Pro-X Piston Clearance

[Economy Cycle John]

What's the general consensus of piston to wall clearance for Pro-x pistons in an AC RD? There is no acutal spec as these are technically watercooled pistons. I've always used .002 for most rebores. Both Scott Clough and Garrett have done lots of boring for me and .002 was ok with them as well. Confirmed that again with Scott today. And Chuck Q has posted here saying .0019-.002 is fine as well.

I'm dealing with quite the shit storm on the air cooled UK forum with some of them insisting .002 is wrong causing piston slap and should be per the original Yamaha spec of .0015 and the've told my customer the same as well.

They diagnosed this from a cell phone video the customer posted, the bike had no cyl/head dampers.

And of course my customer now thinks his bike had a bad bore job thanks to them and is busy dragging me online in at least 2 forums in spite of the fact his bike runs great.

I've never ran away from these kinds of issue becuase I want to do the best job possible for my customers and if I'm wrong I'll man up and fix it for free, I just don't think I'm wrong and .002 is quite acceptable. 

[sav0r]

If there is slap there is too much clearance. .002" seems pretty well understood. I bet whomever did the bore read the paper with the pistons and did .003".

[Economy Cycle John]

5port on that forum says he can determine clearance by ear to the 10 thousandths i.e. .002, .00175 .0015, etc. He said he listened to the video, determined the clearance was .002 but that is too much for prox pistons in an AC RD and is the reason there's piston slap (there isn't). He says the original yamaha spec of .0015 is the correct spec and at .002 there's piston slap.

[m in sc]

I had someone tell me once (prob the same guy) that the 'rings in my motor were rattling and it was going to blow up'. this was on the lc topped bike... also by a cell video.    that topend was fine for 6 years until i blew the headgasket which was almost 8 years ago.


anyone that can 'hear a spec to the .0000 place' should stay off the forums and be busy saving the universe from Lex Luthor.

I used the prox spec. oem pistons are slightly thicker than the modern ones, so i spec it to the that due to the material composition and construction of the piston.  the guy that has bored all my motors for years always uses the piston mfg specs and has never gotten one wrong IMHO. 

https://www.pro-x.com/wp-content/uploads/2014/03/Piston-Kit-Instructions.pdf

[mnein]

Can tell clearance by ear to the nearest .0000? Really?

[Striker1423]

All except my prox top end from the 250 were loud. I attributed it to Wiseco pistons. Never really had a clearance issue.

[85RZwade]

John, maybe suggest your customer have a reputable, established shop/tuner/machinist pull his top end and make the call. If that party says the clearance is wrong, you pay the bill. If it's right, the customer is on the hook. 

[teazer]

Using pistons designed for a water cooled motor in an air cooled top end can be an interesting exercise.  Let's talk about some of the differences.  Air cooled top ends tend to run hotter so they need slightly more clearance than in a comparable water cooled top end.

On an RD that's ridden fairly hard without careful warm up, the chances of a 4 corners seizure rises significantly, so I tend to increase clearances a little on any ported motor.

The flip side of that is that when an AC motor is carefully warmed up it will tend to have a hotter barrel which in theory will run a little looser than a W/C.

So there are lots of variables.  My take is that Chuck Q and Scott C both say .002 is good, then .002" is good. They are experts and know what they are doing. I get what the OP is saying about the change in noise as clearance increases and it's interesting and until he can document that, it's just another internet story.  Stick to what the experts say and if they say 2 thou, then that's what I would use.

The internet unfortunately is a dangerous place where it's easy to harm someone's reputation.

Out of idle curiosity,  are those "experts" in the UK or the US where running conditions tend to be very different?

[m in sc]

I would guarantee its the UK.   

fwiw.. having both ac and lc motors i've never seen a need of difference of spec using pro-x between the 2. Just haven't.  But, to that point, then you would think the air-cooled bikes would require MORE clearance, right?    their logic just doesn't hold up. and, the piston materials now is different than they were in 1974. 

[Striker1423]

Seems like a headache that isn't necessary.

[m in sc]

Seems like a headache that isn't necessary.

or.. fuck that guy.   

[1976RD400C]

I think any shop that is doing work for a customer is asking for trouble setting  clearance at .0015. If that bike's timing or jetting is not quite right or gets run hard and hot, it could lock up and the rider end up on the ground. Then what would they post about the shop on the internet?  Also, the sound in videos seem to make engine noises more pronounced than standing next to it listening.

[Yamaha 179]

Several years ago when we were racing (air cooled) TD3s and TR3 we had trouble sourcing TD and TR pistons but could find TZ (water cooled) pistons with the same exact bore sizes.  I was concerned because there might be differences in the metal the pistons were cast from and asked Kevin Cameron about the situation.  He told me the percentage of the metal in both pistons, silicon and aluminum, and they were different but he said that I shouldn't worry; run the TZ pistons in our air cooled bikes.  He knew that we tended to run a bit on the rich side, as insurance, anyway but didn't think the mix meant anything.  We did run the TZ pistons and had an occasional seizure but it was for other reasons, mostly brain dead tuners/riders.  Of course these were OEM pistons, not aftermarket, so who knows about that.

As for determining piston clearance; I think I'd rather rely on my dial bore gage and micrometer than some idiots hearing.
Lyn Garland 

[elliottles1]

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.

[Arrow]

I would guarantee its the UK.   

fwiw.. having both ac and lc motors i've never seen a need of difference of spec using pro-x between the 2. Just haven't.  But, to that point, then you would think the air-cooled bikes would require MORE clearance, right?    their logic just doesn't hold up. and, the piston materials now is different than they were in 1974.

Just to be clear, it's not based on logic.

Just to be clear.

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