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Pistons + rings and clearance.


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#1 JonasD

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Posted 20 February 2011 - 06:35 AM

Hi!

I am about to order new pistons from Ural-Zentrale.
When i measure the cylinders they are about 78,12 and 78,24.
I guess it would be enough bore up for the 78,48 pistons.
But...
Perhaps it would be good to put in the "Piston ring sv engines Schoettle 79 mm"
that they have in most sizes but not in 78,5. So i guess it will be the 78,9 pistons.

Does anyone have an opinion about those rings compared to the original ones?
And what clearance should i tell them to make when i leave it away for boring?

/Jonas

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K-750M 1969
Always something to repair, upgrade or to rebuild just for fun...

#2 JonasD

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Posted 20 February 2011 - 06:40 AM

Perhaps i should say its a K-750 (not using the sidecar)!  :biggrin: :rolleyes:
K-750M 1969
Always something to repair, upgrade or to rebuild just for fun...

#3 DneprCossack

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Posted 20 February 2011 - 12:15 PM

Don't bore the cylinders before you get the new pistons. Deliver the pistons together with the cylinders to the machine shop (and the tolerances you get from Ural-Zentrale). The pistons is not always that accurate as stated....
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#4 64treefrog

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Posted 20 February 2011 - 02:06 PM

I have just fitted new pistons (the expensive ones) from ural zentrale and one piston had circlip grove only partly cut. The circlip had no seat at the top! Very delicate work with a Dremel.
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#5 immulmen

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Posted 20 February 2011 - 02:58 PM

 JonasD, on 20 February 2011 - 06:35 AM, said:

Hi!

I am about to order new pistons from Ural-Zentrale.
When i measure the cylinders they are about 78,12 and 78,24.
I guess it would be enough bore up for the 78,48 pistons.
But...
Perhaps it would be good to put in the "Piston ring sv engines Schoettle 79 mm"
that they have in most sizes but not in 78,5. So i guess it will be the 78,9 pistons.

Does anyone have an opinion about those rings compared to the original ones?
And what clearance should i tell them to make when i leave it away for boring?

/Jonas

The original BMW R71 manual has piston clearance at .003 inches without a sidecar and .006 with. I don't know about the ring clearance.

#6 Cold War Trophies

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Posted 21 February 2011 - 01:09 PM

 JonasD, on 20 February 2011 - 06:35 AM, said:

Hi!

I am about to order new pistons from Ural-Zentrale.
When i measure the cylinders they are about 78,12 and 78,24.
I guess it would be enough bore up for the 78,48 pistons.
But...
Perhaps it would be good to put in the "Piston ring sv engines Schoettle 79 mm"
that they have in most sizes but not in 78,5. So i guess it will be the 78,9 pistons.

Does anyone have an opinion about those rings compared to the original ones?
And what clearance should i tell them to make when i leave it away for boring?

/Jonas

K750 piston clearance must be 0.07-0.09mm.  Service/wear limit is 0.25mm.  Piston ring end gap can be 0.15-0.6mm.

If at all possible, use the bad looking ill reputed dull original USSR made pistons if you have the option.  Contrary to popular opinion, USSR engineers really were good at their work, and their designs are sound.  Original factory pistons could be ordered as a balanced set(identical weight), which is ideally what you want to use to avoid crank stress.  I've never had any problems with the OEM pistons when I set up the clearances correctly and made sure the ignition and fuel system were within specs.

Clint
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#7 JonasD

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Posted 21 February 2011 - 03:00 PM

Thank you very much for that!

I will order the original pistons, but with the optional rings...
If there is not anyone that would say that also the rings should
be the original one's.

Thanks again!
/Jonas
K-750M 1969
Always something to repair, upgrade or to rebuild just for fun...

#8 Cold War Trophies

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Posted 21 February 2011 - 06:00 PM

 JonasD, on 21 February 2011 - 03:00 PM, said:

Thank you very much for that!

I will order the original pistons, but with the optional rings...
If there is not anyone that would say that also the rings should
be the original one's.

Thanks again!
/Jonas

If they are CJ750 rings, or chrome plated/hardened steel I'd avoid them.  In general, a cast iron ring on cast iron cylinder will pose few problems as they should expand at very similar rates and value.  The question becomes one of metallurgical quality, harder cast iron is more brittle and is more prone to fatigue cracking due to the heating and cooling cycle of a motor, plus the bending from compression & power stroke to relaxed.  The Russian rings work, but aren't known for going the distance unless you get lucky.  I set Russian ring gap towards minimum side, but on an aftermarket ring I'd err on the side of caution and go to the max side of tolerance; if the ring gap is too tight and the ends butt together you'll seize the motor and destroy the piston again.  Also make sure they turn freely in their grooves and are not deeper then the groove!  Rings generally need about 0.02-0.03mm clearance in the ring groove and should be able to seat flush with the piston wall.
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#9 Serious Black

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Posted 22 February 2011 - 03:44 PM

My experience of newer Russian rings has not been good. They appear to lose their tension. Try the pistons from this guy - http://www.chang-jiang.com/ . I bought a set, Thinner rings from the Chang Jiang OHC high performance pistons. Not tried them yet but they look good.
The devil is in the detail.

#10 madoc

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Posted 20 March 2011 - 02:57 PM

Quote

K750 piston clearance must be 0.07-0.09mm. Service/wear limit is 0.25mm. Piston ring end gap can be 0.15-0.6mm.

I think the Ural is supposed to be .04mm - .08mm. That's an awful lot smaller ??

How bad are russian rings ?
The ones that came out, of mine, are around 0.6mm with only maybe 3k miles on them.

I'm just about to refit a new piston and will need to check all the gaps etc
John Nash

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1943 Ariel WNG
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#11 Serious Black

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Posted 20 March 2011 - 03:36 PM

Thinner pistons are lighter and so have inertia. This allows them to do there job better. A little too simple an explanation really, try Wikipedia for a better one.
The devil is in the detail.

#12 madoc

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Posted 20 March 2011 - 05:40 PM

I have the MT16 manual up and it's giving a ring gap of 0.25-0.5mm:
http://curtperedina...._ops_manual.pdf

why is the Ural gap so small at 0.04mm ? Manual is wrong ..right ?
http://multiply.com/...:photos:175/127

Compared to my 1940 350cc enfield (0.27mm to 0.38mm - iron barrel) and 1947 500cc ariel ( 0.15mm to 0.2mm - iron barrel) then the 0.04 gap in the ural book seems even more implausible.

<update>

Someone pointed out that the 0.04mm value might be the up and down clearance, I read further through the manual and another bit, although as ambiguous (i.e. as poorly translated) might give the right values of 0.25-0.45mm, with a max is 0.7mm.
Of course, I only know these look "more right" as I have now have the other values, from similar engines to compare to.
John Nash

2005 BMW F650GS
1968 chang jiang 750 sidevalve solo
1965 K750 project
1963 M62 combination
1950 Ariel KH500
1947 Ariel OHV 600 twinport
1944 Ariel WNG
1943 Ariel WNG
1940 Royal Enfield WD/CO

#13 barrydavies

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Posted 27 December 2017 - 08:00 PM

Hi Chaps
Going to rebore the cylinders on my CJ750  OHV sidecar 78 mm bore  (when the pistons arrive),  after the reading below comments I have come to the conclusion .006" ( thou ) piston clearance would be about right , the old pistons had "picked up " being too tight from new , don't want it to happen again , also will check what they are now to get a bit of an idea, any comment would be welcome
Regards Barry

#14 luca.stere

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Posted 28 December 2017 - 03:02 AM

The piston clearance is established by the piston manufacturer, the original old soviet pistons were with 0.008 clearance, polish ALMOT pistons works with 0.006 for example.




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