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 Post subject: Time for first oversize or not?
PostPosted: Fri Nov 04, 2016 5:27 pm 
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Hi,

I was discussing this week with a colleague about oversize pistons.
His opinion: As far as the cylinders are not out of spec (56,4-56,42 is standard, wear limit is 56,5mm) he would give the cylinder a fresh hone and put in a new standard piston.

My RD has 40000km with the first pistons in close to perfect condition but I think they are already out of specification.
Cylinders are not measured up to now but I think i will take them to the company next week and ask if they can measure them.

I pulled the cylinders because the front Power Valves were stuck, otherwise there was no need to disassemble the engine. It was running quite nice.


If the cylinders are still below 56,5mm what would you do with this one?
It`s the only scratch I found on all 4 cylinders and I can feel it clearly with my finger nail (I hope not too deep for +0,25mm which is only 0,125mm in radius, not really much).
Would you ignore it and put a set of standard pistons back in it? Or should I search for one cylinder which is at the same wear level as the other 3 and put this one to my 2 other "oversize needed" cylinders for my second engine?
Image

By the way, the specifications are a bit funny in the service manual.
Piston clearance should be 0,06-0,065mm, standard piston is 56,39-56,4mm, standard bore should be 56,4-56,42.
This would mean on a new standard engine there is a piston clearance of 0,00 to 0,03mm :smt017

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 Post subject: Re: Time for first oversize or not?
PostPosted: Fri Nov 04, 2016 5:43 pm 
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You have 40,000 km on the original pistons?


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 Post subject: Re: Time for first oversize or not?
PostPosted: Fri Nov 04, 2016 6:12 pm 
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Yes, I think the engine was never opened, everything looked untouched.
As far as I remember I have measured the pistons after disassembly and they were far out of spec at the skirt where it should be measured according to the workshop manual.
I will measure them with more accurate tools next week together with the cylinders.

This video was made after I got the bike - carbs revised, new air filter and fuel in and then started.
Carbs were not syncronized at this stage, bike was in storage for 2 years but used all the time by it`s 7 or 8 owners since 1988 where it was first registered.
After a test ride I disassembled the bike because of a lot of little problems like the PV`s.
https://www.youtube.com/watch?v=z4yve8q2eeE

I have planned to rebore the cylinders (+0,25 piston set already bought).
But if it`s not necessary to rebore I would put them on the shelve and use std pistons now.

By the way, the NS400R has 60000km on the first pistons and they look great, only piston rings were changed at 34000km - engine died from the main bearing.
For the Honda NSR125 it`s also common that you can use the first piston for 50000km, I have sold mine with ~45000km and first piston, still running 155km/h on the straight as with 12000km where I have bought it.
My 350 got new oversize pistons at 38000km.

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 Post subject: Re: Time for first oversize or not?
PostPosted: Fri Nov 04, 2016 6:27 pm 
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You did very well. Typically 20,000 km is considered a lot for 2 stroke pistons. With my RGV 10,000 km was thought to be the time to replace them.

Did you do a compression test before opening the cylinders? Just wondering what the cylinder pressure was with 40,000 km pistons and rings.

I think you made the right decision to go up to 56.65 bores. Your bores are likely out of round now. I bet you'll notice a difference in performance.

When I opened my TZR recently to replace leaking crank seals at 15,000 kms (failure related to storage) I rebuilt the whole engine, including new pistons/rings/pins/bearings. Both bores were at 56.41 and still perfect using my digital bore gauges. I could have left the top end as it was, but once you have the engine opened you might as well do the job right.


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 Post subject: Re: Time for first oversize or not?
PostPosted: Fri Nov 04, 2016 6:44 pm 
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I agree, the best option from technical view is to rebore it, then everything is as new.
But then there is the opinion of my colleague: If you rebore it the cylinders are one step closer to the garbage.
I think in 20 years not much parts are left over...

Even more interesting, one guy in Germany (RD350LC) recommends to just slightly rebore the cylinders if they are out of spec and use coated pistons to get the right clearance...
Another way to avoid early first oversize.

Sadly I did no compression test before opening it as it was running fine after synchronizing the carbs and it was clear that I will revise the engine when it is open.

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RD350 YPVS 31K 1985
Honda CBR 1000 RR SC57
Yamaha R1 RN04


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 Post subject: Re: Time for first oversize or not?
PostPosted: Fri Nov 04, 2016 6:50 pm 
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If you go one size up every 40,000 km and max bore is 58.00 you have a very long way to go before you have to worry about it :grin:


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 Post subject: Re: Time for first oversize or not?
PostPosted: Fri Nov 04, 2016 7:01 pm 
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But Yamaha pistons are only available up to +0,5mm - and I need to buy a set of them before they are not longer available.
After that the bike is not 100% standard anymore :smt003

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RD350 YPVS 31K 1985
Honda CBR 1000 RR SC57
Yamaha R1 RN04


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 Post subject: Re: Time for first oversize or not?
PostPosted: Fri Nov 04, 2016 9:28 pm 
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Lol...yes, I see your dilemma.

I have + 0.5 Mitaka pistons. I wonder how much that will devalue my bike :smt003


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 Post subject: Re: Time for first oversize or not?
PostPosted: Sat Nov 05, 2016 8:18 am 
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By the time you get around to needing replacements bigger than .5 mm oversize, you will be able to purchase a set of 3D printed pistons from a vending machine at your local refueling station.

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Last edited by Rick Lance on Sat Nov 05, 2016 4:41 pm, edited 1 time in total.

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 Post subject: Re: Time for first oversize or not?
PostPosted: Sat Nov 05, 2016 11:41 am 
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I've seen a RS150 from Malaysia with over 75,000km that had never been opened snd still ran pretty well.

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 Post subject: Re: Time for first oversize or not?
PostPosted: Sat Nov 05, 2016 4:01 pm 
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24,000 miles or 40km on stock piston, I've viewed many, many times especially on the " so called inherently out of balance" " RZ 350YPVS.

As mentioned in another post, owned & ridden several that survived a certain young teen/early adults abuse of every possible kind :smt109
Each time the bikes ran great, with 30,000 miles or more on each.

While technically speaking if one was to have disassembled them, its likley they would have been out of spec.probably guaranteed...lol

The old adage, "dont fix it if it aint broken" comes to mind. But only to a certain point

As Capn. Obvious would say, " better to be safe than sorry " w preventative maintenance & scheduled service intervals, especially with the V-4 a complex and pain in the ass endevor that could get very expensive if something catastrophic were to happen.

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 Post subject: Re: Time for first oversize or not?
PostPosted: Sat Nov 05, 2016 4:57 pm 
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They all work well, until they don't. The longer 2 stroke pistons are in use the closer they are to failure. Pay a little now or pay a lot later.


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 Post subject: Re: Time for first oversize or not?
PostPosted: Sat Nov 05, 2016 9:07 pm 
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silverstrom wrote:
They all work well, until they don't. The longer 2 stroke pistons are in use the closer they are to failure.


Kind of applies to all of us....


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 Post subject: Re: Time for first oversize or not?
PostPosted: Sun Nov 06, 2016 1:16 am 
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Some more than others Russ. Before you know it your long rod stroker days are gone and you compressor stall.


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 Post subject: Re: Time for first oversize or not?
PostPosted: Tue Mar 13, 2018 3:59 pm 
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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

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My bikes:
RD500 YPVS 1GE
RD350 YPVS 31K 1985
Honda CBR 1000 RR SC57
Yamaha R1 RN04


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