind. If they were.. they wouldn't have the tabs still on them.. .02

[Arrow]

I agree with you, please don't think I'm not. I'm quite aware that the OEM clearance is not really achievable now, as it was in the day.
Just saying what was, back in the day.
If there is no reference to 'what was' then we have nothing to compare to.

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[m in sc]

Oh, I didn't take it that way. And.. for the record, wasn't bashing on the brits (my best friend is British actually) at all but there are some obstinate fellows in that group.

My whole point for my (probably unnecessary) rant was to hopefully prevent somebody from locking up their motor.

(I'm looking at you Frank.  )

[Arrow]

No worries.

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[teazer]

teazer,

actually, its the other way round, the aircooled runs hotter, so the clearance can be less, as the barrel and the piston are expanding at roughly similar rates so the clearance remains predictable.
watercooled barrels run cooler so the piston expands more in comparison and reduces the clearance more, i believe that to be the reason that recommended clearances for water cooled pistons become debatable when the piston is going in an aircooled barrel.

even the above quoted pro x paper states that the manufacturers recommendation supercedes all of that information.

just my 2 euros worth.

les.

Yes and no... Pistons and barrels expand at different rates and one reason is the ability to transfer heat from piston to barrel to cooling material (air/water) and more clearance usually means less heat transfer.  I wasn't really trying to calculate the actual expansion rates but rather just drawing attention to the many factors (variables) that we can't effectively measure outside of an engine lab.

My refernece to the UK wasn't to put anyone down.  It's just that thrashing a bike down a US freeway in summer is a very different thermal load that tooling around the average British road where temps tend to be lower and engines typically spend less time at high load. Lots of issues with those generalizations of course because that's the nature of anecdotes. 

But point taken and lots of other relevant input.     

[Jspooner]

I've had many cylinders done by a local shop and he uses .002 as well. Never had a problem. Not really sure how anyone can diagnose anything from a cell phone video. I think you should do just what Wade said and then tell the guy to shuv his phone up his ass and see if anyone can diagnose whats wrong with his brain.

[OnlyCrimnson]

How many freakin Prox pistons have been put in RD's? I'm gonna guess hundreds on this forum alone, if not thousands, and a lot of them probably had .002 clearance. Totally fine.

This guy can can hear a tenth clearance from a video?! 

[IR8D8R]

I remember Chuck harping many times on making the measurement on a piston for boring with any coating scrubbed off with Scotch Brite at the measurement spot. He was very adamant about it.

Do you think the spec sheet has the piston diameter recorded from measurements with the coating in place? I expect so.

RD's always sound a little like they have piston slap. It helps to put in the fin rubbers.

IR8D8R

[2 Smoker]

Maybe he is a professional safe cracker.

[SoCal250]

This is simple. I would follow info from Clough & Quenzler as gospel. They are experts, they do this for a living, and have been doing it many years. Why argue with more than 80 years of collective experience? For someone to make an accurate analysis of bore clearance with an internet video is just ludicrous (especially with cylinder dampers missing). How many engines has this UK "expert" built and how many years has he been doing it? How many championship engines and how many land speed world records has he accounted for?

PS - I agree with Joe's comment above.

[Jspooner]

PS - I agree with Joe's comment above.

I've been waiting for someone to comment on that. 

[85RZwade]

Well, when you're right you're right 

[m in sc]

maybe he needs one of these?

[dgorms]

Marc, if that's a "shit for brains" detector........................don't point it at me.....................D

[Yamanatic]

Hi everyone - long time no see!
Some observations to consider; forged pistons 'grow' more than cast pistons. A loose top end is faster than a tight one - race motors are fastest right before they blow up from being worn out. Those little rubber buttons Yamaha put between the fins on the A/C 2T motors were to quiet piston slap and top end rattle, which are not necessarily bad noises. Don't worry as much if a top end makes some noise; when the noise goes away is the time to start worrying.

Warren