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 Post subject: Re: Time for first oversize or not?
PostPosted: Sat Mar 17, 2018 11:47 pm 
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Speed Freak wrote:
More than one year has passed...

As the temperature is going up again I went to my garage and used the parts washing machine which I have fixed over winter :mrgreen:
After I have cleaned the parts I took them to the company and measured it.

What should I say?
The maximum diameter measured was 56,425mm, approximately the same on all 4 cylinders.
Measured ~2-3mm below the exhaust port.
All 4 cylinders show approximately the same wear, only the one with the scatch shows slightly more wear at TDC.

At TDC where the piston rings change direction I measured ~56,403mm in direction of in/ex port and 90° to this direction ~56,405mm.

Below the exhaust ports I also measured the area where the rings change the direction at BDC.
In direction in/ex ports I measured 56,425mm and 90° rotated 56,412mm


Measured with 2 tools - mine (Mitutoyo) 0,01mm scale and the one from my company (0,001mm scale).
Both show the same result and I can repeat the measurement as often as I want.



As I mentioned in the first post, the piston clearance should be 0,06-0,065mm according to the manual.
Dimensions (out of the manual):
Standard piston diameter: 56,39-56,4mm
Standard cylinder diameter: 56,4-56,42mm

There is something wrong with the dimensions...

I`m wondering if the std. pistons really have 56,4mm at the skirt.
Would mean it`s possible to get the cylinders honed to 56,46mm and the clearance should be correct??? :smt017

Anyone measured a Yamaha standard piston?
I`m thinking about buying one to measure it...
But the std. rear pistons are NLA, I only get front pistons with standard size. :smt013



I've recently grabbed up quite a few NOS Yamaha pistons for my recent RZ500 purchase. The following is what I got for measurements at the lower skirt with a 0.01mm accurate Mitutoyo micrometer. If the measurement was about between 2 lines I called it to .005mm.

Std Size Rear #1: 56.350mm
Std Size Rear #2: 56.345mm
Std Size Front #1: 56.345mm
Std Size Front #2: 56.345mm

O/S1 Rear #1: 56.640mm
O/S1 Rear #2: 56.640mm
O/S1 Rear #3: 56.640mm
O/S1 Rear #4: 56.640mm

I also measured the 2 Std TDR pistons I have:

Std TDR #1: 56.350mm
Std TDR #2: 56.345mm


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 Post subject: Re: Time for first oversize or not?
PostPosted: Sun Mar 18, 2018 6:21 am 
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Ok, thanks for the measurement.

This confirms that the values in the service manual are wrong for the standard piston diameter.
On the other hand it means the 1st OS pistons are ~+0,3mm, not 0,25

My measurements of the cylinders together with the piston diameters (considering measuring tolerances) => ~0,06mm clearance.

I`m still not really sure what to do...
1) Put 4x Mitaka std. pistons back into the cylinders as they are (~200€)
2) Buy Yamaha standard pistons (I have sources for all parts) (~ 550€ with piston pin and rings)
3) 1st OS (~280€ for the cylidners)

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 Post subject: Re: Time for first oversize or not?
PostPosted: Sun Mar 18, 2018 1:22 pm 
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I think honing the scratch out will take you out of spec.
Just O/S .25 and get the pistons you are happy paying for.
You will have a fresh cross hatch to break in the rings and many more oversizes available thanks to the aftermarket.

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 Post subject: Re: Time for first oversize or not?
PostPosted: Sun Mar 18, 2018 10:26 pm 
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I would be very hesitant to bore a cylinder that is within tolerance for it's current bore size. Especially uppers.... Especially just to run cheap pistons... I seem to remember at least a couple specific instances of Mitaka piston seizures on this forum. Not saying the Mitakas were the culprit, but when you got 40,000km out of one set of OEM pistons, why mess with success?

2 of the 3 well known engine builders in the US have both recommended TKRJ pistons for the RZ500 if going aftermarket. The other one uses Wiseco's it sounds like, but in more specialized applications it appears. With the price of TKRJ's, if you are considering them, you might as well try to scrounge up some NOS Yamaha pistons for not much more money. If you hope to achieve close to the same 40K km that the first set of pistons gave you with your next set, and you divide out the cost savings the Mitakas give you, the per km savings is negligible. And that is assuming the Mitakas give you the same life as OEM.

You say you have a few extra cylinders for a spare engine that need to be bored. If you are dead set on boring you should try to use as many of those as you can, even if you have to go larger than +0.25mm.

Did you do a pressure check before tearing the engine down? I would be concerned about the remaining useful life of 30+ year old crank seals with 40K on them. I think the worst case scenario might be boring everything up to OS1 just to have a crank seal induced seizure 1000km later, and before you know it, what were Std bore cylinders are now on the 2nd oversize.

If that were my bike, I'd find some Std OEM rear pistons, and get an expert's diagnosis on the scratched front cylinder. If it gets the thumbs up, run Std OEM pistons all around. If the scratched cylinder must be bored, bore it +0.25mm and use another front cylinder that is at, or needs to be bored to +0.25mm for the other front. (if you have one or can find one) Otherwise just leave the left front on the Std bore and run it with a new piston. Balancing an OS1 piston with a Std front should be no problem. I can't see there being any significant power difference between cylinders just because one is +0.25mm. TDR front pistons are reasonably priced.

And do the crank seals...


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 Post subject: Re: Time for first oversize or not?
PostPosted: Mon Mar 19, 2018 9:27 am 
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Some facts to my project:

- Bike has 40000km and was running (engine was never opened)
- Engine was disassembled because of damaged power valves, everything else looks good.
- Engine will be rebuilt completely with 100% Yamaha parts except some minor improvements which I have planned (new gaskets, everything checked, cases painted etc.)
- crankshafts are good and will be reused

I have 2x +0,25 and 1x +0,5 piston sets at home (Yamaha)
If I go to +0,25 it`s clear that I will use Yamaha parts.

If I want to go std. size I have to buy a set of standard pistons
- Mitaka (200€)
- Yamaha (550€, not so easy to get the rear pistons)

The scratch is too deep to remove it by honing. +0,25 (or 0,3) will remove it.

The technical perfect solution is to go to +0,25
Scratch removed and fresh honing for break in.

But the cylinders are at +0,25 - one step closer to scrap.
I think with new pistons it might be possible to do another 30-40000km
At this stage I think it will be also necessary to rebuild the crankshafts.
To be honest, I`m using my bikes on regular basis but I will never do 30-40000km with this bike...

Already bought one standard Yamaha rear piston... Seems that I have decided somehow what to do :smt003
If someone has a second one so sell it I would be happy to get an offer.

All other parts are still available from Yamaha

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Honda CBR 1000 RR SC57
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 Post subject: Re: Time for first oversize or not?
PostPosted: Mon Mar 19, 2018 11:45 am 
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Twidget wrote:
I would be very hesitant to bore a cylinder that is within tolerance for it's current bore size. Especially uppers.... Especially just to run cheap pistons... I seem to remember at least a couple specific instances of Mitaka piston seizures on this forum. Not saying the Mitakas were the culprit, but when you got 40,000km out of one set of OEM pistons, why mess with success?

2 of the 3 well known engine builders in the US have both recommended TKRJ pistons for the RZ500 if going aftermarket. The other one uses Wiseco's it sounds like, but in more specialized applications it appears. With the price of TKRJ's, if you are considering them, you might as well try to scrounge up some NOS Yamaha pistons for not much more money. If you hope to achieve close to the same 40K km that the first set of pistons gave you with your next set, and you divide out the cost savings the Mitakas give you, the per km savings is negligible. And that is assuming the Mitakas give you the same life as OEM.

You say you have a few extra cylinders for a spare engine that need to be bored. If you are dead set on boring you should try to use as many of those as you can, even if you have to go larger than +0.25mm.

Did you do a pressure check before tearing the engine down? I would be concerned about the remaining useful life of 30+ year old crank seals with 40K on them. I think the worst case scenario might be boring everything up to OS1 just to have a crank seal induced seizure 1000km later, and before you know it, what were Std bore cylinders are now on the 2nd oversize.

If that were my bike, I'd find some Std OEM rear pistons, and get an expert's diagnosis on the scratched front cylinder. If it gets the thumbs up, run Std OEM pistons all around. If the scratched cylinder must be bored, bore it +0.25mm and use another front cylinder that is at, or needs to be bored to +0.25mm for the other front. (if you have one or can find one) Otherwise just leave the left front on the Std bore and run it with a new piston. Balancing an OS1 piston with a Std front should be no problem. I can't see there being any significant power difference between cylinders just because one is +0.25mm. TDR front pistons are reasonably priced.

And do the crank seals...


Twidget:

He has to hone all his cylinders for his new pistons/rings so they will break in and do their job for 40000 kms right?
But his bores are already past the limits in the manual, right?
What happens when we put STD pistons in big holes?
Slapaty Slap that's what.

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 Post subject: Re: Time for first oversize or not?
PostPosted: Mon Mar 19, 2018 4:15 pm 
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It should be easier than this. All this fuss over std bore size. Go up one size and get the cylinders round again and in proper spec. Then you have another 40,000 km with no worries. If the last 40,000 took 33 years do you think it will be an issue in your ownership again?

One step closer to scrap is a bit dramatic. Max bore size is 58.00 before you run into problems. At 56.65 you have several lifetimes worth of material left.

And regardless of what Twidget says, there is nothing wrong with Mitaka pistons. They are as good as the cast OEM pistons. If there have been Mitaka failures because they are "cheap", it isn't openly discussed on this forum. There are failures for other reasons, including self-inflicted causes, but there are no quality issues that should cause any concern.

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 Post subject: Re: Time for first oversize or not?
PostPosted: Mon Mar 19, 2018 6:23 pm 
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And when you will have 200 000 kms, you'll still be able to re-sleeve your cylinders.
Did it with sleeves from L.A.Sleeve.


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 Post subject: Re: Time for first oversize or not?
PostPosted: Mon Mar 19, 2018 11:37 pm 
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When I recommended using the Std bore cylinders as-is, I was comparing their 56.425mm dimension to the manual's upper limit of 56.5mm mentioned by Speed Freak, and neglected actually doing the math to determine clearance.

56.425mm bore - 56.345mm piston = 0.080mm clearance

Yeah, that's getting up there.

Did you say what the diameters of the removed pistons were? I'm interested in knowing what clearance the engine had at the 40K mark.

I am starting to see Speed Freak's concern with the manual numbers. If 56.5mm is the upper wear limit of the cylinder, then that assumes a piston of at least a nominal diameter of 56.435mm would have be available to maintain acceptable clearance.

Are there any aftermarket pistons that tend to run a little large? Let's say your cylinders hone out to 56.430mm. A 56.365mm piston would keep the clearance in spec. Saving a bore might even be enough to get me to use a Mitaka if they happened to run that big. I've heard of some guys using hard piston coatings to get clearances to where they need to be but that seems a little extreme.

I guess I'm just a little more concerned about using up bore sizes than most. You can say there are enough usable bores available in a set of cylinders to go 200,000km, but that's assuming you'll get 40,000 trouble-free km out of every set of pistons. It sure is a good thing that these engines don't have a reputation for unexplained seizures. ;-)


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 Post subject: Re: Time for first oversize or not?
PostPosted: Tue Mar 20, 2018 2:16 am 
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Twidget wrote:


Are there any aftermarket pistons that tend to run a little large? Let's say your cylinders hone out to 56.430mm. A 56.365mm piston would keep the clearance in spec. Saving a bore might even be enough to get me to use a Mitaka if they happened to run that big. I've heard of some guys using hard piston coatings to get clearances to where they need to be but that seems a little extreme.

I guess I'm just a little more concerned about using up bore sizes than most. You can say there are enough usable bores available in a set of cylinders to go 200,000km, but that's assuming you'll get 40,000 trouble-free km out of every set of pistons. It sure is a good thing that these engines don't have a reputation for unexplained seizures. ;-)


If the bore is slightly larger after a good honing I suppose you could use a forged piston, like Wiseco, as they do require more clearance than cast pistons.

Back in the 80s and 90s we didn't have the high quality synthetic oils that we have now and cylinders would experience some wear through normal use. Going forward from this point in time using modern full synthetic 2 stroke oil will keep cylinder liner wear to a minimum. The next 40,000 km may show little or no wear at all. Bore to 56.65 and use the best oil you can get and you may never have to rebore again unless something fails or something goes terribly wrong.

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1985 RZ500 47X Spec Project Bike
1986 RG500 ATR
1986 NS400R
1987 TZR250
1988 Honda RC31
2007 DL650 VStrom
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 Post subject: Re: Time for first oversize or not?
PostPosted: Wed Mar 21, 2018 3:25 pm 
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Been riding my 500 a bit so thought id check the forum.

After I went to first oversize I seem to remember that Weisco comes in 56mm 56.25mm 56.5mm pistons so are in between Yamaha sizes, could just hone to fit and still be under first over. Did I remember right?

Used many Weisco never a problem


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 Post subject: Re: Time for first oversize or not?
PostPosted: Thu Mar 22, 2018 3:03 pm 
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Talked to a colleague in the company who is responsible for prototype engine assembly (also 2-stroke addicted :mrgreen: )
And I measured one std. piston...
Now I came to the result that I will go to +0,25

Measured the standard piston and it has 56,33mm (funny how different they are...)
Would mean this piston has 0,07mm clearance to the new standard bore.
+0,03mm = 0,1mm (out of spec)

Together with the missing crosshatch below the exhaust port the colleague didn`t recommend to reuse the cylinder.
His opinion was that the risk is too high to destroy crankshaft, conrod and cylinder head in case of a failure.

Let`s see how deep the scratch is.
Maybe they can move the bore center slightly to the exhaust side during machining, would mean more material will be removed on the exhaust side than on the intake side.
Will not have influence to the function of the engine.
Modern engines have up to 5mm offset between crankshaft and cylinder axis.

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 Post subject: Re: Time for first oversize or not?
PostPosted: Thu Mar 22, 2018 4:19 pm 
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How far offset can you go and still maintain proper clearance at the rod big end, or is that not really a concern?

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1985 RZ500 47X Spec Project Bike
1986 RG500 ATR
1986 NS400R
1987 TZR250
1988 Honda RC31
2007 DL650 VStrom
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 Post subject: Re: Time for first oversize or not?
PostPosted: Thu Mar 22, 2018 5:54 pm 
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+0,25 is 0,125mm radial => max. offset ~0,1mm or even less, otherwise the intake side will not get a new surface.
I think there should not be a problem except the "bad" conrods are used where the machining is not correct.

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 Post subject: Re: Time for first oversize or not?
PostPosted: Fri Mar 23, 2018 1:46 am 
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Is it just me or does metric suck for measuring these small tolerances?
Standard just works better.
.25 O/S????
Don't you like slapady slap??
Lol.

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