TZ/ SPR pistons

[epa police]

When the bike arrives I will be pulling the motor down and putting new pistons in regardless. SPR pistons are $130 each plus rings and 4DP pistons are $83 each plus ring so obviously I want to go down the 4DP route. This will be a track bike only so the single ring doesn't concern me but assuming the bore is good which size TZ pistons should I use A,B,C or D size. Does anyone know the size difference between the four TZ pistons. The ring peg position is 6 o clock on the TZ and SPR piston so thats ok.

[EEKNOWS]

A,B etc refers to the size a being the smallest.The sizes ore on the TZ forum "somewhere"

[Paul]

am i mistaken or can you only buy the A size TZ pistons

[EEKNOWS]

Get B's from Japan no problems, imagine the others wouldn't be hard to find.

[epa police]

B size are available for sure. I know the letters mean different sizes I didnt know what the increments were.     

[Neal]

0.005mm If I am correct

[Paul]

My parts supplier informs me only the A's are available (A for available??!)
FWIW that's all I bought for spares. They came from Japan on back order.

[Warwick]

Bs are the closest to stock size from memory. I've always used Bs if running TZ pistons myself.

[epa police]

Thats interesting Paul because Accu Performance have B size pistons listed. The difference must be nothing anyway as the same rings are used for all and .005mm as suggested by Neal sounds about right.
I have a few good used pistons TZ here but there is no way of telling what size they are without a micrometer which I dont have.

[Paul]

ja, i am just talking of yamaha not listing the parts
i honestly dont think the difference can be that vital for what i use the bike for
besides, it is virtually impossible to measure that small, any measurement you get has to be seen in that light, it can only be theoretical and taking a measurement a number of times would have to lead to varying results

[epa police]

I just ordered some B size pistons as spares for the TZ anyway. I will be expecting a replate on the SP/SPR/? bike when it gets here so this way I'm prepared 

[Jim Lyon]

quote author=Paul link=topic=586.msg6074#msg6074 date=1360735399]

ja, i am just talking of yamaha not listing the parts
i honestly dont think the difference can be that vital for what i use the bike for

JL - Ah, now there's a voice indulging in wishful thinking! - If Gordon Jennings were to only hear that, he'd
be turning in his grave!
Now I understand & sympathise with why you're saying what you are, because the alternatives are difficult.
But then technical excellence, @ whatever level, don't come easy. - So why do it? To put it simply, tight piston - bore clearances give more power, & loose ones allow hot gas blowby to burn the lubrication off the bore & cause seizures!

besides, it is virtually impossible to measure that small,

JL - It's not impossible, just difficult. The whole reason for developing Nicasil is to get tighter tolerances in order to aid heat transfer, thus obtaining more power & greater reliability.

any measurement you get has to be seen in that light,
it can only be theoretical

JL - Not @ all. There are entirely practical consequences of getting this wrong, as I've pointed out above e.g. a seized engine.

and taking a measurement a number of times would have to lead to varying results

JL - True, to a degree. But, as in many endeavours, "practice makes perfect". It's a great pity that you don't have access to the December 1979 issue of "Cycle", containing Phil Schilling's article "The TZ Papers" in which he explains the importance of "To measure is to know". The new tools I got for this job are a 50-75 m.m. Oxford external micrometer ( £20 ) & a 50-160 m.m. Format ( brand name of German group of companies ) dial bore gauge ( £97 - those prices are fairly cheap for 8-10 years ago in the UK, & it took quite a bit of searching to find them, but still cheaper & less inconvenient than seizing engines ). The technique is to measure up the piston with the micrometer, lock it @ that setting. Set up the bore gauge to the micrometer, then set the gauge to zero. Running the bore gauge up & down the bore will give you the exact clearances between piston & bore.
In passing, some brand new TZs come with throw away rattlers for pistons, & just because a piston comes in a box stating it's such & such a
size, means nothing - unless of course you're so naive as to trust a complete stranger on the opposite side of the world?

[/quote]                 See JL comments x 4, above

[Neal]

JL - What's the better clearance to use for eg - 0.03 or 0.04 or 0.05mm ?
For racing only .
Thank you 

[Jim Lyon]

JL - What's the better clearance to use for eg - 0.03 or 0.04 or 0.05mm ?
For racing only .
Thank you

I'm not familiar with your particular set up, but as a default position, I'd automatically set the clearance for whatever minimum the manufacturer recommends as a tolerance. Even with a perfect fit, pistons come with a LOT of stresses built in from manufacture, so at least a couple of good heat cycles ( & if you don't have a huge fan @ a dyno, then NON stationary heat cycles as that allows the engine to be cooled as the manufaturers intended ) to allow them to relax & adapt to the new bore goes a long way to a good fit. And, as I keep on saying, follow this link

                                     http://edj.net/2stroke/jennings

& while ANYthing by Gordon Jennings is well worth reading  ( several times ! ), in particular read the articles on Basics Race Preparation & Oil Premix Ratios. Put simplistically, use as  rich a petrol & oil ratio as your ignition system can tolerate. And if that doesn't work, read MY comments on my H.E.I.S thread ! - HTH?

[Jim Lyon]

ADDENDUM

Re my earlier comment "I'd automatically set the clearance for whatever minimum the manufacturer recommends as a tolerance"

The manufacturers don't always get it right. Some years back on the RD Forum there was some discussion about problems with Mitaka pistons. the problem that was discussed most was the fact that the expander ring behind the top ring tended to cause the ring to stick. The eventual conclusion was that it didn't work well with modern oils, & it was recommended by Mick Abbey not to use it.
The other problem that was hardly ever discussed was the piston tended to rock. My view was "Then why not run it @ a tighter clearance?" Only one person thought similarly & was prepared to do something about it. So, with his next rebore he had the minimum recommended clearance reduced by 2 thou - & after quite a few miles it was working just fine!
I think there's a lesson there?