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 Post subject: letters on pistons
PostPosted: Tue Jan 02, 2018 4:13 pm 

Joined: Wed Jan 18, 2012 10:36 pm
Posts: 95
I took the top end off of my S2 today and the left cylinder's piston had an "A" marking, while the other two had a "B" marking.

Either of the "B" pistons fit just fine in the "A" jug....and the "A" piston does not feel sloppy in either of the "B" bores.
I have a set of digital calipers, but lost the damn battery cover and the battery, Ill get a new set soon.

In general though, what does this mean when it comes time for new pistons? Or if I just go with rings? Is B just the first oversize? In my experience with 2 stroke dirt bikes for the last 30 odd years its either been no marking or STD for standard, then .020 etc on up for the oversizes.

Thanks!

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1972 S2 350, 1972 F9 350 Bighorn, 1975 KX400, 1981 YZ465, 1980 XT500 (Built for MX) , 1987 KD80, 1995 MZ Skorpion, and a Doodle Bug


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 Post subject: Re: letters on pistons
PostPosted: Tue Jan 02, 2018 4:19 pm 

Joined: Wed Jan 18, 2012 10:36 pm
Posts: 95
Heh. I looked on ebay to see some pics of s2 pistons....most noteably I saw a standard piston marked "std" AND "B"
and there was an .040 over piston stamped with an "A".....so now Im really intrigued.

I suppose Ill just have to get some good measurements on all of them and go from there. Seems the letters are not indicative of the size.

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1972 S2 350, 1972 F9 350 Bighorn, 1975 KX400, 1981 YZ465, 1980 XT500 (Built for MX) , 1987 KD80, 1995 MZ Skorpion, and a Doodle Bug


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 Post subject: Re: letters on pistons
PostPosted: Tue Jan 02, 2018 4:30 pm 
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Joined: Thu Jun 09, 2011 6:53 am
Posts: 120
Location: Rochester,NY
I can measure tomorrow. I have NOS pistons here. Can measure tomorrow, with a A and B on them


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 Post subject: Re: letters on pistons
PostPosted: Tue Jan 02, 2018 10:16 pm 
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Joined: Tue Jun 07, 2011 6:36 am
Posts: 2370
Location: Dandridge, TN, USA
The letters reference a fit tolerance for the bore. It does not have anything to do with "oversize" bore steps.
Let's say piston dia spec is 5 to 7... STD would be 6, A would be 5, B would be 7.
I don't know specifics but you get the idea.

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 Post subject: Re: letters on pistons
PostPosted: Wed Jan 03, 2018 12:13 am 

Joined: Thu Jun 09, 2011 11:25 am
Posts: 2487
There is also what was called a "magic number". This number was 32. Most people have never seen it, some have only seen one part of it. Magic numbers were used on the two stroke engine build lines.

All magic numbers were done in Marks-A-Lot. One part was on the piston dome, the other in the cylinders skirt that fits down into the case. The piston marks go away soon after the engine is running, most skirt numbers stay in place.

To use the number, an engine builder would find the skirt number, let's say 18 as an example. The correct piston he would select would be a number 14. Perfect piston clearance

The magic number would never be used once the engine was put into service, they went from there, to A, B, C, etc.


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 Post subject: Re: letters on pistons
PostPosted: Wed Jan 03, 2018 3:13 pm 

Joined: Wed Jan 18, 2012 10:36 pm
Posts: 95
I got a pair of calipers working, not the best Im sure, but all the pistons are coming in around 52.89mm, and given the lack of any oversize on the domes, Ill assume Ive got standard bores here.

Ill have to make the decision at some point whether Im going with new standard pistons, or going oversize. For arguments sake, I suppose if I were to go with new standard units, Id look for 2 "B"s and an "A"....in reality I think I need to have them bored, even if it will add some time and expense.

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1972 S2 350, 1972 F9 350 Bighorn, 1975 KX400, 1981 YZ465, 1980 XT500 (Built for MX) , 1987 KD80, 1995 MZ Skorpion, and a Doodle Bug


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 Post subject: Re: letters on pistons
PostPosted: Wed Jan 03, 2018 4:26 pm 
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Joined: Tue Jun 07, 2011 6:36 am
Posts: 2370
Location: Dandridge, TN, USA
If it were mine I'd try to put the pistons back in the same holes they came out of but I wouldn't be too concerned about it. The piston dia tolerance is +0.0002/-0.0010in.

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