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ABC of Pistons


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#1 Greg

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Posted 21 November 2006 - 01:09 AM

Since there is a great deal of confusion over Ural pistons and cylinders, here are the dimensions taken from a Russian manual. Note these are for 650 engines. Since the 750 engines have only been around for eight years, nothing has been published for them yet.

Original bore pistons.
A -  Piston diameter - 77.91-77.92 - cylinder diameter - 78.00-78.01 - clearance 0.08-0.10
B -  Piston diameter - 77.92-77.93 - cylinder diameter - 78.01-78.02 - clearance 0.08-0.10
C -  Piston diameter - 77.93-77.94 - cylinder diameter - 78.02-78.03 - clearance 0.08-0.10

Oversize Pistons.
R1 -  Piston diameter - 78.11-78.14 - cylinder diameter - 78.20-78.23 - clearance 0.09
R2 -  Piston diameter - 78.41-78.44 - cylinder diameter - 78.50-78.53 - clearance 0.09
R3 -  Piston diameter - 78.91-77.94 - cylinder diameter - 79.00-79.03 - clearance 0.09

Wrist Pins.
White - 20.9975-21.0000
Black - 20.9950-20.9975
Red - 20.9925-20.9950
Green - 20.9900-20.9925

Now the piston clearances are for an aluminum piston in a cast iron barrel. The 750 has an aluminum piston in a steel lined cast aluminum barrel (similar to the Dnepr 650) so I'd be inclined to use the Dnepr clearance of 0.05-0.07.

I can't recall ever seeing any dimensions for the Taiwanese pistons used by CSMI, but I very much doubt that they came in A,B and C sizes. I also don't know what oversizes they came in.

Now why IMZ persisyed with this byzantine system of piston sizing when the 750 engine came out, I'll never understand, but it seems they do.

Anyway what is the capacity difference between the smallest A cylinder and the largest C cylinder for the 750 - 0.2865 ml (that's assuming that the same tolerances were kept as for the 650).

Hopefully, this will clear up some of the confusion and will explain why there's often a painted mark on the wrist pin bosses.

#2 Serious Black

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Posted 21 November 2006 - 11:04 AM

You will find that modern motor manufacturers still use this ' Byzantine' method of matching the correct piston to the correct cylinder.
The devil is in the detail.

#3 Ace

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Posted 21 November 2006 - 09:10 PM

Hey Greg do you have a similar set of specs for Dnepr pistons wrist pins and cylinders?  I would sure like to know.
Ace
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#4 valicaddy

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Posted 22 November 2006 - 03:59 AM

tattoocurator, on Nov 22 2006, 05:10 AM, said:

I would sure like to know.
And you're not the only one!! This may help me not to ruin my engine..
M72, FLHP

#5 Sam

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Posted 27 November 2006 - 03:51 PM

Similar specs on the 750's are in the back of the "Katalog" on the "unofficial" web site-- Bill's site, I believe?

http://www.myural.co...log_diagram.htm

click on the .pdf, it'll open up the parts diagram, the list of crank, cylinder, and piston sizes is the last page.

#6 IVANGROZNEY

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Posted 01 December 2006 - 04:59 PM

Oh your killing me. I've got a 2002 CSMI 750 so none of this applies and if the need arises I'm going to have a heck of a time finding out what will work.
New Jersey Ken,
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#7 greenmachine

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Posted 01 December 2006 - 06:27 PM

I think that CMSI was using some Taiwan made pistons with Hastings rings but I could be mistaken yet again.

I'm thinkin that nobody changed the BORES on the cylinders, but just used some pistons and rings which might not have been available through IMZ at the time and were suited to the USA......   I would then conclude that factory pistons and rings would be suitable without any big hassle. ?

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#8 Sam Simons

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Posted 04 December 2006 - 04:20 PM

Piston to cylinder tolerances aside,this strategy/method makes perfect sense IF the engines were of chrome bore.....as many Jap engines are/were....it doesn't make sense otherwise,in fact,they created
a larger parts support inventory requirement than needed....

Sam in IN
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#9 Greg

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Posted 04 December 2006 - 11:43 PM

View PostSam, on Nov 27 2006, 03:51 PM, said:

Similar specs on the 750's are in the back of the "Katalog" on the "unofficial" web site-- Bill's site, I believe?

http://www.myural.co...log_diagram.htm

click on the .pdf, it'll open up the parts diagram, the list of crank, cylinder, and piston sizes is the last page.

I'd be wary of trusting that data as it's lifted straight from the 650 catalog - even includes the 650 crankshaft bearings and seals rather than the larger 750 items.

As to the Dnepr data, it's been recently posted to the bCozz site.

Finally, within the last 10 years, IMZ have used Taiwanese diecast pistons, Russian diecast pistons, Russian sandcast pistons and maybe some forged pistons with various combinations of piston rings. I'd be prepared to bet that NO research was done into what those differences meant for bore clearances! The A,B,C system was designed for diecast pistons in sandcast iron barrels, not sandcast pistons in steel cast-in liners in diecast barrels.

#10 Ed Paynter

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Posted 05 December 2006 - 12:30 AM

This is just pure speculation, but a thought occurred to me.....
do you think it might have been a compromise between quality control and productivity demands?

I'm thinking that rather than discarding slightly undersized or oversized cast iron jugs, you just size them into 3 subsets, and match them up with similarly off standard pistons?  This type of "quality control" would keep production output at maximum while making only minimal concessions to off standard losses (it's got to be WAAYYY  out before you discard it!)

It's similar to something benchrest shooters do when reloading......
since they constitute a large percentage of the cost, weigh all of your resized and trimmed cases and then all of your slugs and separate them each into lots of 5 of matching weights (or 6, if you want a spotter) since a match consists of 5 rounds per target.   Although there may be significant variations in point of impact between each separate 5 shot string, the rounds in each individual group will have very similar pressures, thus ballistics and point of impact.  

Some of the diehards shoot an entire match with a single case, weighing and grouping bullets and sometimes even primers and reloading THE CASE.... for each round of fire.

It may sound a bit anal, but the resulting accuracy is measured in .000ths of an inch, literally putting all 5 of the rounds in a single hole, usually requiring calipers to measure the hole then subtract the bullet diameter to determine group size.

OK, maybe it's not that similar?!   :)

ed
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#11 zach

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Posted 06 December 2006 - 08:27 PM

3.0677, 3.0681, and 3.0685 inches respectively.

#12 Sam Simons

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Posted 06 December 2006 - 09:29 PM

Hello Mr. Cob,

I'll try to help,based upon my experience in rebuilding various high performance Yamaha & Kawasaki
multi-cylinder two stoke engines (i.e. RD,R5,RZ,LC and H1 engines). But,2 or 4 stroke,the proceedure
applies to either. Here goes-

I also don't understand the Ural piston size 'proceedure', but, I haven't seen/heard anything about the
piston to cylinder wall recommended clearances-what are they? As you likely already know,the
clearance will vary from factors such as piston type and metalurgy,intended use,etc.
All of that aside,in your case,I'm guessing that your cylinders are worn...hence,this engine teardown.
Now,if that's the case,then the cyl's aren't round(if they ever were..),and you should rebore them;so,if I were in this situation,I'd select the size of piston that would allow me to bore and hone to the desired
final cylinder clearance...to hell with that ABC bullsh*t......it makes no sense to me unless those cyl's were
either chromed or  cyro-hardened,and,that's not a bad idea to do the cyrogenic process and pick up
a second set of whatever size you choose now....Boretech in OHIO does very good work.

Hope this helps in some manner....

Sam in IN
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#13 greenmachine

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Posted 07 December 2006 - 12:03 AM

Each size is 4 thousandths of an inch larger.....   right?


I can't measure stuff that close even with a digital caliper. I'm always a couple thousandths off in any case. I probably wouldn't make a very good living as a machinist I'm thinkin....



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#14 zach

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Posted 07 December 2006 - 12:03 AM

Mr. Cob,
The most accurate way to convert MM to inches is to use the 25.4 mm to an inch. You are slighty rounding up the last digit calculating the opposite way (use .03937 instead of .0394).  So - use MM's in an inch instead of inches in a MM for an easier to remember conversion number.

Yes the difference between the three is slight - .0004, but it coincides with the numbers Greg posted. It does seem like a very tiny tolerance for the application!
-Zach

View PostMr. Cob, on Dec 6 2006, 07:34 PM, said:

View PostPossum, on Dec 6 2006, 08:27 PM, said:

3.0677, 3.0681, and 3.0685 of an inch respectively. I say wrong numbers fella.


Howdy Possum,

Even using your numbers the difference between 3.0677 and 3.0685 is LESS THEN 1/1000 of an inch, 0.0008 to be exact.

I arrived at my numbers by multiplying MM's by 0.0394, thus 77.92MM times 0.0394 Inches is equal to 3.07005 inches.  What am I doing wrong?


#15 plaviator

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Posted 19 February 2017 - 05:21 AM

My M72 has 20mm wrist pins and I am finding it difficult to find oversize pistons for 78.4 bore. Does anyone know of a source?




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